// Copyright 2020 The Wuffs Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//    https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

pub struct decoder? implements base.token_decoder(
        quirks : array[QUIRKS_COUNT] base.bool,

        allow_leading_ars  : base.bool,
        allow_leading_ubom : base.bool,

        end_of_data : base.bool,

        trailer_stop : base.u8,

        // comment_type is set as a side-effect of decode_comment?.
        //  - 0 means no comment.
        //  - 1 means a block comment.
        //  - 2 means a line  comment.
        comment_type : base.u8,

        util : base.utility,
) + (
        // stack is conceptually an array of bits, implemented as an array of u32.
        // The N'th bit being 0 or 1 means that we're in an array or object, where
        // N is the recursion depth.
        //
        // Parsing JSON involves recursion: containers (arrays and objects) can
        // hold other containers. As child elements are completed, the parser needs
        // to remember 1 bit of state per recursion depth: whether the parent
        // container was an array or an object. When continuing to parse the
        // parent's elements, `, "key": value` is only valid for objects.
        //
        // Note that we explicitly track our own stack and depth. We do not use the
        // call stack to hold this state and the decoder.decode_tokens function is
        // not recursive per se.
        //
        // Wuffs code does not have the capability to dynamically allocate memory,
        // so the maximum depth is hard-coded at compile time. In this case, the
        // maximum is 1024 (stack is 1024 bits or 128 bytes), also known as
        // DECODER_DEPTH_MAX_INCL.
        //
        // The [JSON spec](https://www.ietf.org/rfc/rfc8259.txt) clearly states,
        // "an implementation may set limits on the maximum depth of nesting".
        //
        // In comparison, as of February 2020, the Chromium web browser's JSON
        // parser's maximum recursion depth is 200:
        // https://source.chromium.org/chromium/chromium/src/+/3dece34cde622faa0daac07156c25d92c9897d1e:base/json/json_common.h;l=18
        //
        // Other languages and libraries' maximum depths (determined empirically)
        // are listed at https://github.com/lovasoa/bad_json_parsers#results
        stack : array[1024 / 32] base.u32,
)

pub func decoder.set_quirk!(key: base.u32, value: base.u64) base.status {
    if args.key >= QUIRKS_BASE {
        args.key -= QUIRKS_BASE
        if args.key < QUIRKS_COUNT {
            this.quirks[args.key] = args.value > 0
            return ok
        }
    }
    return base."#unsupported option"
}

pub func decoder.workbuf_len() base.range_ii_u64 {
    return this.util.empty_range_ii_u64()
}

pub func decoder.decode_tokens?(dst: base.token_writer, src: base.io_reader, workbuf: slice base.u8) {
    // This is a very, very long function, and it is tempting to refactor it.
    // Be careful of performance impacts when doing so. For example, commit
    // 86d3b89f "Factor out json.decoder.decode_string" pulled out a 500 line
    // decode_string function, which was certainly cleaner structurally, but
    // also regressed performance by 1.1x to 1.2x. For details, see
    // https://github.com/google/wuffs/commit/86d3b89f9a6578d964a4b6d71e21dfc9bb702b44

    var vminor            : base.u32[..= 0xFF_FFFF]
    var number_length     : base.u32[..= 0x3FF]
    var number_status     : base.u32[..= 0x3]
    var string_length     : base.u32[..= 0xFFFB]
    var whitespace_length : base.u32[..= 0xFFFE]
    var depth             : base.u32[..= 1024]
    var stack_byte        : base.u32[..= (1024 / 32) - 1]
    var stack_bit         : base.u32[..= 31]
    var match             : base.u32[..= 2]
    var c4                : base.u32
    var c                 : base.u8
    var backslash         : base.u8
    var char              : base.u8
    var class             : base.u8[..= 0x0F]
    var multi_byte_utf8   : base.u32

    var backslash_x_ok     : base.u8
    var backslash_x_value  : base.u8
    var backslash_x_string : base.u32

    var uni4_ok             : base.u8
    var uni4_string         : base.u64
    var uni4_value          : base.u32[..= 0xFFFF]
    var uni4_high_surrogate : base.u32[..= 0x10_FC00]

    var uni8_ok     : base.u8
    var uni8_string : base.u64
    var uni8_value  : base.u32[..= 0xFFFF_FFFF]

    // expect is a bitmask of what the next character class can be.
    //
    // expect_after_value is what to expect after seeing a value (a literal,
    // number, string, array or object). For depth 0, this is ignored.
    // Otherwise, it should be (EXPECT_CLOSE_FOO | EXPECT_COMMA), for some
    // value of FOO.
    var expect             : base.u32
    var expect_after_value : base.u32

    if this.end_of_data {
        return base."@end of data"
    }

    if this.quirks[QUIRK_EXPECT_TRAILING_NEW_LINE_OR_EOF - QUIRKS_BASE] {
        if this.quirks[QUIRK_ALLOW_COMMENT_BLOCK - QUIRKS_BASE] or
                this.quirks[QUIRK_ALLOW_COMMENT_LINE - QUIRKS_BASE] or
                this.quirks[QUIRK_ALLOW_TRAILING_FILLER - QUIRKS_BASE] {
            return "#bad quirk combination"
        }
    }

    if this.quirks[QUIRK_ALLOW_LEADING_ASCII_RECORD_SEPARATOR - QUIRKS_BASE] or
            this.quirks[QUIRK_ALLOW_LEADING_UNICODE_BYTE_ORDER_MARK - QUIRKS_BASE] {
        this.decode_leading?(dst: args.dst, src: args.src)
    }

    expect = EXPECT_VALUE

    while.outer true {
        while.goto_parsed_a_leaf_value true {{
        if args.dst.length() <= 0 {
            yield? base."$short write"
            continue.outer
        }

        // Consume whitespace.
        whitespace_length = 0
        c = 0
        class = 0
        while.ws true,
                inv args.dst.length() > 0,
                post args.src.length() > 0,
        {
            if args.src.length() <= 0 {
                if whitespace_length > 0 {
                    args.dst.write_simple_token_fast!(
                            value_major: 0,
                            value_minor: 0,
                            continued: 0,
                            length: whitespace_length)
                    whitespace_length = 0
                }
                if args.src.is_closed() {
                    return "#bad input"
                }
                yield? base."$short read"
                continue.outer
            }

            c = args.src.peek_u8()
            class = LUT_CLASSES[c]
            if class <> CLASS_WHITESPACE {
                break.ws
            }
            args.src.skip_u32_fast!(actual: 1, worst_case: 1)

            if whitespace_length >= 0xFFFE {
                args.dst.write_simple_token_fast!(
                        value_major: 0,
                        value_minor: 0,
                        continued: 0,
                        length: 0xFFFF)
                whitespace_length = 0
                continue.outer
            }
            whitespace_length += 1
        } endwhile.ws

        // Emit whitespace.
        if whitespace_length > 0 {
            args.dst.write_simple_token_fast!(
                    value_major: 0,
                    value_minor: 0,
                    continued: 0,
                    length: whitespace_length)
            whitespace_length = 0
            if args.dst.length() <= 0 {
                continue.outer
            }
        }

        // Check expected character classes.
        if 0 == (expect & ((1 as base.u32) << class)) {
            return "#bad input"
        }

        // These assertions are redundant (the Wuffs compiler should already
        // know these facts; deleting these assertions should still compile)
        // but are listed explicitly to guard against future edits to the code
        // above inadvertently invalidating these assertions.
        assert args.dst.length() > 0
        assert args.src.length() > 0

        if class == CLASS_STRING {
            // -------- BEGIN parse strings.
            // Emit the leading '"'.
            args.dst.write_simple_token_fast!(
                    value_major: 0,
                    value_minor: (base.TOKEN__VBC__STRING << 21) |
                    base.TOKEN__VBD__STRING__DEFINITELY_UTF_8 |
                    base.TOKEN__VBD__STRING__CHAIN_MUST_BE_UTF_8 |
                    base.TOKEN__VBD__STRING__DEFINITELY_ASCII |
                    base.TOKEN__VBD__STRING__CONVERT_0_DST_1_SRC_DROP,
                    continued: 1,
                    length: 1)
            args.src.skip_u32_fast!(actual: 1, worst_case: 1)

            while.string_loop_outer true {
                if args.dst.length() <= 0 {
                    yield? base."$short write"
                    continue.string_loop_outer
                }

                string_length = 0
                while.string_loop_inner true,
                        pre args.dst.length() > 0,
                {
                    if args.src.length() <= 0 {
                        if string_length > 0 {
                            args.dst.write_simple_token_fast!(
                                    value_major: 0,
                                    value_minor: (base.TOKEN__VBC__STRING << 21) |
                                    base.TOKEN__VBD__STRING__DEFINITELY_UTF_8 |
                                    base.TOKEN__VBD__STRING__CHAIN_MUST_BE_UTF_8 |
                                    base.TOKEN__VBD__STRING__CONVERT_1_DST_1_SRC_COPY,
                                    continued: 1,
                                    length: string_length)
                            string_length = 0
                        }
                        if args.src.is_closed() {
                            return "#bad input"
                        }
                        yield? base."$short read"
                        continue.string_loop_outer
                    }

                    // As an optimization, consume non-special ASCII 4 bytes at
                    // a time.
                    while args.src.length() > 4,
                            inv args.dst.length() > 0,
                            inv args.src.length() > 0,
                    {
                        c4 = args.src.peek_u32le()
                        if 0x00 <> (LUT_CHARS[0xFF & (c4 >> 0)] |
                                LUT_CHARS[0xFF & (c4 >> 8)] |
                                LUT_CHARS[0xFF & (c4 >> 16)] |
                                LUT_CHARS[0xFF & (c4 >> 24)]) {
                            break
                        }
                        args.src.skip_u32_fast!(actual: 4, worst_case: 4)
                        if string_length > (0xFFFB - 4) {
                            args.dst.write_simple_token_fast!(
                                    value_major: 0,
                                    value_minor: (base.TOKEN__VBC__STRING << 21) |
                                    base.TOKEN__VBD__STRING__DEFINITELY_UTF_8 |
                                    base.TOKEN__VBD__STRING__CHAIN_MUST_BE_UTF_8 |
                                    base.TOKEN__VBD__STRING__CONVERT_1_DST_1_SRC_COPY,
                                    continued: 1,
                                    length: string_length + 4)
                            string_length = 0
                            continue.string_loop_outer
                        }
                        string_length += 4
                    } endwhile

                    c = args.src.peek_u8()
                    char = LUT_CHARS[c]

                    if char == 0x00 {  // Non-special ASCII.
                        args.src.skip_u32_fast!(actual: 1, worst_case: 1)
                        if string_length >= 0xFFFB {
                            args.dst.write_simple_token_fast!(
                                    value_major: 0,
                                    value_minor: (base.TOKEN__VBC__STRING << 21) |
                                    base.TOKEN__VBD__STRING__DEFINITELY_UTF_8 |
                                    base.TOKEN__VBD__STRING__CHAIN_MUST_BE_UTF_8 |
                                    base.TOKEN__VBD__STRING__CONVERT_1_DST_1_SRC_COPY,
                                    continued: 1,
                                    length: 0xFFFC)
                            string_length = 0
                            continue.string_loop_outer
                        }
                        string_length += 1
                        continue.string_loop_inner

                    } else if char == 0x01 {  // '"'
                        if string_length <> 0 {
                            args.dst.write_simple_token_fast!(
                                    value_major: 0,
                                    value_minor: (base.TOKEN__VBC__STRING << 21) |
                                    base.TOKEN__VBD__STRING__DEFINITELY_UTF_8 |
                                    base.TOKEN__VBD__STRING__CHAIN_MUST_BE_UTF_8 |
                                    base.TOKEN__VBD__STRING__CONVERT_1_DST_1_SRC_COPY,
                                    continued: 1,
                                    length: string_length)
                            string_length = 0
                        }
                        break.string_loop_outer

                    } else if char == 0x02 {  // '\\'.
                        if string_length > 0 {
                            args.dst.write_simple_token_fast!(
                                    value_major: 0,
                                    value_minor: (base.TOKEN__VBC__STRING << 21) |
                                    base.TOKEN__VBD__STRING__DEFINITELY_UTF_8 |
                                    base.TOKEN__VBD__STRING__CHAIN_MUST_BE_UTF_8 |
                                    base.TOKEN__VBD__STRING__CONVERT_1_DST_1_SRC_COPY,
                                    continued: 1,
                                    length: string_length)
                            string_length = 0
                            if args.dst.length() <= 0 {
                                continue.string_loop_outer
                            }
                        }
                        assert args.dst.length() > 0

                        if args.src.length() < 2 {
                            if args.src.is_closed() {
                                return "#bad backslash-escape"
                            }
                            yield? base."$short read"
                            continue.string_loop_outer
                        }
                        c = (args.src.peek_u16le() >> 8) as base.u8
                        backslash = LUT_BACKSLASHES[c]
                        if (backslash & 0x80) <> 0 {
                            args.src.skip_u32_fast!(actual: 2, worst_case: 2)
                            args.dst.write_simple_token_fast!(
                                    value_major: 0,
                                    value_minor: (base.TOKEN__VBC__UNICODE_CODE_POINT << 21) |
                                    ((backslash & 0x7F) as base.u32),
                                    continued: 1,
                                    length: 2)
                            continue.string_loop_outer

                        } else if backslash <> 0 {
                            if this.quirks[LUT_QUIRKY_BACKSLASHES_QUIRKS[backslash & 7]] {
                                args.src.skip_u32_fast!(actual: 2, worst_case: 2)
                                args.dst.write_simple_token_fast!(
                                        value_major: 0,
                                        value_minor: (base.TOKEN__VBC__UNICODE_CODE_POINT << 21) |
                                        (LUT_QUIRKY_BACKSLASHES_CHARS[backslash & 7] as base.u32),
                                        continued: 1,
                                        length: 2)
                                continue.string_loop_outer
                            }

                        } else if c == 'u' {
                            // -------- BEGIN backslash-u.
                            if args.src.length() < 6 {
                                if args.src.is_closed() {
                                    return "#bad backslash-escape"
                                }
                                yield? base."$short read"
                                continue.string_loop_outer
                            }

                            uni4_string = args.src.peek_u48le_as_u64() >> 16
                            uni4_value = 0
                            uni4_ok = 0x80

                            c = LUT_HEXADECIMAL_DIGITS[0xFF & (uni4_string >> 0)]
                            uni4_ok &= c
                            uni4_value |= ((c & 0x0F) as base.u32) << 12
                            c = LUT_HEXADECIMAL_DIGITS[0xFF & (uni4_string >> 8)]
                            uni4_ok &= c
                            uni4_value |= ((c & 0x0F) as base.u32) << 8
                            c = LUT_HEXADECIMAL_DIGITS[0xFF & (uni4_string >> 16)]
                            uni4_ok &= c
                            uni4_value |= ((c & 0x0F) as base.u32) << 4
                            c = LUT_HEXADECIMAL_DIGITS[0xFF & (uni4_string >> 24)]
                            uni4_ok &= c
                            uni4_value |= ((c & 0x0F) as base.u32) << 0

                            if uni4_ok == 0 {
                                // It wasn't 4 hexadecimal digits. No-op (and
                                // fall through to "#bad backslash-escape").

                            } else if (uni4_value < 0xD800) or (0xDFFF < uni4_value) {
                                // Not a Unicode surrogate. We're good.
                                args.src.skip_u32_fast!(actual: 6, worst_case: 6)
                                args.dst.write_simple_token_fast!(
                                        value_major: 0,
                                        value_minor: (base.TOKEN__VBC__UNICODE_CODE_POINT << 21) |
                                        uni4_value,
                                        continued: 1,
                                        length: 6)
                                continue.string_loop_outer

                            } else if uni4_value >= 0xDC00 {
                                // Low surrogate. No-op (and fall through to
                                // "#bad backslash-escape").

                            } else {
                                // High surrogate, which needs to be followed
                                // by a "\\u1234" low surrogate. We've already
                                // peeked 6 bytes for the high surrogate. We
                                // need 12 in total: another 8 bytes at an
                                // offset of 4.
                                if args.src.length() < 12 {
                                    if args.src.is_closed() {
                                        if this.quirks[QUIRK_REPLACE_INVALID_UNICODE - QUIRKS_BASE] {
                                            args.src.skip_u32_fast!(actual: 6, worst_case: 6)
                                            args.dst.write_simple_token_fast!(
                                                    value_major: 0,
                                                    value_minor: (base.TOKEN__VBC__UNICODE_CODE_POINT << 21) |
                                                    base.UNICODE__REPLACEMENT_CHARACTER,
                                                    continued: 1,
                                                    length: 6)
                                            continue.string_loop_outer
                                        }
                                        return "#bad backslash-escape"
                                    }
                                    yield? base."$short read"
                                    continue.string_loop_outer
                                }
                                uni4_string = args.src.peek_u64le_at(offset: 4) >> 16

                                // Look for the low surrogate's "\\u".
                                if ((0xFF & (uni4_string >> 0)) <> '\\') or
                                        ((0xFF & (uni4_string >> 8)) <> 'u') {
                                    uni4_high_surrogate = 0
                                    uni4_value = 0
                                    uni4_ok = 0
                                } else {
                                    uni4_high_surrogate =
                                            0x1_0000 + ((uni4_value - 0xD800) << 10)
                                    uni4_value = 0
                                    uni4_ok = 0x80
                                    uni4_string >>= 16

                                    c = LUT_HEXADECIMAL_DIGITS[0xFF & (uni4_string >> 0)]
                                    uni4_ok &= c
                                    uni4_value |= ((c & 0x0F) as base.u32) << 12
                                    c = LUT_HEXADECIMAL_DIGITS[0xFF & (uni4_string >> 8)]
                                    uni4_ok &= c
                                    uni4_value |= ((c & 0x0F) as base.u32) << 8
                                    c = LUT_HEXADECIMAL_DIGITS[0xFF & (uni4_string >> 16)]
                                    uni4_ok &= c
                                    uni4_value |= ((c & 0x0F) as base.u32) << 4
                                    c = LUT_HEXADECIMAL_DIGITS[0xFF & (uni4_string >> 24)]
                                    uni4_ok &= c
                                    uni4_value |= ((c & 0x0F) as base.u32) << 0
                                }

                                if (uni4_ok <> 0) and
                                        (0xDC00 <= uni4_value) and (uni4_value <= 0xDFFF) {

                                    // Emit a single token for the surrogate
                                    // pair.
                                    uni4_value -= 0xDC00
                                    args.src.skip_u32_fast!(actual: 12, worst_case: 12)
                                    args.dst.write_simple_token_fast!(
                                            value_major: 0,
                                            value_minor: (base.TOKEN__VBC__UNICODE_CODE_POINT << 21) |
                                            uni4_high_surrogate | uni4_value,
                                            continued: 1,
                                            length: 12)
                                    continue.string_loop_outer
                                }
                            }

                            if this.quirks[QUIRK_REPLACE_INVALID_UNICODE - QUIRKS_BASE] {
                                if args.src.length() < 6 {
                                    return "#internal error: inconsistent I/O"
                                }
                                args.src.skip_u32_fast!(actual: 6, worst_case: 6)
                                args.dst.write_simple_token_fast!(
                                        value_major: 0,
                                        value_minor: (base.TOKEN__VBC__UNICODE_CODE_POINT << 21) |
                                        base.UNICODE__REPLACEMENT_CHARACTER,
                                        continued: 1,
                                        length: 6)
                                continue.string_loop_outer
                            }
                            // -------- END   backslash-u.

                        } else if (c == 'U') and
                                this.quirks[QUIRK_ALLOW_BACKSLASH_CAPITAL_U - QUIRKS_BASE] {
                            // -------- BEGIN backslash-capital-u.
                            if args.src.length() < 10 {
                                if args.src.is_closed() {
                                    return "#bad backslash-escape"
                                }
                                yield? base."$short read"
                                continue.string_loop_outer
                            }
                            uni8_string = args.src.peek_u64le_at(offset: 2)
                            uni8_value = 0
                            uni8_ok = 0x80

                            c = LUT_HEXADECIMAL_DIGITS[0xFF & (uni8_string >> 0)]
                            uni8_ok &= c
                            uni8_value |= ((c & 0x0F) as base.u32) << 28
                            c = LUT_HEXADECIMAL_DIGITS[0xFF & (uni8_string >> 8)]
                            uni8_ok &= c
                            uni8_value |= ((c & 0x0F) as base.u32) << 24
                            c = LUT_HEXADECIMAL_DIGITS[0xFF & (uni8_string >> 16)]
                            uni8_ok &= c
                            uni8_value |= ((c & 0x0F) as base.u32) << 20
                            c = LUT_HEXADECIMAL_DIGITS[0xFF & (uni8_string >> 24)]
                            uni8_ok &= c
                            uni8_value |= ((c & 0x0F) as base.u32) << 16
                            c = LUT_HEXADECIMAL_DIGITS[0xFF & (uni8_string >> 32)]
                            uni8_ok &= c
                            uni8_value |= ((c & 0x0F) as base.u32) << 12
                            c = LUT_HEXADECIMAL_DIGITS[0xFF & (uni8_string >> 40)]
                            uni8_ok &= c
                            uni8_value |= ((c & 0x0F) as base.u32) << 8
                            c = LUT_HEXADECIMAL_DIGITS[0xFF & (uni8_string >> 48)]
                            uni8_ok &= c
                            uni8_value |= ((c & 0x0F) as base.u32) << 4
                            c = LUT_HEXADECIMAL_DIGITS[0xFF & (uni8_string >> 56)]
                            uni8_ok &= c
                            uni8_value |= ((c & 0x0F) as base.u32) << 0

                            if uni8_ok == 0 {
                                // It wasn't 8 hexadecimal digits. No-op (and
                                // fall through to "#bad backslash-escape").

                            } else if (uni8_value < 0xD800) or (
                                    (0xDFFF < uni8_value) and (uni8_value <= 0x10_FFFF)) {
                                // Not a Unicode surrogate. We're good.
                                args.src.skip_u32_fast!(actual: 10, worst_case: 10)
                                args.dst.write_simple_token_fast!(
                                        value_major: 0,
                                        value_minor: (base.TOKEN__VBC__UNICODE_CODE_POINT << 21) |
                                        (uni8_value & 0x1F_FFFF),
                                        continued: 1,
                                        length: 10)
                                continue.string_loop_outer
                            } else if this.quirks[QUIRK_REPLACE_INVALID_UNICODE - QUIRKS_BASE] {
                                args.src.skip_u32_fast!(actual: 10, worst_case: 10)
                                args.dst.write_simple_token_fast!(
                                        value_major: 0,
                                        value_minor: (base.TOKEN__VBC__UNICODE_CODE_POINT << 21) |
                                        base.UNICODE__REPLACEMENT_CHARACTER,
                                        continued: 1,
                                        length: 10)
                                continue.string_loop_outer
                            }
                            // -------- END   backslash-capital-u.

                        } else if (c == 'x') and
                                this.quirks[QUIRK_ALLOW_BACKSLASH_X_AS_CODE_POINTS - QUIRKS_BASE] {
                            // -------- BEGIN backslash-x
                            if args.src.length() < 4 {
                                if args.src.is_closed() {
                                    return "#bad backslash-escape"
                                }
                                yield? base."$short read"
                                continue.string_loop_outer
                            }

                            backslash_x_string = args.src.peek_u32le()
                            backslash_x_ok = 0x80

                            c = LUT_HEXADECIMAL_DIGITS[0xFF & (backslash_x_string >> 16)]
                            backslash_x_ok &= c
                            backslash_x_value = ((c & 0x0F) << 4) as base.u8
                            c = LUT_HEXADECIMAL_DIGITS[0xFF & (backslash_x_string >> 24)]
                            backslash_x_ok &= c
                            backslash_x_value = (backslash_x_value | (c & 0x0F)) as base.u8

                            if (backslash_x_ok == 0) or
                                    ((backslash_x_string & 0xFFFF) <> 0x785C) {
                                // It wasn't "\\x34", for some hexadecimal
                                // digits "34".
                                return "#bad backslash-escape"
                            }
                            args.src.skip_u32_fast!(actual: 4, worst_case: 4)
                            args.dst.write_simple_token_fast!(
                                    value_major: 0,
                                    value_minor: (base.TOKEN__VBC__UNICODE_CODE_POINT << 21) |
                                    (backslash_x_value as base.u32),
                                    continued: 1,
                                    length: 4)
                            continue.string_loop_outer
                            // -------- END   backslash-x
                        }

                        return "#bad backslash-escape"

                    } else if char == 0x03 {  // 2-byte UTF-8.
                        if args.src.length() < 2 {
                            if string_length > 0 {
                                args.dst.write_simple_token_fast!(
                                        value_major: 0,
                                        value_minor: (base.TOKEN__VBC__STRING << 21) |
                                        base.TOKEN__VBD__STRING__DEFINITELY_UTF_8 |
                                        base.TOKEN__VBD__STRING__CHAIN_MUST_BE_UTF_8 |
                                        base.TOKEN__VBD__STRING__CONVERT_1_DST_1_SRC_COPY,
                                        continued: 1,
                                        length: string_length)
                                string_length = 0
                                if args.dst.length() <= 0 {
                                    continue.string_loop_outer
                                }
                            }
                            if args.src.is_closed() {
                                if this.quirks[QUIRK_REPLACE_INVALID_UNICODE - QUIRKS_BASE] {
                                    args.dst.write_simple_token_fast!(
                                            value_major: 0,
                                            value_minor: (base.TOKEN__VBC__UNICODE_CODE_POINT << 21) |
                                            base.UNICODE__REPLACEMENT_CHARACTER,
                                            continued: 1,
                                            length: 1)
                                    args.src.skip_u32_fast!(actual: 1, worst_case: 1)
                                    continue.string_loop_outer
                                }
                                return "#bad UTF-8"
                            }
                            yield? base."$short read"
                            continue.string_loop_outer
                        }
                        multi_byte_utf8 = args.src.peek_u16le_as_u32()
                        if (multi_byte_utf8 & 0xC000) == 0x8000 {
                            multi_byte_utf8 = (0x00_07C0 & (multi_byte_utf8 ~mod<< 6)) |
                                    (0x00_003F & (multi_byte_utf8 >> 8))
                            args.src.skip_u32_fast!(actual: 2, worst_case: 2)
                            if string_length >= 0xFFF8 {
                                args.dst.write_simple_token_fast!(
                                        value_major: 0,
                                        value_minor: (base.TOKEN__VBC__STRING << 21) |
                                        base.TOKEN__VBD__STRING__DEFINITELY_UTF_8 |
                                        base.TOKEN__VBD__STRING__CHAIN_MUST_BE_UTF_8 |
                                        base.TOKEN__VBD__STRING__CONVERT_1_DST_1_SRC_COPY,
                                        continued: 1,
                                        length: string_length + 2)
                                string_length = 0
                                continue.string_loop_outer
                            }
                            string_length += 2
                            continue.string_loop_inner
                        }

                    } else if char == 0x04 {  // 3-byte UTF-8.
                        if args.src.length() < 3 {
                            if string_length > 0 {
                                args.dst.write_simple_token_fast!(
                                        value_major: 0,
                                        value_minor: (base.TOKEN__VBC__STRING << 21) |
                                        base.TOKEN__VBD__STRING__DEFINITELY_UTF_8 |
                                        base.TOKEN__VBD__STRING__CHAIN_MUST_BE_UTF_8 |
                                        base.TOKEN__VBD__STRING__CONVERT_1_DST_1_SRC_COPY,
                                        continued: 1,
                                        length: string_length)
                                string_length = 0
                                if args.dst.length() <= 0 {
                                    continue.string_loop_outer
                                }
                            }
                            if args.src.is_closed() {
                                if this.quirks[QUIRK_REPLACE_INVALID_UNICODE - QUIRKS_BASE] {
                                    args.dst.write_simple_token_fast!(
                                            value_major: 0,
                                            value_minor: (base.TOKEN__VBC__UNICODE_CODE_POINT << 21) |
                                            base.UNICODE__REPLACEMENT_CHARACTER,
                                            continued: 1,
                                            length: 1)
                                    args.src.skip_u32_fast!(actual: 1, worst_case: 1)
                                    continue.string_loop_outer
                                }
                                return "#bad UTF-8"
                            }
                            yield? base."$short read"
                            continue.string_loop_outer
                        }
                        multi_byte_utf8 = args.src.peek_u24le_as_u32()
                        if (multi_byte_utf8 & 0xC0_C000) == 0x80_8000 {
                            multi_byte_utf8 = (0x00_F000 & (multi_byte_utf8 ~mod<< 12)) |
                                    (0x00_0FC0 & (multi_byte_utf8 >> 2)) |
                                    (0x00_003F & (multi_byte_utf8 >> 16))
                            if (0x07FF < multi_byte_utf8) and
                                    ((multi_byte_utf8 < 0xD800) or (0xDFFF < multi_byte_utf8)) {

                                args.src.skip_u32_fast!(actual: 3, worst_case: 3)
                                if string_length >= 0xFFF8 {
                                    args.dst.write_simple_token_fast!(
                                            value_major: 0,
                                            value_minor: (base.TOKEN__VBC__STRING << 21) |
                                            base.TOKEN__VBD__STRING__DEFINITELY_UTF_8 |
                                            base.TOKEN__VBD__STRING__CHAIN_MUST_BE_UTF_8 |
                                            base.TOKEN__VBD__STRING__CONVERT_1_DST_1_SRC_COPY,
                                            continued: 1,
                                            length: string_length + 3)
                                    string_length = 0
                                    continue.string_loop_outer
                                }
                                string_length += 3
                                continue.string_loop_inner
                            }
                        }

                    } else if char == 0x05 {  // 4-byte UTF-8.
                        if args.src.length() < 4 {
                            if string_length > 0 {
                                args.dst.write_simple_token_fast!(
                                        value_major: 0,
                                        value_minor: (base.TOKEN__VBC__STRING << 21) |
                                        base.TOKEN__VBD__STRING__DEFINITELY_UTF_8 |
                                        base.TOKEN__VBD__STRING__CHAIN_MUST_BE_UTF_8 |
                                        base.TOKEN__VBD__STRING__CONVERT_1_DST_1_SRC_COPY,
                                        continued: 1,
                                        length: string_length)
                                string_length = 0
                                if args.dst.length() <= 0 {
                                    continue.string_loop_outer
                                }
                            }
                            if args.src.is_closed() {
                                if this.quirks[QUIRK_REPLACE_INVALID_UNICODE - QUIRKS_BASE] {
                                    args.dst.write_simple_token_fast!(
                                            value_major: 0,
                                            value_minor: (base.TOKEN__VBC__UNICODE_CODE_POINT << 21) |
                                            base.UNICODE__REPLACEMENT_CHARACTER,
                                            continued: 1,
                                            length: 1)
                                    args.src.skip_u32_fast!(actual: 1, worst_case: 1)
                                    continue.string_loop_outer
                                }
                                return "#bad UTF-8"
                            }
                            yield? base."$short read"
                            continue.string_loop_outer
                        }
                        multi_byte_utf8 = args.src.peek_u32le()
                        if (multi_byte_utf8 & 0xC0C0_C000) == 0x8080_8000 {
                            multi_byte_utf8 = (0x1C_0000 & (multi_byte_utf8 ~mod<< 18)) |
                                    (0x03_F000 & (multi_byte_utf8 ~mod<< 4)) |
                                    (0x00_0FC0 & (multi_byte_utf8 >> 10)) |
                                    (0x00_003F & (multi_byte_utf8 >> 24))
                            if (0xFFFF < multi_byte_utf8) and (multi_byte_utf8 <= 0x10_FFFF) {
                                args.src.skip_u32_fast!(actual: 4, worst_case: 4)
                                if string_length >= 0xFFF8 {
                                    args.dst.write_simple_token_fast!(
                                            value_major: 0,
                                            value_minor: (base.TOKEN__VBC__STRING << 21) |
                                            base.TOKEN__VBD__STRING__DEFINITELY_UTF_8 |
                                            base.TOKEN__VBD__STRING__CHAIN_MUST_BE_UTF_8 |
                                            base.TOKEN__VBD__STRING__CONVERT_1_DST_1_SRC_COPY,
                                            continued: 1,
                                            length: string_length + 4)
                                    string_length = 0
                                    continue.string_loop_outer
                                }
                                string_length += 4
                                continue.string_loop_inner
                            }
                        }
                    }

                    if string_length > 0 {
                        args.dst.write_simple_token_fast!(
                                value_major: 0,
                                value_minor: (base.TOKEN__VBC__STRING << 21) |
                                base.TOKEN__VBD__STRING__DEFINITELY_UTF_8 |
                                base.TOKEN__VBD__STRING__CHAIN_MUST_BE_UTF_8 |
                                base.TOKEN__VBD__STRING__CONVERT_1_DST_1_SRC_COPY,
                                continued: 1,
                                length: string_length)
                        string_length = 0
                        if args.dst.length() <= 0 {
                            continue.string_loop_outer
                        }
                    }
                    if (char & 0x80) <> 0 {
                        if this.quirks[QUIRK_ALLOW_ASCII_CONTROL_CODES - QUIRKS_BASE] {
                            args.dst.write_simple_token_fast!(
                                    value_major: 0,
                                    value_minor: (base.TOKEN__VBC__UNICODE_CODE_POINT << 21) |
                                    ((char & 0x7F) as base.u32),
                                    continued: 1,
                                    length: 1)
                            args.src.skip_u32_fast!(actual: 1, worst_case: 1)
                            continue.string_loop_outer
                        }
                        if char == 0x8A {
                            return "#bad new-line in a string"
                        }
                        return "#bad C0 control code"
                    }
                    if this.quirks[QUIRK_REPLACE_INVALID_UNICODE - QUIRKS_BASE] {
                        args.dst.write_simple_token_fast!(
                                value_major: 0,
                                value_minor: (base.TOKEN__VBC__UNICODE_CODE_POINT << 21) |
                                base.UNICODE__REPLACEMENT_CHARACTER,
                                continued: 1,
                                length: 1)
                        args.src.skip_u32_fast!(actual: 1, worst_case: 1)
                        continue.string_loop_outer
                    }
                    return "#bad UTF-8"
                } endwhile.string_loop_inner
            } endwhile.string_loop_outer

            // Emit the trailing '"'.
            while true {
                if args.src.length() <= 0 {
                    if args.src.is_closed() {
                        return "#bad input"
                    }
                    yield? base."$short read"
                    continue
                }
                if args.dst.length() <= 0 {
                    yield? base."$short write"
                    continue
                }
                args.src.skip_u32_fast!(actual: 1, worst_case: 1)
                args.dst.write_simple_token_fast!(
                        value_major: 0,
                        value_minor: (base.TOKEN__VBC__STRING << 21) |
                        base.TOKEN__VBD__STRING__DEFINITELY_UTF_8 |
                        base.TOKEN__VBD__STRING__CHAIN_MUST_BE_UTF_8 |
                        base.TOKEN__VBD__STRING__DEFINITELY_ASCII |
                        base.TOKEN__VBD__STRING__CONVERT_0_DST_1_SRC_DROP,
                        continued: 0,
                        length: 1)
                break
            } endwhile

            // As above, expect must have contained EXPECT_STRING. If it didn't
            // also contain EXPECT_NUMBER (excluding EXPECT_COMMENT) then we
            // were parsing an object key and the next token should be ':'.
            if 0 == (expect & ((1 as base.u32) << CLASS_NUMBER)) {
                expect = EXPECT_COLON
                continue.outer
            }
            break.goto_parsed_a_leaf_value
            // -------- END   parse strings.

        } else if class == CLASS_COMMA {
            args.src.skip_u32_fast!(actual: 1, worst_case: 1)
            // The ',' is punctuation (filler).
            args.dst.write_simple_token_fast!(
                    value_major: 0,
                    value_minor: (base.TOKEN__VBC__FILLER << 21) |
                    base.TOKEN__VBD__FILLER__PUNCTUATION,
                    continued: 0,
                    length: 1)
            // What's valid after a comma depends on whether or not we're in an
            // array or an object.
            if 0 == (expect & ((1 as base.u32) << CLASS_CLOSE_SQUARE_BRACKET)) {
                if this.quirks[QUIRK_ALLOW_EXTRA_COMMA - QUIRKS_BASE] {
                    expect = EXPECT_STRING | EXPECT_CLOSE_CURLY_BRACE
                } else {
                    expect = EXPECT_STRING
                }
            } else {
                if this.quirks[QUIRK_ALLOW_EXTRA_COMMA - QUIRKS_BASE] {
                    expect = EXPECT_VALUE | EXPECT_CLOSE_SQUARE_BRACKET
                } else {
                    expect = EXPECT_VALUE
                }
            }
            continue.outer

        } else if class == CLASS_COLON {
            args.src.skip_u32_fast!(actual: 1, worst_case: 1)
            // The ':' is punctuation (filler).
            args.dst.write_simple_token_fast!(
                    value_major: 0,
                    value_minor: (base.TOKEN__VBC__FILLER << 21) |
                    base.TOKEN__VBD__FILLER__PUNCTUATION,
                    continued: 0,
                    length: 1)
            expect = EXPECT_VALUE
            continue.outer

        } else if class == CLASS_NUMBER {
            // -------- BEGIN parse numbers.
            while true,
                    pre args.dst.length() > 0,
            {
                number_length = this.decode_number!(src: args.src)
                number_status = number_length >> 8
                vminor = (base.TOKEN__VBC__NUMBER << 21) |
                        base.TOKEN__VBD__NUMBER__CONTENT_FLOATING_POINT |
                        base.TOKEN__VBD__NUMBER__CONTENT_INTEGER_SIGNED |
                        base.TOKEN__VBD__NUMBER__FORMAT_TEXT
                if (number_length & 0x80) <> 0 {
                    vminor = (base.TOKEN__VBC__NUMBER << 21) |
                            base.TOKEN__VBD__NUMBER__CONTENT_FLOATING_POINT |
                            base.TOKEN__VBD__NUMBER__FORMAT_TEXT
                }
                number_length = number_length & 0x7F
                if number_status == 0 {
                    args.dst.write_simple_token_fast!(
                            value_major: 0,
                            value_minor: vminor,
                            continued: 0,
                            length: number_length)
                    break
                }

                while number_length > 0 {
                    number_length -= 1
                    if args.src.can_undo_byte() {
                        args.src.undo_byte!()
                    } else {
                        return "#internal error: inconsistent I/O"
                    }
                } endwhile

                if number_status == 1 {
                    if this.quirks[QUIRK_ALLOW_INF_NAN_NUMBERS - QUIRKS_BASE] {
                        this.decode_inf_nan?(dst: args.dst, src: args.src)
                        break
                    }
                    return "#bad input"
                } else if number_status == 2 {
                    return "#unsupported number length"
                } else {
                    yield? base."$short read"
                    while args.dst.length() <= 0,
                            post args.dst.length() > 0,
                    {
                        yield? base."$short write"
                    } endwhile
                }
            } endwhile
            break.goto_parsed_a_leaf_value
            // -------- END   parse numbers.

        } else if class == CLASS_OPEN_CURLY_BRACE {
            vminor = (base.TOKEN__VBC__STRUCTURE << 21) |
                    base.TOKEN__VBD__STRUCTURE__PUSH |
                    base.TOKEN__VBD__STRUCTURE__FROM_NONE |
                    base.TOKEN__VBD__STRUCTURE__TO_DICT
            if depth == 0 {
                // No-op.
            } else if 0 <> (expect_after_value & ((1 as base.u32) << CLASS_CLOSE_CURLY_BRACE)) {
                vminor = (base.TOKEN__VBC__STRUCTURE << 21) |
                        base.TOKEN__VBD__STRUCTURE__PUSH |
                        base.TOKEN__VBD__STRUCTURE__FROM_DICT |
                        base.TOKEN__VBD__STRUCTURE__TO_DICT
            } else {
                vminor = (base.TOKEN__VBC__STRUCTURE << 21) |
                        base.TOKEN__VBD__STRUCTURE__PUSH |
                        base.TOKEN__VBD__STRUCTURE__FROM_LIST |
                        base.TOKEN__VBD__STRUCTURE__TO_DICT
            }
            if depth >= 1024 {
                return "#unsupported recursion depth"
            }
            stack_byte = depth / 32
            stack_bit = depth & 31
            this.stack[stack_byte] |= (1 as base.u32) << stack_bit
            depth += 1

            args.src.skip_u32_fast!(actual: 1, worst_case: 1)
            args.dst.write_simple_token_fast!(
                    value_major: 0,
                    value_minor: vminor,
                    continued: 0,
                    length: 1)
            expect = EXPECT_CLOSE_CURLY_BRACE | EXPECT_STRING
            expect_after_value = EXPECT_CLOSE_CURLY_BRACE | EXPECT_COMMA
            continue.outer

        } else if class == CLASS_CLOSE_CURLY_BRACE {
            args.src.skip_u32_fast!(actual: 1, worst_case: 1)
            if depth <= 1 {
                args.dst.write_simple_token_fast!(
                        value_major: 0,
                        value_minor: (base.TOKEN__VBC__STRUCTURE << 21) |
                        base.TOKEN__VBD__STRUCTURE__POP |
                        base.TOKEN__VBD__STRUCTURE__FROM_DICT |
                        base.TOKEN__VBD__STRUCTURE__TO_NONE,
                        continued: 0,
                        length: 1)
                break.outer
            }
            depth -= 1
            stack_byte = (depth - 1) / 32
            stack_bit = (depth - 1) & 31
            if 0 == (this.stack[stack_byte] & ((1 as base.u32) << stack_bit)) {
                args.dst.write_simple_token_fast!(
                        value_major: 0,
                        value_minor: (base.TOKEN__VBC__STRUCTURE << 21) |
                        base.TOKEN__VBD__STRUCTURE__POP |
                        base.TOKEN__VBD__STRUCTURE__FROM_DICT |
                        base.TOKEN__VBD__STRUCTURE__TO_LIST,
                        continued: 0,
                        length: 1)
                expect = EXPECT_CLOSE_SQUARE_BRACKET | EXPECT_COMMA
                expect_after_value = EXPECT_CLOSE_SQUARE_BRACKET | EXPECT_COMMA
            } else {
                args.dst.write_simple_token_fast!(
                        value_major: 0,
                        value_minor: (base.TOKEN__VBC__STRUCTURE << 21) |
                        base.TOKEN__VBD__STRUCTURE__POP |
                        base.TOKEN__VBD__STRUCTURE__FROM_DICT |
                        base.TOKEN__VBD__STRUCTURE__TO_DICT,
                        continued: 0,
                        length: 1)
                expect = EXPECT_CLOSE_CURLY_BRACE | EXPECT_COMMA
                expect_after_value = EXPECT_CLOSE_CURLY_BRACE | EXPECT_COMMA
            }
            continue.outer

        } else if class == CLASS_OPEN_SQUARE_BRACKET {
            vminor = (base.TOKEN__VBC__STRUCTURE << 21) |
                    base.TOKEN__VBD__STRUCTURE__PUSH |
                    base.TOKEN__VBD__STRUCTURE__FROM_NONE |
                    base.TOKEN__VBD__STRUCTURE__TO_LIST
            if depth == 0 {
                // No-op.
            } else if 0 <> (expect_after_value & ((1 as base.u32) << CLASS_CLOSE_CURLY_BRACE)) {
                vminor = (base.TOKEN__VBC__STRUCTURE << 21) |
                        base.TOKEN__VBD__STRUCTURE__PUSH |
                        base.TOKEN__VBD__STRUCTURE__FROM_DICT |
                        base.TOKEN__VBD__STRUCTURE__TO_LIST
            } else {
                vminor = (base.TOKEN__VBC__STRUCTURE << 21) |
                        base.TOKEN__VBD__STRUCTURE__PUSH |
                        base.TOKEN__VBD__STRUCTURE__FROM_LIST |
                        base.TOKEN__VBD__STRUCTURE__TO_LIST
            }
            if depth >= 1024 {
                return "#unsupported recursion depth"
            }
            stack_byte = depth / 32
            stack_bit = depth & 31
            this.stack[stack_byte] &= 0xFFFF_FFFF ^ ((1 as base.u32) << stack_bit)
            depth += 1

            args.src.skip_u32_fast!(actual: 1, worst_case: 1)
            args.dst.write_simple_token_fast!(
                    value_major: 0,
                    value_minor: vminor,
                    continued: 0,
                    length: 1)
            expect = EXPECT_CLOSE_SQUARE_BRACKET | EXPECT_VALUE
            expect_after_value = EXPECT_CLOSE_SQUARE_BRACKET | EXPECT_COMMA
            continue.outer

        } else if class == CLASS_CLOSE_SQUARE_BRACKET {
            args.src.skip_u32_fast!(actual: 1, worst_case: 1)
            if depth <= 1 {
                args.dst.write_simple_token_fast!(
                        value_major: 0,
                        value_minor: (base.TOKEN__VBC__STRUCTURE << 21) |
                        base.TOKEN__VBD__STRUCTURE__POP |
                        base.TOKEN__VBD__STRUCTURE__FROM_LIST |
                        base.TOKEN__VBD__STRUCTURE__TO_NONE,
                        continued: 0,
                        length: 1)
                break.outer
            }
            depth -= 1
            stack_byte = (depth - 1) / 32
            stack_bit = (depth - 1) & 31
            if 0 == (this.stack[stack_byte] & ((1 as base.u32) << stack_bit)) {
                args.dst.write_simple_token_fast!(
                        value_major: 0,
                        value_minor: (base.TOKEN__VBC__STRUCTURE << 21) |
                        base.TOKEN__VBD__STRUCTURE__POP |
                        base.TOKEN__VBD__STRUCTURE__FROM_LIST |
                        base.TOKEN__VBD__STRUCTURE__TO_LIST,
                        continued: 0,
                        length: 1)
                expect = EXPECT_CLOSE_SQUARE_BRACKET | EXPECT_COMMA
                expect_after_value = EXPECT_CLOSE_SQUARE_BRACKET | EXPECT_COMMA
            } else {
                args.dst.write_simple_token_fast!(
                        value_major: 0,
                        value_minor: (base.TOKEN__VBC__STRUCTURE << 21) |
                        base.TOKEN__VBD__STRUCTURE__POP |
                        base.TOKEN__VBD__STRUCTURE__FROM_LIST |
                        base.TOKEN__VBD__STRUCTURE__TO_DICT,
                        continued: 0,
                        length: 1)
                expect = EXPECT_CLOSE_CURLY_BRACE | EXPECT_COMMA
                expect_after_value = EXPECT_CLOSE_CURLY_BRACE | EXPECT_COMMA
            }
            continue.outer

        } else if class == CLASS_FALSE {
            match = args.src.match7(a: '\x05false'le)
            if match == 0 {
                args.dst.write_simple_token_fast!(
                        value_major: 0,
                        value_minor: (base.TOKEN__VBC__LITERAL << 21) |
                        base.TOKEN__VBD__LITERAL__FALSE,
                        continued: 0,
                        length: 5)
                if args.src.length() < 5 {
                    return "#internal error: inconsistent I/O"
                }
                args.src.skip_u32_fast!(actual: 5, worst_case: 5)
                break.goto_parsed_a_leaf_value
            } else if match == 1 {
                yield? base."$short read"
                continue.outer
            }

        } else if class == CLASS_TRUE {
            match = args.src.match7(a: '\x04true'le)
            if match == 0 {
                args.dst.write_simple_token_fast!(
                        value_major: 0,
                        value_minor: (base.TOKEN__VBC__LITERAL << 21) |
                        base.TOKEN__VBD__LITERAL__TRUE,
                        continued: 0,
                        length: 4)
                if args.src.length() < 4 {
                    return "#internal error: inconsistent I/O"
                }
                args.src.skip_u32_fast!(actual: 4, worst_case: 4)
                break.goto_parsed_a_leaf_value
            } else if match == 1 {
                yield? base."$short read"
                continue.outer
            }

        } else if class == CLASS_NULL_NAN_INF {
            match = args.src.match7(a: '\x04null'le)
            if match == 0 {
                args.dst.write_simple_token_fast!(
                        value_major: 0,
                        value_minor: (base.TOKEN__VBC__LITERAL << 21) |
                        base.TOKEN__VBD__LITERAL__NULL,
                        continued: 0,
                        length: 4)
                if args.src.length() < 4 {
                    return "#internal error: inconsistent I/O"
                }
                args.src.skip_u32_fast!(actual: 4, worst_case: 4)
                break.goto_parsed_a_leaf_value
            } else if match == 1 {
                yield? base."$short read"
                continue.outer
            }

            if this.quirks[QUIRK_ALLOW_INF_NAN_NUMBERS - QUIRKS_BASE] {
                this.decode_inf_nan?(dst: args.dst, src: args.src)
                break.goto_parsed_a_leaf_value
            }

        } else if class == CLASS_COMMENT {
            if this.quirks[QUIRK_ALLOW_COMMENT_BLOCK - QUIRKS_BASE] or
                    this.quirks[QUIRK_ALLOW_COMMENT_LINE - QUIRKS_BASE] {
                this.decode_comment?(dst: args.dst, src: args.src)
                if this.comment_type > 0 {
                    continue.outer
                }
            }
        }

        return "#bad input"
        }} endwhile.goto_parsed_a_leaf_value

        // We've just parsed a leaf (non-container) value: literal (null,
        // false, true), number or string.
        if depth == 0 {
            break.outer
        }
        expect = expect_after_value
    } endwhile.outer

    if this.quirks[QUIRK_ALLOW_TRAILING_FILLER - QUIRKS_BASE] or
            this.quirks[QUIRK_EXPECT_TRAILING_NEW_LINE_OR_EOF - QUIRKS_BASE] {
        this.decode_trailer?(dst: args.dst, src: args.src)
    }

    this.end_of_data = true
}

pri func decoder.decode_number!(src: base.io_reader) base.u32[..= 0x3FF] {
    var c              : base.u8
    var n              : base.u32[..= 0x3FF]
    var floating_point : base.u32[..= 0x80]

    while.goto_done true {{
    n = 0

    // Peek.
    if args.src.length() <= 0 {
        if not args.src.is_closed() {
            n |= 0x300
        }
        break.goto_done
    }
    c = args.src.peek_u8()

    // Scan the optional minus sign.
    if c <> '-' {
        assert args.src.length() > 0
        assert n <= 1
    } else {
        n += 1
        args.src.skip_u32_fast!(actual: 1, worst_case: 1)

        // Peek.
        if args.src.length() <= 0 {
            if not args.src.is_closed() {
                n |= 0x300
            }
            n |= 0x100  // A '-' without digits is invalid.
            break.goto_done
        }
        c = args.src.peek_u8()

        assert args.src.length() > 0
        assert n <= 1
    }

    // Scan the opening digits.
    if c == '0' {
        n += 1
        args.src.skip_u32_fast!(actual: 1, worst_case: 1)
        assert n <= 99
    } else {
        n = this.decode_digits!(src: args.src, n: n)
        if n > 99 {
            break.goto_done
        }
        assert n <= 99
    }

    // Peek.
    if args.src.length() <= 0 {
        if not args.src.is_closed() {
            n |= 0x300
        }
        break.goto_done
    }
    c = args.src.peek_u8()

    // Scan the optional fraction.
    if c <> '.' {
        assert args.src.length() > 0
        assert n <= 99
    } else {
        if n >= 99 {
            n |= 0x200
            break.goto_done
        }
        n += 1
        args.src.skip_u32_fast!(actual: 1, worst_case: 1)
        floating_point = 0x80

        n = this.decode_digits!(src: args.src, n: n)
        if n > 99 {
            break.goto_done
        }

        // Peek.
        if args.src.length() <= 0 {
            if not args.src.is_closed() {
                n |= 0x300
            }
            break.goto_done
        }
        c = args.src.peek_u8()

        assert args.src.length() > 0
        assert n <= 99
    }

    // Scan the optional 'E' or 'e'.
    if (c <> 'E') and (c <> 'e') {
        break.goto_done
    }
    if n >= 99 {
        n |= 0x200
        break.goto_done
    }
    n += 1
    args.src.skip_u32_fast!(actual: 1, worst_case: 1)
    floating_point = 0x80
    assert n <= 99

    // Peek.
    if args.src.length() <= 0 {
        if not args.src.is_closed() {
            n |= 0x300
        }
        n |= 0x100  // An 'E' or 'e' without digits is invalid.
        break.goto_done
    }
    c = args.src.peek_u8()

    // Scan the optional '+' or '-'.
    if (c <> '+') and (c <> '-') {
        assert n <= 99
    } else {
        if n >= 99 {
            n |= 0x200
            break.goto_done
        }
        n += 1
        args.src.skip_u32_fast!(actual: 1, worst_case: 1)
        assert n <= 99
    }

    // Scan the exponent digits.
    n = this.decode_digits!(src: args.src, n: n)

    break.goto_done
    }} endwhile.goto_done

    return n | floating_point
}

pri func decoder.decode_digits!(src: base.io_reader, n: base.u32[..= 99]) base.u32[..= 0x3FF] {
    var c : base.u8
    var n : base.u32[..= 0x3FF]

    n = args.n
    while true {
        if args.src.length() <= 0 {
            if not args.src.is_closed() {
                n |= 0x300
            }
            break
        }
        c = args.src.peek_u8()
        if 0x00 == LUT_DECIMAL_DIGITS[c] {
            break
        }
        // Cap DECODER_NUMBER_LENGTH_MAX_INCL at an arbitrary value, 99. The
        // caller's src.data.len should therefore be at least 100, also known
        // as DECODER_SRC_IO_BUFFER_LENGTH_MIN_INCL.
        //
        // An example of a JSON number that is 81 bytes long is:
        // https://github.com/nst/JSONTestSuite/blob/master/test_parsing/y_number_double_close_to_zero.json
        //
        // Note that 99 (in hex, 0x63) is less than 0x80, so we can use 0x80 as
        // a flag bit in func decoder.decode_number.
        if n >= 99 {
            n |= 0x200
            break
        }
        n += 1
        args.src.skip_u32_fast!(actual: 1, worst_case: 1)
    } endwhile
    if n == args.n {
        n |= 0x100
    }
    return n
}

pri func decoder.decode_leading?(dst: base.token_writer, src: base.io_reader) {
    var c : base.u8
    var u : base.u32

    this.allow_leading_ars =
            this.quirks[QUIRK_ALLOW_LEADING_ASCII_RECORD_SEPARATOR - QUIRKS_BASE]
    this.allow_leading_ubom =
            this.quirks[QUIRK_ALLOW_LEADING_UNICODE_BYTE_ORDER_MARK - QUIRKS_BASE]

    while this.allow_leading_ars or this.allow_leading_ubom {
        if args.dst.length() <= 0 {
            yield? base."$short write"
            continue
        }
        if args.src.length() <= 0 {
            if args.src.is_closed() {
                break
            }
            yield? base."$short read"
            continue
        }
        c = args.src.peek_u8()
        if (c == 0x1E) and this.allow_leading_ars {
            this.allow_leading_ars = false
            args.src.skip_u32_fast!(actual: 1, worst_case: 1)
            args.dst.write_simple_token_fast!(
                    value_major: 0, value_minor: 0, continued: 0, length: 1)
            continue
        } else if (c == 0xEF) and this.allow_leading_ubom {
            if args.src.length() < 3 {
                if args.src.is_closed() {
                    break
                }
                yield? base."$short read"
                continue
            }
            u = args.src.peek_u24le_as_u32()
            if u == 0xBF_BBEF {
                this.allow_leading_ubom = false
                args.src.skip_u32_fast!(actual: 3, worst_case: 3)
                args.dst.write_simple_token_fast!(
                        value_major: 0, value_minor: 0, continued: 0, length: 3)
                continue
            }
        }
        break
    } endwhile
}

pri func decoder.decode_comment?(dst: base.token_writer, src: base.io_reader) {
    var c      : base.u8
    var c2     : base.u16
    var length : base.u32[..= 0xFFFD]

    this.comment_type = 0

    while (args.dst.length() <= 0) or (args.src.length() <= 1),
            post args.dst.length() > 0,
            post args.src.length() > 1,
    {
        if args.dst.length() <= 0 {
            yield? base."$short write"
            continue
        }
        if args.src.is_closed() {
            return ok
        }
        yield? base."$short read"
    } endwhile
    c2 = args.src.peek_u16le()

    if (c2 == '/*'le) and this.quirks[QUIRK_ALLOW_COMMENT_BLOCK - QUIRKS_BASE] {
        args.src.skip_u32_fast!(actual: 2, worst_case: 2)
        length = 2

        while.comment_block true,
                pre args.dst.length() > 0,
        {
            if args.src.length() <= 1 {
                if length > 0 {
                    args.dst.write_simple_token_fast!(
                            value_major: 0,
                            value_minor: (base.TOKEN__VBC__FILLER << 21) |
                            base.TOKEN__VBD__FILLER__COMMENT_BLOCK,
                            continued: 1,
                            length: length)
                }
                if args.src.is_closed() {
                    return "#bad input"
                }
                yield? base."$short read"
                while args.dst.length() <= 0,
                        post args.dst.length() > 0,
                {
                    yield? base."$short write"
                } endwhile
                length = 0
                continue.comment_block
            }

            c2 = args.src.peek_u16le()
            if c2 == '*/'le {
                args.src.skip_u32_fast!(actual: 2, worst_case: 2)
                args.dst.write_simple_token_fast!(
                        value_major: 0,
                        value_minor: (base.TOKEN__VBC__FILLER << 21) |
                        base.TOKEN__VBD__FILLER__COMMENT_BLOCK,
                        continued: 0,
                        length: length + 2)
                this.comment_type = 1
                return ok
            }

            args.src.skip_u32_fast!(actual: 1, worst_case: 1)
            if length >= 0xFFFD {
                args.dst.write_simple_token_fast!(
                        value_major: 0,
                        value_minor: (base.TOKEN__VBC__FILLER << 21) |
                        base.TOKEN__VBD__FILLER__COMMENT_BLOCK,
                        continued: 1,
                        length: length + 1)
                while args.dst.length() <= 0,
                        post args.dst.length() > 0,
                {
                    yield? base."$short write"
                } endwhile
                length = 0
                continue.comment_block
            }
            length += 1
        } endwhile.comment_block

    } else if (c2 == '//'le) and this.quirks[QUIRK_ALLOW_COMMENT_LINE - QUIRKS_BASE] {
        args.src.skip_u32_fast!(actual: 2, worst_case: 2)
        length = 2

        while.comment_line true,
                pre args.dst.length() > 0,
        {
            if args.src.length() <= 0 {
                if args.src.is_closed() {
                    args.dst.write_simple_token_fast!(
                            value_major: 0,
                            value_minor: (base.TOKEN__VBC__FILLER << 21) |
                            base.TOKEN__VBD__FILLER__COMMENT_LINE,
                            continued: 0,
                            length: length)
                    this.comment_type = 2
                    return ok
                } else if length > 0 {
                    args.dst.write_simple_token_fast!(
                            value_major: 0,
                            value_minor: (base.TOKEN__VBC__FILLER << 21) |
                            base.TOKEN__VBD__FILLER__COMMENT_LINE,
                            continued: 1,
                            length: length)
                }
                yield? base."$short read"
                while args.dst.length() <= 0,
                        post args.dst.length() > 0,
                {
                    yield? base."$short write"
                } endwhile
                length = 0
                continue.comment_line
            }

            c = args.src.peek_u8()
            if c == '\n' {
                args.dst.write_simple_token_fast!(
                        value_major: 0,
                        value_minor: (base.TOKEN__VBC__FILLER << 21) |
                        base.TOKEN__VBD__FILLER__COMMENT_LINE,
                        continued: 0,
                        length: length)
                this.comment_type = 2
                return ok
            }

            args.src.skip_u32_fast!(actual: 1, worst_case: 1)
            if length >= 0xFFFD {
                args.dst.write_simple_token_fast!(
                        value_major: 0,
                        value_minor: (base.TOKEN__VBC__FILLER << 21) |
                        base.TOKEN__VBD__FILLER__COMMENT_LINE,
                        continued: 1,
                        length: length + 1)
                while args.dst.length() <= 0,
                        post args.dst.length() > 0,
                {
                    yield? base."$short write"
                } endwhile
                length = 0
                continue.comment_line
            }
            length += 1
        } endwhile.comment_line
    }
}

pri func decoder.decode_inf_nan?(dst: base.token_writer, src: base.io_reader) {
    var c4  : base.u32
    var neg : base.u32[..= 1]

    while true {
        if args.dst.length() <= 0 {
            yield? base."$short write"
            continue
        }
        if args.src.length() <= 2 {
            if args.src.is_closed() {
                return "#bad input"
            }
            yield? base."$short read"
            continue
        }

        // Bitwise or'ing with 0x20 converts upper case ASCII to lower case.

        c4 = args.src.peek_u24le_as_u32()
        if (c4 | 0x20_2020) == 'inf'le {
            if args.src.length() > 7 {
                if (args.src.peek_u64le() | 0x2020_2020_2020_2020) == 'infinity'le {
                    args.dst.write_simple_token_fast!(
                            value_major: 0,
                            value_minor: (base.TOKEN__VBC__NUMBER << 21) |
                            base.TOKEN__VBD__NUMBER__CONTENT_POS_INF,
                            continued: 0,
                            length: 8)
                    args.src.skip_u32_fast!(actual: 8, worst_case: 8)
                    return ok
                }
            } else if not args.src.is_closed() {
                yield? base."$short read"
                continue
            }
            args.dst.write_simple_token_fast!(
                    value_major: 0,
                    value_minor: (base.TOKEN__VBC__NUMBER << 21) |
                    base.TOKEN__VBD__NUMBER__CONTENT_POS_INF,
                    continued: 0,
                    length: 3)
            args.src.skip_u32_fast!(actual: 3, worst_case: 3)
            return ok

        } else if (c4 | 0x20_2020) == 'nan'le {
            args.dst.write_simple_token_fast!(
                    value_major: 0,
                    value_minor: (base.TOKEN__VBC__NUMBER << 21) |
                    base.TOKEN__VBD__NUMBER__CONTENT_POS_NAN,
                    continued: 0,
                    length: 3)
            args.src.skip_u32_fast!(actual: 3, worst_case: 3)
            return ok
        } else if (c4 & 0xFF) == '+' {
            neg = 0
        } else if (c4 & 0xFF) == '-' {
            neg = 1
        } else {
            return "#bad input"
        }

        if args.src.length() <= 3 {
            if args.src.is_closed() {
                return "#bad input"
            }
            yield? base."$short read"
            continue
        }

        c4 = args.src.peek_u32le() >> 8
        if (c4 | 0x20_2020) == 'inf'le {
            if args.src.length() > 8 {
                if (args.src.peek_u64le_at(offset: 1) | 0x2020_2020_2020_2020) == 'infinity'le {
                    args.dst.write_simple_token_fast!(
                            value_major: 0,
                            value_minor: (base.TOKEN__VBC__NUMBER << 21) |
                            (base.TOKEN__VBD__NUMBER__CONTENT_POS_INF >> neg),
                            continued: 0,
                            length: 9)
                    args.src.skip_u32_fast!(actual: 9, worst_case: 9)
                    return ok
                }
            } else if not args.src.is_closed() {
                yield? base."$short read"
                continue
            }
            args.dst.write_simple_token_fast!(
                    value_major: 0,
                    value_minor: (base.TOKEN__VBC__NUMBER << 21) |
                    (base.TOKEN__VBD__NUMBER__CONTENT_POS_INF >> neg),
                    continued: 0,
                    length: 4)
            args.src.skip_u32_fast!(actual: 4, worst_case: 4)
            return ok

        } else if (c4 | 0x20_2020) == 'nan'le {
            args.dst.write_simple_token_fast!(
                    value_major: 0,
                    value_minor: (base.TOKEN__VBC__NUMBER << 21) |
                    (base.TOKEN__VBD__NUMBER__CONTENT_POS_NAN >> neg),
                    continued: 0,
                    length: 4)
            args.src.skip_u32_fast!(actual: 4, worst_case: 4)
            return ok
        }

        return "#bad input"
    } endwhile
}

pri func decoder.decode_trailer?(dst: base.token_writer, src: base.io_reader) {
    var c                 : base.u8
    var whitespace_length : base.u32[..= 0xFFFE]

    if this.quirks[QUIRK_EXPECT_TRAILING_NEW_LINE_OR_EOF - QUIRKS_BASE] {
        this.trailer_stop = '\n'
    } else {
        this.trailer_stop = 0
    }

    while.outer true {
        if args.dst.length() <= 0 {
            yield? base."$short write"
            continue.outer
        }

        whitespace_length = 0
        while.inner true,
                pre args.dst.length() > 0,
        {
            if args.src.length() <= 0 {
                if whitespace_length > 0 {
                    args.dst.write_simple_token_fast!(
                            value_major: 0, value_minor: 0, continued: 0, length: whitespace_length)
                }
                if args.src.is_closed() {
                    break.outer
                }
                yield? base."$short read"
                continue.outer
            }

            c = args.src.peek_u8()
            if LUT_CLASSES[c] <> CLASS_WHITESPACE {
                if whitespace_length > 0 {
                    args.dst.write_simple_token_fast!(
                            value_major: 0, value_minor: 0, continued: 0, length: whitespace_length)
                }
                if this.trailer_stop > 0 {
                    return "#bad input"
                }
                this.decode_comment?(dst: args.dst, src: args.src)
                if this.comment_type > 0 {
                    continue.outer
                }
                return ok
            }

            args.src.skip_u32_fast!(actual: 1, worst_case: 1)
            if (whitespace_length >= 0xFFFE) or (c == this.trailer_stop) {
                args.dst.write_simple_token_fast!(
                        value_major: 0, value_minor: 0, continued: 0, length: whitespace_length + 1)
                if c == this.trailer_stop {
                    return ok
                }
                continue.outer
            }
            whitespace_length += 1
        } endwhile.inner
    } endwhile.outer
}