// Copyright 2023 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 status "#bad DHT marker"
pub status "#bad DQT marker"
pub status "#bad DRI marker"
pub status "#bad SOF marker"
pub status "#bad SOS marker"
pub status "#bad header"
pub status "#bad marker"
pub status "#missing Huffman table"
pub status "#missing Quantization table"
pub status "#truncated input"
pub status "#unsupported DQT after SOF markers"
pub status "#unsupported arithmetic coding"
pub status "#unsupported fractional sampling"
pub status "#unsupported hierarchical coding"
pub status "#unsupported implicit height"
pub status "#unsupported lossless coding"
pub status "#unsupported marker"
pub status "#unsupported precision (12 bits)"
pub status "#unsupported precision (16 bits)"
pub status "#unsupported precision"

pri status "#internal error: inconsistent decoder state"

pub const DECODER_WORKBUF_LEN_MAX_INCL_WORST_CASE : base.u64 = 0x4_0040_0100

pub struct decoder? implements base.image_decoder(
        width  : base.u32[..= 0xFFFF],
        height : base.u32[..= 0xFFFF],

        width_in_mcus  : base.u32[..= 0x2000],
        height_in_mcus : base.u32[..= 0x2000],

        // The call sequence state machine is discussed in
        // (/doc/std/image-decoders-call-sequence.md).
        call_sequence : base.u8,

        sof_marker          : base.u8,
        next_restart_marker : base.u8[..= 7],

        max_incl_components_h : base.u8[..= 4],
        max_incl_components_v : base.u8[..= 4],

        num_components : base.u32[..= 4],
        components_c   : array[4] base.u8,
        components_h   : array[4] base.u8[..= 4],
        components_v   : array[4] base.u8[..= 4],
        components_tq  : array[4] base.u8[..= 3],

        // components_workbuf_offsets, indexed by (b) and (b + 1), is the i and
        // j bounds in the workbuf[i .. j] slice holding the post-IDCT values.
        // components_workbuf_widths[b] is the row width, which is also the row
        // stride (there is no slack).
        //
        // For example, a 4:2:0 chroma-subsampled, 36 pixels wide × 28 pixels
        // high image has 6 blocks (4 Y, 1 Cb, 1 Cr; 8×8 samples each) per MCU.
        // Each MCU is 16×16 pixels. The image is 3 MCUs wide × 2 MCUs high.
        // The post-IDCT image dimensions round up to 48 × 32 pixels.
        //
        // The components_workbuf_widths array is:
        //   0: 0x30 = 48  // Y
        //   1: 0x18 = 24  // Cb
        //   2: 0x18 = 24  // Cr
        //   3: 0x00 =  0  // Unused
        //
        // The components_workbuf_offsets array is:
        //   0: 0x000 =    0
        //   1: 0x600 = 1536 = previous + (48 * 32)
        //   2: 0x790 = 1920 = previous + (24 * 16)
        //   3: 0x900 = 2304 = previous + (24 * 16)
        //   4: 0x900 = 2304 = previous + ( 0 *  0)
        components_workbuf_widths  : array[4] base.u32[..= 0x1_0008],
        components_workbuf_heights : array[4] base.u32[..= 0x1_0008],
        components_workbuf_offsets : array[5] base.u64[..= 0x4_0040_0100],  // 4 * 0x1_0008 * 0x1_0008.

        scan_num_components  : base.u32[..= 4],
        scan_comps_cselector : array[4] base.u8[..= 3],
        scan_comps_td        : array[4] base.u8[..= 3],
        scan_comps_ta        : array[4] base.u8[..= 3],

        scan_ss : base.u8[..= 63],
        scan_se : base.u8[..= 63],
        scan_ah : base.u8[..= 13],
        scan_al : base.u8[..= 13],

        scan_width_in_mcus  : base.u32[..= 0x2000],
        scan_height_in_mcus : base.u32[..= 0x2000],

        scan_comps_bx_offset : array[16] base.u8[..= 3],
        scan_comps_by_offset : array[16] base.u8[..= 3],

        mcu_num_blocks       : base.u32[..= 10],
        mcu_current_block    : base.u32[..= 10],
        mcu_blocks_sselector : array[16] base.u8[..= 3],
        mcu_zig_index        : base.u32[..= 63],

        mcu_previous_dc_values : array[4] base.u16,

        restart_interval       : base.u16,
        saved_restart_interval : base.u16,
        restarts_remaining     : base.u16,

        frame_config_io_position : base.u64,

        payload_length : base.u32[..= 0xFFFF],

        seen_dqt : array[4] base.bool,
        seen_dht : array[8] base.bool,

        // These fields yield bitstream bits in Most Significant Bits order.
        bitstream_bits   : base.u64,
        bitstream_n_bits : base.u32,
        bitstream_ri     : base.u32[..= 0x800],
        bitstream_wi     : base.u32[..= 0x800],

        quant_tables : array[4] array[64] base.u8,

        // huff_tables_symbols[(tc*4)|th][i] is the i'th Huffman code's symbol.
        huff_tables_symbols : array[8] array[256] base.u8,

        // huff_tables_slow[(tc*4)|th][n] is a u32 such that:
        //  - the high 24 bits hold the maximum (exclusive) bit-string of the
        //    codes of bit-length (n+1).
        //  - the low 8 bits hold a huff_tables_symbols[(tc*4)|th] index bias.
        huff_tables_slow : array[8] array[16] base.u32,

        // huff_tables_fast[(tc*4)|th][b] is a u16 that decodes the bit-length
        // and symbol when the MSB-first bit-stream starts with b.
        //  - the high 8 bits hold the bit-length. Zero means no fast path.
        //  - the low 8 bits hold the symbol.
        huff_tables_fast : array[8] array[256] base.u16,

        swizzler : base.pixel_swizzler,
        util     : base.utility,
) + (
        bitstream_buffer : array[0x800] base.u8,  // 0x800 = 2048.

        mcu_blocks : array[10] array[64] base.u16,

        // The dht_temp_etc fields are decode_dht temporary values.

        // dht_temp_counts[n] is the number of codes with bit-length (n+1).
        dht_temp_counts : array[16] base.u8,
        // dht_temp_bit_lengths[i] is the bit-length of the i'th code.
        dht_temp_bit_lengths : array[256] base.u8,
        // dht_temp_bit_strings[i] is the bit-string of the i'th code.
        dht_temp_bit_strings : array[256] base.u16,

        dst_palette : array[4 * 256] base.u8,
)

pub func decoder.set_quirk!(key: base.u32, value: base.u64) base.status {
    return base."#unsupported option"
}

pub func decoder.decode_image_config?(dst: nptr base.image_config, src: base.io_reader) {
    var status : base.status

    while true {
        status =? this.do_decode_image_config?(dst: args.dst, src: args.src)
        if (status == base."$short read") and args.src.is_closed() {
            return "#truncated input"
        }
        yield? status
    } endwhile
}

pri func decoder.do_decode_image_config?(dst: nptr base.image_config, src: base.io_reader) {
    var c      : base.u8
    var marker : base.u8
    var pixfmt : base.u32

    if this.call_sequence <> 0x00 {
        return base."#bad call sequence"
    }

    c = args.src.read_u8?()
    if c <> 0xFF {
        return "#bad header"
    }
    c = args.src.read_u8?()
    if c <> 0xD8 {  // SOI (Start Of Image).
        return "#bad header"
    }

    // Process chunks (markers and their payloads).
    while true {
        // Read the marker (a two-byte 0xFF 0x?? sequence).
        while true {
            c = args.src.read_u8?()
            if c == 0xFF {
                break
            }
            // Getting here is invalid according to the JPEG spec, but libjpeg
            // treats this as a warning (JWRN_EXTRANEOUS_DATA), not an error.
        } endwhile
        while true {
            c = args.src.read_u8?()
            if c <> 0xFF {
                marker = c
                break
            }
            // Section B.1.1.2: "Any marker may optionally be preceded by any
            // number of [0xFF] fill bytes".
        } endwhile

        if marker == 0x00 {
            // Ignore byte stuffing.
            continue
        } else if (0xD0 <= marker) and (marker <= 0xD7) {
            // RSTn (Restart) markers have no payload.
            continue
        }

        // Payload length includes the 2 bytes for the u16be payload length.
        this.payload_length = args.src.read_u16be_as_u32?()
        if this.payload_length < 2 {
            return "#bad marker"
        }
        this.payload_length -= 2

        // Switch on the marker.
        if marker < 0xC0 {
            return "#unsupported marker"

        } else if marker < 0xD0 {  // SOFn (Start of Frame) and friends.
            if marker <= 0xC2 {
                if this.sof_marker <> 0 {
                    return "#bad SOF marker"
                }
                this.sof_marker = marker
                this.decode_sof?(src: args.src)
                break

            } else if marker == 0xC3 {
                return "#unsupported lossless coding"

            } else if marker == 0xC4 {  // DHT (Define Huffman Table).
                // We shouldn't see DHT before SOF.
                return "#bad DHT marker"

            } else if (0xC5 <= marker) and (marker <= 0xC7) {
                return "#unsupported hierarchical coding"

            } else if marker == 0xC8 {  // JPG (JPEG extension).
                return "#unsupported marker"

            } else {
                return "#unsupported arithmetic coding"
            }

        } else if marker < 0xE0 {
            if marker < 0xDA {
                // RSTn markers are already handled above. We either have 0xD8
                // (SOI: Start Of Image) or 0xD9 (EOI: End Of Image), neither
                // of which are valid here.
                return "#bad marker"

            } else if marker == 0xDA {  // SOS (Start Of Scan).
                // We shouldn't see SOS before SOF.
                return "#bad SOS marker"

            } else if marker == 0xDB {  // DQT (Define Quantization Table).
                this.decode_dqt?(src: args.src)
                continue

            } else if marker == 0xDD {  // DRI (Define Restart Interval).
                this.decode_dri?(src: args.src)
                continue

            } else {
                // 0xDC (DNL: Define Number of Lines).
                // 0xDE (DHP: Define Hierarchical Progression).
                // 0xDF (EXP: Expand Reference Components).
                return "#unsupported marker"
            }

        } else if marker < 0xF0 {  // APPn (Application specific).
            // No-op.

        } else {
            if marker == 0xFE {  // COM (Comment).
                // No-op.

            } else {
                return "#unsupported marker"
            }
        }

        args.src.skip_u32?(n: this.payload_length)
        this.payload_length = 0
    } endwhile

    this.frame_config_io_position = args.src.position()

    if args.dst <> nullptr {
        pixfmt = base.PIXEL_FORMAT__Y
        if this.num_components > 1 {
            // TODO: base.PIXEL_FORMAT__YCBCR is probably more correct,
            // although possibly less convenient for the caller.
            pixfmt = base.PIXEL_FORMAT__BGRX
        }
        args.dst.set!(
                pixfmt: pixfmt,
                pixsub: 0,
                width: this.width,
                height: this.height,
                first_frame_io_position: this.frame_config_io_position,
                first_frame_is_opaque: true)
    }

    this.call_sequence = 0x20
}

pri func decoder.decode_dqt?(src: base.io_reader) {
    var c : base.u8
    var q : base.u8[..= 3]
    var i : base.u32

    while this.payload_length > 0 {
        this.payload_length -= 1
        c = args.src.read_u8?()
        if (c & 0x0F) > 3 {
            return "#bad DQT marker"
        }
        q = c & 0x0F
        if (c >> 4) == 1 {
            return "#unsupported precision"
        } else if ((c >> 4) > 1) or (this.payload_length < 64) {
            return "#bad DQT marker"
        }
        this.payload_length -= 64

        i = 0
        while i < 64 {
            this.quant_tables[q][UNZIG[i]] = args.src.read_u8?()
            i += 1
        } endwhile
        this.seen_dqt[q] = true
    } endwhile
}

pri func decoder.decode_dri?(src: base.io_reader) {
    if this.payload_length <> 2 {
        return "#bad DRI marker"
    }
    this.payload_length = 0

    this.restart_interval = args.src.read_u16be?()
    if this.sof_marker == 0 {
        this.saved_restart_interval = this.restart_interval
    }
}

pri func decoder.decode_sof?(src: base.io_reader) {
    var c      : base.u8
    var comp_h : base.u8
    var comp_v : base.u8
    var i      : base.u32
    var j      : base.u32

    var has_h24 : base.bool
    var has_h3  : base.bool
    var has_v24 : base.bool
    var has_v3  : base.bool

    var upper_bound : base.u32[..= 0x1_0008]

    if this.payload_length < 6 {
        return "#bad SOF marker"
    }
    this.payload_length -= 6
    c = args.src.read_u8?()
    if c == 8 {
        // No-op.
    } else if c == 12 {
        return "#unsupported precision (12 bits)"
    } else if c == 16 {
        return "#unsupported precision (16 bits)"
    } else {
        return "#unsupported precision"
    }
    this.height = args.src.read_u16be_as_u32?()
    if this.height == 0 {
        return "#unsupported implicit height"
    }
    this.width = args.src.read_u16be_as_u32?()
    if this.width == 0 {
        return base."#unsupported image dimension"
    }
    c = args.src.read_u8?()
    if (c == 0) or (c > 4) {
        return "#bad SOF marker"
    }
    this.num_components = c as base.u32
    if this.payload_length <> (3 * this.num_components) {
        return "#bad SOF marker"
    }
    this.payload_length = 0

    i = 0
    while i < this.num_components {
        assert i < 4 via "a < b: a < c; c <= b"(c: this.num_components)
        this.components_c[i] = args.src.read_u8?()
        c = args.src.read_u8?()
        comp_h = c >> 4
        comp_v = c & 0x0F
        if (comp_h == 0) or (comp_h > 4) or (comp_v == 0) or (comp_v > 4) {
            return "#bad SOF marker"
        }
        this.components_h[i] = comp_h
        if this.max_incl_components_h < this.components_h[i] {
            this.max_incl_components_h = this.components_h[i]
        }
        this.components_v[i] = comp_v
        if this.max_incl_components_v < this.components_v[i] {
            this.max_incl_components_v = this.components_v[i]
        }
        c = args.src.read_u8?()
        if c >= 4 {
            return "#bad SOF marker"
        }
        this.components_tq[i] = c

        // Section B.2.2: "the value of C_i shall be different from the values
        // of C_1 through C_(i-1)".
        j = 0
        while j < i,
                inv i < 4,
        {
            assert j < 4 via "a < b: a < c; c < b"(c: i)
            if this.components_c[j] == this.components_c[i] {
                return "#bad SOF marker"
            }
            j += 1
        } endwhile

        i += 1
    } endwhile

    if this.num_components == 1 {
        // The first (and only) component's H and V factors are effectively
        // always 1. Section A.2.2: "When Ns = 1 (where Ns is the number of
        // components in a scan), the order of data units [8x8 blocks] within a
        // scan shall be left-to-right and top-to-bottom... regardless of the
        // values of H1 and V1.".
        this.max_incl_components_h = 1
        this.max_incl_components_v = 1
        this.components_h[0] = 1
        this.components_v[0] = 1

    } else {
        has_h24 = false
        has_h3 = false
        has_v24 = false
        has_v3 = false
        i = 0
        while i < this.num_components {
            assert i < 4 via "a < b: a < c; c <= b"(c: this.num_components)
            has_h24 = has_h24 or (this.components_h[i] == 2) or (this.components_h[i] == 4)
            has_h3 = has_h3 or (this.components_h[i] == 3)
            has_v24 = has_v24 or (this.components_v[i] == 2) or (this.components_v[i] == 4)
            has_v3 = has_v3 or (this.components_v[i] == 3)
            i += 1
        } endwhile
        if (has_h24 and has_h3) or (has_v24 and has_v3) {
            return "#unsupported fractional sampling"
        }
    }

    if this.max_incl_components_h == 1 {
        this.width_in_mcus = (this.width + 0x07) / 0x08
    } else if this.max_incl_components_h == 2 {
        this.width_in_mcus = (this.width + 0x0F) / 0x10
    } else if this.max_incl_components_h == 3 {
        this.width_in_mcus = (this.width + 0x17) / 0x18
    } else {
        this.width_in_mcus = (this.width + 0x1F) / 0x20
    }

    if this.max_incl_components_v == 1 {
        this.height_in_mcus = (this.height + 0x07) / 0x08
    } else if this.max_incl_components_v == 2 {
        this.height_in_mcus = (this.height + 0x0F) / 0x10
    } else if this.max_incl_components_v == 3 {
        this.height_in_mcus = (this.height + 0x17) / 0x18
    } else {
        this.height_in_mcus = (this.height + 0x1F) / 0x20
    }

    upper_bound = 0x1_0008

    this.components_workbuf_widths[0] = upper_bound.min(no_more_than:
            8 * this.width_in_mcus * (this.components_h[0] as base.u32))
    this.components_workbuf_widths[1] = upper_bound.min(no_more_than:
            8 * this.width_in_mcus * (this.components_h[1] as base.u32))
    this.components_workbuf_widths[2] = upper_bound.min(no_more_than:
            8 * this.width_in_mcus * (this.components_h[2] as base.u32))
    this.components_workbuf_widths[3] = upper_bound.min(no_more_than:
            8 * this.width_in_mcus * (this.components_h[3] as base.u32))

    this.components_workbuf_heights[0] = upper_bound.min(no_more_than:
            8 * this.height_in_mcus * (this.components_v[0] as base.u32))
    this.components_workbuf_heights[1] = upper_bound.min(no_more_than:
            8 * this.height_in_mcus * (this.components_v[1] as base.u32))
    this.components_workbuf_heights[2] = upper_bound.min(no_more_than:
            8 * this.height_in_mcus * (this.components_v[2] as base.u32))
    this.components_workbuf_heights[3] = upper_bound.min(no_more_than:
            8 * this.height_in_mcus * (this.components_v[3] as base.u32))

    this.components_workbuf_offsets[0] = 0
    this.components_workbuf_offsets[1] = this.components_workbuf_offsets[0] +
            ((this.components_workbuf_widths[0] as base.u64) * (this.components_workbuf_heights[0] as base.u64))
    this.components_workbuf_offsets[2] = this.components_workbuf_offsets[1] +
            ((this.components_workbuf_widths[1] as base.u64) * (this.components_workbuf_heights[1] as base.u64))
    this.components_workbuf_offsets[3] = this.components_workbuf_offsets[2] +
            ((this.components_workbuf_widths[2] as base.u64) * (this.components_workbuf_heights[2] as base.u64))
    this.components_workbuf_offsets[4] = this.components_workbuf_offsets[3] +
            ((this.components_workbuf_widths[3] as base.u64) * (this.components_workbuf_heights[3] as base.u64))
}

pub func decoder.decode_frame_config?(dst: nptr base.frame_config, src: base.io_reader) {
    var status : base.status

    while true {
        status =? this.do_decode_frame_config?(dst: args.dst, src: args.src)
        if (status == base."$short read") and args.src.is_closed() {
            return "#truncated input"
        }
        yield? status
    } endwhile
}

pri func decoder.do_decode_frame_config?(dst: nptr base.frame_config, src: base.io_reader) {
    if this.call_sequence == 0x20 {
        // No-op.
    } else if this.call_sequence < 0x20 {
        this.do_decode_image_config?(dst: nullptr, src: args.src)
    } else if this.call_sequence == 0x28 {
        if this.frame_config_io_position <> args.src.position() {
            return base."#bad restart"
        }
    } else if this.call_sequence == 0x40 {
        this.call_sequence = 0x60
        return base."@end of data"
    } else {
        return base."@end of data"
    }

    if args.dst <> nullptr {
        args.dst.set!(bounds: this.util.make_rect_ie_u32(
                min_incl_x: 0,
                min_incl_y: 0,
                max_excl_x: this.width,
                max_excl_y: this.height),
                duration: 0,
                index: 0,
                io_position: this.frame_config_io_position,
                disposal: 0,
                opaque_within_bounds: true,
                overwrite_instead_of_blend: false,
                background_color: 0xFF00_0000)
    }

    this.call_sequence = 0x40
}

pub func decoder.decode_frame?(dst: ptr base.pixel_buffer, src: base.io_reader, blend: base.pixel_blend, workbuf: slice base.u8, opts: nptr base.decode_frame_options) {
    var status : base.status

    while true {
        status =? this.do_decode_frame?(dst: args.dst, src: args.src, blend: args.blend, workbuf: args.workbuf, opts: args.opts)
        if (status == base."$short read") and args.src.is_closed() {
            return "#truncated input"
        }
        yield? status
    } endwhile
}

pri func decoder.do_decode_frame?(dst: ptr base.pixel_buffer, src: base.io_reader, blend: base.pixel_blend, workbuf: slice base.u8, opts: nptr base.decode_frame_options) {
    var pixfmt : base.u32
    var status : base.status
    var c      : base.u8
    var marker : base.u8

    if this.call_sequence == 0x40 {
        // No-op.
    } else if this.call_sequence < 0x40 {
        this.do_decode_frame_config?(dst: nullptr, src: args.src)
    } else {
        return base."@end of data"
    }

    pixfmt = base.PIXEL_FORMAT__Y
    if this.num_components > 1 {
        pixfmt = base.PIXEL_FORMAT__BGRX
    }
    status = this.swizzler.prepare!(
            dst_pixfmt: args.dst.pixel_format(),
            dst_palette: args.dst.palette_or_else(fallback: this.dst_palette[..]),
            src_pixfmt: this.util.make_pixel_format(repr: pixfmt),
            src_palette: this.util.empty_slice_u8(),
            blend: args.blend)
    if not status.is_ok() {
        return status
    }

    // Process chunks (markers and their payloads).
    while true {
        // Read the marker (a two-byte 0xFF 0x?? sequence).
        while true {
            c = args.src.read_u8?()
            if c == 0xFF {
                break
            }
            // Getting here is invalid according to the JPEG spec, but libjpeg
            // treats this as a warning (JWRN_EXTRANEOUS_DATA), not an error.
        } endwhile
        while true {
            c = args.src.read_u8?()
            if c <> 0xFF {
                marker = c
                break
            }
            // Section B.1.1.2: "Any marker may optionally be preceded by any
            // number of [0xFF] fill bytes".
        } endwhile

        if marker == 0x00 {
            // Ignore byte stuffing.
            continue
        } else if (0xD0 <= marker) and (marker <= 0xD7) {
            // RSTn (Restart) markers have no payload.
            continue
        }

        // Payload length includes the 2 bytes for the u16be payload length.
        this.payload_length = args.src.read_u16be_as_u32?()
        if this.payload_length < 2 {
            return "#bad marker"
        }
        this.payload_length -= 2

        // Switch on the marker.
        if marker < 0xC0 {
            return "#unsupported marker"

        } else if marker < 0xD0 {  // SOFn (Start of Frame) and friends.
            if marker == 0xC4 {  // DHT (Define Huffman Table).
                this.decode_dht?(src: args.src)
                continue

            } else if marker == 0xC8 {  // JPG (JPEG extension).
                return "#unsupported marker"
            }
            return "#bad SOF marker"

        } else if marker < 0xE0 {
            if marker < 0xDA {
                // RSTn markers are already handled above. We either have 0xD8
                // (SOI: Start Of Image) or 0xD9 (EOI: End Of Image), neither
                // of which are valid here.
                return "#bad marker"

            } else if marker == 0xDA {  // SOS (Start Of Scan).
                this.decode_sos?(dst: args.dst, src: args.src, workbuf: args.workbuf)
                break

            } else if marker == 0xDB {  // DQT (Define Quantization Table).
                return "#unsupported DQT after SOF markers"

            } else if marker == 0xDD {  // DRI (Define Restart Interval).
                this.decode_dri?(src: args.src)
                continue

            } else {
                // 0xDC (DNL: Define Number of Lines).
                // 0xDE (DHP: Define Hierarchical Progression).
                // 0xDF (EXP: Expand Reference Components).
                return "#unsupported marker"
            }

        } else if marker < 0xF0 {  // APPn (Application specific).
            // No-op.

        } else {
            if marker == 0xFE {  // COM (Comment).
                // No-op.

            } else {
                return "#unsupported marker"
            }
        }

        args.src.skip_u32?(n: this.payload_length)
        this.payload_length = 0
    } endwhile

    this.call_sequence = 0x60
}

pri func decoder.decode_dht?(src: base.io_reader) {
    var c                   : base.u8
    var tc                  : base.u8[..= 1]
    var th                  : base.u8[..= 3]
    var tc4_th              : base.u8[..= 7]
    var working_total_count : base.u32[..= 0xFFFF]
    var total_count         : base.u32[..= 256]
    var i                   : base.u32
    var failed              : base.bool

    if this.sof_marker == 0 {
        return "#bad DHT marker"
    }
    while this.payload_length > 0 {
        if this.payload_length < 17 {
            return "#bad DHT marker"
        }
        this.payload_length -= 17
        c = args.src.read_u8?()
        if ((c >> 4) > 1) or ((c & 0x0F) > 3) {
            return "#bad DHT marker"
        }
        tc = c >> 4
        th = c & 0x0F
        tc4_th = ((tc * 4) | th) as base.u8

        // SOF0 (which is 0xC0) means a Baseline JPEG. The Baseline (th <= 1)
        // restriction is specified in table B.5.
        if (this.sof_marker == 0xC0) and ((tc4_th & 3) > 1) {
            return "#bad DHT marker"
        }

        // Read dht_temp_counts.
        i = 0
        while i < 16 {
            this.dht_temp_counts[i] = args.src.read_u8?()
            i += 1
        } endwhile
        working_total_count = 0
        i = 0
        while i < 16 {
            working_total_count =
                    (working_total_count + (this.dht_temp_counts[i] as base.u32)) & 0xFFFF
            i += 1
        } endwhile
        if (working_total_count <= 0) or (256 < working_total_count) {
            return "#bad DHT marker"
        }
        total_count = working_total_count

        // Read huff_tables_symbols[tc4_th].
        if this.payload_length < total_count {
            return "#bad DHT marker"
        }
        this.payload_length -= total_count
        i = 0
        while i < total_count {
            assert i < 256 via "a < b: a < c; c <= b"(c: total_count)
            this.huff_tables_symbols[tc4_th][i] = args.src.read_u8?()
            i += 1
        } endwhile
        while i < 256 {
            this.huff_tables_symbols[tc4_th][i] = 0
            i += 1
        } endwhile

        // For tc == 0 (DC, not AC tables), the symbols must not exceed 15 (for
        // lossy JPEG) or 16 (for lossless JPEG). We only support lossy.
        if (tc4_th & 4) == 0 {
            i = 0
            while i < total_count {
                assert i < 256 via "a < b: a < c; c <= b"(c: total_count)
                if this.huff_tables_symbols[tc4_th][i] > 15 {
                    return "#bad DHT marker"
                }
                i += 1
            } endwhile
        }

        failed = this.calculate_huff_tables!(tc4_th: tc4_th, total_count: total_count)
        if failed {
            return "#bad DHT marker"
        }

        this.seen_dht[tc4_th] = true
    } endwhile
}

pri func decoder.calculate_huff_tables!(tc4_th: base.u8[..= 7], total_count: base.u32[..= 256]) base.bool {
    var i                    : base.u32
    var j                    : base.u8
    var k                    : base.u8
    var bit_length_minus_one : base.u32[..= 15]
    var bit_length           : base.u8[..= 16]
    var bit_string           : base.u32
    var slow                 : base.u32
    var prefix               : base.u8
    var fast                 : base.u16
    var reps                 : base.u32

    // Calculate dht_temp_bit_lengths.
    i = 0
    k = 0
    bit_length_minus_one = 0
    while i < args.total_count {
        assert i < 256 via "a < b: a < c; c <= b"(c: args.total_count)
        while k >= this.dht_temp_counts[bit_length_minus_one],
                inv i < 256,
                post k < this.dht_temp_counts[bit_length_minus_one],
        {
            k = 0
            bit_length_minus_one = (bit_length_minus_one + 1) & 15
        } endwhile
        assert k < 255 via "a < b: a < c; c <= b"(c: this.dht_temp_counts[bit_length_minus_one])
        k += 1
        this.dht_temp_bit_lengths[i] = (bit_length_minus_one + 1) as base.u8
        i += 1
    } endwhile

    // Calculate dht_temp_bit_strings.
    bit_length = 0
    bit_string = 0
    i = 0
    while i < args.total_count {
        assert i < 256 via "a < b: a < c; c <= b"(c: args.total_count)
        while bit_length < this.dht_temp_bit_lengths[i],
                inv i < 256,
        {
            if bit_length >= 16 {
                return true
            }
            bit_length += 1
            bit_string ~mod<<= 1
        } endwhile
        this.dht_temp_bit_strings[i] = (bit_string & 0xFFFF) as base.u16
        bit_string ~mod+= 1

        // Check the bit_string just assigned to dht_temp_bit_strings[i].
        // Section C: "the codes shall be generated such that the
        // all-1-bits code word of any length is reserved".
        if (bit_string >> bit_length) > 0 {
            return true
        }

        i += 1
    } endwhile

    // Calculate huff_tables_slow[args.tc4_th].
    k = 0
    bit_length_minus_one = 0
    while true {
        if this.dht_temp_counts[bit_length_minus_one] == 0 {
            this.huff_tables_slow[args.tc4_th][bit_length_minus_one] = 0
        } else {
            slow = 0xFF & ((k as base.u32) ~mod-
                    (this.dht_temp_bit_strings[k] as base.u32))
            k ~mod+= this.dht_temp_counts[bit_length_minus_one]
            this.huff_tables_slow[args.tc4_th][bit_length_minus_one] = slow |
                    (((this.dht_temp_bit_strings[k ~mod- 1] as base.u32) + 1) << 8)
        }

        bit_length_minus_one = (bit_length_minus_one + 1) & 15
        if bit_length_minus_one == 0 {
            break
        }
    } endwhile

    // Calculate huff_tables_fast[args.tc4_th].
    i = 0
    while i < 256 {
        this.huff_tables_fast[args.tc4_th][i] = 0xFFFF
        i += 1
    } endwhile
    j = 0
    bit_length_minus_one = 0
    while bit_length_minus_one < 8 {
        k = 0
        while k < this.dht_temp_counts[bit_length_minus_one],
                inv bit_length_minus_one < 8,
        {
            assert k < 255 via "a < b: a < c; c <= b"(c: this.dht_temp_counts[bit_length_minus_one])
            prefix = (((this.dht_temp_bit_strings[j] as base.u32) <<
                    (7 - bit_length_minus_one)) & 0xFF) as base.u8
            fast = ((((bit_length_minus_one + 1) as base.u32) << 8) |
                    (this.huff_tables_symbols[args.tc4_th][j] as base.u32)) as base.u16
            reps = (1 as base.u32) << (7 - bit_length_minus_one)
            while reps > 0,
                    inv bit_length_minus_one < 8,
                    inv k < 255,
            {
                this.huff_tables_fast[args.tc4_th][prefix] = fast
                prefix ~mod+= 1
                reps -= 1
            } endwhile
            k += 1
            j ~mod+= 1
        } endwhile

        bit_length_minus_one += 1
    } endwhile

    return false
}

pri func decoder.decode_sos?(dst: ptr base.pixel_buffer, src: base.io_reader, workbuf: slice base.u8) {
    var my : base.u32
    var mx : base.u32

    var decode_mcu_result : base.u32
    var bitstream_length  : base.u32

    var b      : base.u32
    var i      : base.u32
    var csel   : base.u8[..= 3]
    var h      : base.u64[..= 4]
    var v      : base.u64[..= 4]
    var stride : base.u64[..= 0x4_0000]
    var offset : base.u64

    var status : base.status

    if args.workbuf.length() < this.components_workbuf_offsets[4] {
        return base."#bad workbuf length"
    }

    this.prepare_scan?(src: args.src)

    // Reset.
    this.next_restart_marker = 0
    this.restarts_remaining = this.restart_interval
    this.mcu_previous_dc_values[0] = 0
    this.mcu_previous_dc_values[1] = 0
    this.mcu_previous_dc_values[2] = 0
    this.mcu_previous_dc_values[3] = 0
    this.bitstream_bits = 0
    this.bitstream_n_bits = 0
    this.bitstream_ri = 0
    this.bitstream_wi = 0
    this.fill_bitstream!(src: args.src)

    my = 0
    while my < this.scan_height_in_mcus {
        assert my < 0x2000 via "a < b: a < c; c <= b"(c: this.scan_height_in_mcus)
        mx = 0
        while mx < this.scan_width_in_mcus,
                inv my < 0x2000,
        {
            assert mx < 0x2000 via "a < b: a < c; c <= b"(c: this.scan_width_in_mcus)
            this.mcu_current_block = 0
            this.mcu_zig_index = 0

            while.decode_mcu true,
                    inv my < 0x2000,
                    inv mx < 0x2000,
            {
                decode_mcu_result = this.decode_mcu!()
                if decode_mcu_result == 0 {
                    break.decode_mcu
                } else if decode_mcu_result <> 1 {
                    return "#internal error: inconsistent decoder state"
                }

                while.fill_bitstream true,
                        inv my < 0x2000,
                        inv mx < 0x2000,
                {
                    bitstream_length = this.bitstream_wi ~mod- this.bitstream_ri
                    this.fill_bitstream!(src: args.src)
                    if bitstream_length < (this.bitstream_wi ~mod- this.bitstream_ri) {
                        break.fill_bitstream
                    }
                    yield? base."$short read"
                } endwhile.fill_bitstream
            } endwhile.decode_mcu

            // Apply IDCT.
            b = 0
            while b < this.mcu_num_blocks,
                    inv my < 0x2000,
                    inv mx < 0x2000,
            {
                assert b < 10 via "a < b: a < c; c <= b"(c: this.mcu_num_blocks)
                csel = this.scan_comps_cselector[this.mcu_blocks_sselector[b]]
                h = this.components_h[csel] as base.u64
                v = this.components_v[csel] as base.u64
                stride = this.components_workbuf_widths[csel] as base.u64
                offset = this.components_workbuf_offsets[csel] + (8 *
                        ((((h * (mx as base.u64)) + (this.scan_comps_bx_offset[b] as base.u64))) +
                        ((((v * (my as base.u64)) + (this.scan_comps_by_offset[b] as base.u64))) * stride)))
                if offset <= args.workbuf.length() {
                    this.decode_idct!(
                            dst_buffer: args.workbuf[offset ..],
                            dst_stride: stride,
                            b: b,
                            q: this.components_tq[csel] as base.u32)

                }
                b += 1
            } endwhile

            // Clear this.mcu_blocks.
            b = 0
            while b < this.mcu_num_blocks,
                    inv my < 0x2000,
                    inv mx < 0x2000,
            {
                assert b < 10 via "a < b: a < c; c <= b"(c: this.mcu_num_blocks)
                i = 0
                while i < 64,
                        inv my < 0x2000,
                        inv mx < 0x2000,
                        inv b < 10,
                {
                    this.mcu_blocks[b][i] = 0
                    i += 1
                } endwhile
                b += 1
            } endwhile

            // Check for a restart.
            if this.restarts_remaining > 0 {
                this.restarts_remaining -= 1
                if this.restarts_remaining == 0 {
                    this.skip_past_the_next_restart_marker?(src: args.src)

                    // Reset.
                    this.restarts_remaining = this.restart_interval
                    this.mcu_previous_dc_values[0] = 0
                    this.mcu_previous_dc_values[1] = 0
                    this.mcu_previous_dc_values[2] = 0
                    this.mcu_previous_dc_values[3] = 0
                    this.bitstream_bits = 0
                    this.bitstream_n_bits = 0
                    this.bitstream_ri = 0
                    this.bitstream_wi = 0
                }
            }

            mx += 1
        } endwhile
        my += 1
    } endwhile

    if this.num_components == 1 {
        status = this.swizzle_gray!(dst: args.dst, workbuf: args.workbuf)
        return status
    }
    status = this.swizzle_colorful!(dst: args.dst, workbuf: args.workbuf)
    return status
}

pri func decoder.prepare_scan?(src: base.io_reader) {
    var c         : base.u8
    var i         : base.u32
    var j         : base.u32
    var h         : base.u32[..= 4]
    var v         : base.u32[..= 4]
    var hv        : base.u32[..= 16]
    var total_hv  : base.u32
    var b         : base.u32
    var bx_offset : base.u32
    var by_offset : base.u32

    if (this.payload_length < 6) or (this.payload_length > 12) {
        return "#bad SOS marker"
    }
    c = args.src.read_u8?()
    if (c < 1) or (c > 4) {
        return "#bad SOS marker"
    }
    this.scan_num_components = c as base.u32
    if (this.scan_num_components > this.num_components) or
            ((this.scan_num_components < this.num_components) and (this.sof_marker < 0xC2)) or
            (this.payload_length <> (4 + (2 * this.scan_num_components))) {
        return "#bad SOS marker"
    }
    this.payload_length = 0

    i = 0
    while i < this.scan_num_components {
        assert i < 4 via "a < b: a < c; c <= b"(c: this.scan_num_components)
        c = args.src.read_u8?()

        j = 0
        while true,
                inv i < 4,
        {
            if j >= this.num_components {
                return "#bad SOS marker"
            }
            assert j < 4 via "a < b: a < c; c <= b"(c: this.num_components)
            if c <> this.components_c[j] {
                j += 1
                continue
            }
            if i > 0 {
                if j <= (this.scan_comps_cselector[i - 1] as base.u32) {
                    return "#bad SOS marker"
                }
            }
            if not this.seen_dqt[this.components_tq[j]] {
                return "#missing Quantization table"
            }
            this.scan_comps_cselector[i] = j as base.u8
            break
        } endwhile

        c = args.src.read_u8?()
        if ((c >> 4) > 3) or ((c & 0x0F) > 3) {
            return "#bad SOS marker"
        }
        this.scan_comps_td[i] = c >> 4
        this.scan_comps_ta[i] = c & 0x0F
        // SOF0 (which is 0xC0) means a Baseline JPEG. The Baseline (tx <= 1)
        // restriction is specified in table B.5.
        if this.sof_marker == 0xC0 {
            if (this.scan_comps_td[i] > 1) or (this.scan_comps_ta[i] > 1) {
                return "#bad SOS marker"
            }
        }

        if (not this.seen_dht[0 | this.scan_comps_td[i]]) or
                (not this.seen_dht[4 | this.scan_comps_ta[i]]) {
            // TODO: while not required by the spec, we could fall back to
            // implicit tables? See section K.3 "Typical Huffman tables for
            // 8-bit precision luminance and chrominance" and
            // https://github.com/libjpeg-turbo/libjpeg-turbo/commit/a113506d175d03ae0e40965c3d3d21a5d561e119
            return "#missing Huffman table"
        }

        i += 1
    } endwhile

    c = args.src.read_u8?()
    if c > 63 {
        return "#bad SOS marker"
    }
    this.scan_ss = c

    c = args.src.read_u8?()
    if (c > 63) or (c < this.scan_ss) {
        return "#bad SOS marker"
    }
    this.scan_se = c

    c = args.src.read_u8?()
    if ((c >> 4) > 13) or ((c & 0x0F) > 13) {
        return "#bad SOS marker"
    }
    this.scan_ah = c >> 4
    this.scan_al = c & 0x0F

    // At this point, for sequential (not progressive) JPEGs, we should check
    // that (Ss, Se, Ah, Al) is (0, 63, 0, 0) but libjpeg treats otherwise as a
    // warning (JWRN_NOT_SEQUENTIAL), not an error.

    this.scan_width_in_mcus = this.width_in_mcus
    this.scan_height_in_mcus = this.height_in_mcus

    if this.scan_num_components == 1 {
        this.scan_comps_bx_offset[0] = 0
        this.scan_comps_by_offset[0] = 0
        this.mcu_num_blocks = 1
        this.mcu_blocks_sselector[0] = 0

    } else {
        total_hv = 0
        i = 0
        b = 0
        bx_offset = 0
        by_offset = 0
        while i < this.scan_num_components {
            assert i < 4 via "a < b: a < c; c <= b"(c: this.scan_num_components)
            h = this.components_h[this.scan_comps_cselector[i]] as base.u32
            v = this.components_v[this.scan_comps_cselector[i]] as base.u32
            hv = ((this.components_h[this.scan_comps_cselector[i]] as base.u32) *
                    (this.components_v[this.scan_comps_cselector[i]] as base.u32))
            total_hv ~mod+= hv

            while hv > 0,
                    inv i < 4,
            {
                this.scan_comps_bx_offset[b & 15] = (bx_offset & 3) as base.u8
                this.scan_comps_by_offset[b & 15] = (by_offset & 3) as base.u8
                this.mcu_blocks_sselector[b & 15] = i as base.u8
                b ~mod+= 1
                bx_offset ~mod+= 1
                if bx_offset == h {
                    bx_offset = 0
                    by_offset ~mod+= 1
                    if by_offset == v {
                        by_offset = 0
                    }
                }
                hv -= 1
            } endwhile

            i += 1
        } endwhile
        if total_hv > 10 {
            return "#bad SOS marker"
        }
        this.mcu_num_blocks = total_hv
    }
}

pri func decoder.fill_bitstream!(src: base.io_reader) {
    var i  : base.u32[..= 0x800]
    var wi : base.u32[..= 0x800]
    var c  : base.u8
    var n  : base.u32[..= 0x800]

    // Compact unread bytes to the start of the buffer.
    if this.bitstream_ri <= 0 {
        // No-op.
    } else if this.bitstream_ri == this.bitstream_wi {
        this.bitstream_ri = 0
        this.bitstream_wi = 0
    } else {
        // TODO: have a built-in (calling memmove) to compact a slice?
        i = 0
        assert i < this.bitstream_ri via "a < b: a == c; c < b"(c: 0)
        while this.bitstream_ri < this.bitstream_wi,
                pre i < this.bitstream_ri,
        {
            assert this.bitstream_ri < 0x800 via "a < b: a < c; c <= b"(c: this.bitstream_wi)
            assert i < 0x800 via "a < b: a < c; c < b"(c: this.bitstream_ri)
            this.bitstream_buffer[i] = this.bitstream_buffer[this.bitstream_ri]
            this.bitstream_ri += 1
            i += 1
            // TODO: this if statement should be unnecessary. Instead, we
            // should be able to "assert i < this.bitstream_ri".
            if i >= this.bitstream_ri {
                break  // Unreachable.
            }
        } endwhile
        this.bitstream_ri = 0
        this.bitstream_wi = i
    }

    wi = this.bitstream_wi
    while (wi < 0x800) and (args.src.length() > 0) {
        c = args.src.peek_u8()
        if c < 0xFF {
            this.bitstream_buffer[wi] = c
            wi += 1
            args.src.skip_u32_fast!(actual: 1, worst_case: 1)

        } else if args.src.length() <= 1 {
            break

        } else if (args.src.peek_u16le() >> 8) > 0 {
            break

        } else {
            this.bitstream_buffer[wi] = 0xFF
            wi += 1
            args.src.skip_u32_fast!(actual: 2, worst_case: 2)
        }
    } endwhile

    // If we hit a "\xFF\x??" marker then pad the bitstream. In this method
    // invocation, insert up to 32 bytes of implicit zeroes. Repeated calls to
    // this method can insert even more.
    if args.src.length() > 1 {
        if (args.src.peek_u8() >= 0xFF) and ((args.src.peek_u16le() >> 8) > 0) {
            n = wi.min(no_more_than: 0x800 - 32) + 32
            while wi < n {
                assert wi < 0x800 via "a < b: a < c; c <= b"(c: n)
                this.bitstream_buffer[wi] = 0x00
                wi += 1
            } endwhile
        }
    }

    this.bitstream_wi = wi
}

pri func decoder.skip_past_the_next_restart_marker?(src: base.io_reader) {
    var c : base.u8

    while true {
        if args.src.length() < 2 {
            yield? base."$short read"
            continue
        } else if args.src.peek_u8() < 0xFF {
            args.src.skip_u32_fast!(actual: 1, worst_case: 1)
            continue
        }

        c = (args.src.peek_u16le() >> 8) as base.u8
        if c < 0xC0 {
            // Either a reserved marker or "\xFF\x00" byte stuffing. Consume it
            // and continue.
            args.src.skip_u32_fast!(actual: 2, worst_case: 2)
            continue
        } else if (c < 0xD0) or (0xD7 < c) {
            // Not an RSTn (Restart) marker. Leave it and break.
            break
        }

        c &= 7
        if (this.next_restart_marker == ((c + 1) & 7)) or
                (this.next_restart_marker == ((c + 2) & 7)) {
            // A leading RSTn marker. Leave it and break.
            break
        } else if (this.next_restart_marker == ((c + 7) & 7)) or
                (this.next_restart_marker == ((c + 6) & 7)) {
            // A lagging RSTn marker. Consume it and continue.
            args.src.skip_u32_fast!(actual: 2, worst_case: 2)
            continue
        } else {
            // The RSTn marker is either the expected one or far away from the
            // expected one (neither leading or lagging). Consume it and break.
            args.src.skip_u32_fast!(actual: 2, worst_case: 2)
            break
        }
    } endwhile

    this.next_restart_marker = (this.next_restart_marker ~mod+ 1) & 7
}

pri func decoder.swizzle_gray!(dst: ptr base.pixel_buffer, workbuf: slice base.u8) base.status {
    var dst_pixfmt          : base.pixel_format
    var dst_bits_per_pixel  : base.u32[..= 256]
    var dst_bytes_per_pixel : base.u32[..= 32]
    var dst_length          : base.u64
    var tab                 : table base.u8
    var dst                 : slice base.u8
    var y                   : base.u32
    var stride              : base.u64[..= 0x4_0000]

    // TODO: the dst_pixfmt variable shouldn't be necessary. We should be able
    // to chain the two calls: "args.dst.pixel_format().bits_per_pixel()".
    dst_pixfmt = args.dst.pixel_format()
    dst_bits_per_pixel = dst_pixfmt.bits_per_pixel()
    if (dst_bits_per_pixel & 7) <> 0 {
        return base."#unsupported option"
    }
    dst_bytes_per_pixel = dst_bits_per_pixel / 8
    dst_length = (dst_bytes_per_pixel * this.width) as base.u64

    tab = args.dst.plane(p: 0)
    y = 0
    while y < this.height {
        assert y < 0xFFFF via "a < b: a < c; c <= b"(c: this.height)
        dst = tab.row_u32(y: y)
        if dst_length < dst.length() {
            dst = dst[.. dst_length]
        }
        this.swizzler.swizzle_interleaved_from_slice!(
                dst: dst,
                dst_palette: args.dst.palette_or_else(fallback: this.dst_palette[..]),
                src: args.workbuf)
        stride = this.components_workbuf_widths[0] as base.u64
        if stride <= args.workbuf.length() {
            args.workbuf = args.workbuf[stride ..]
        } else {
            args.workbuf = this.util.empty_slice_u8()
        }
        y += 1
    } endwhile
    return ok
}

pri func decoder.swizzle_colorful!(dst: ptr base.pixel_buffer, workbuf: slice base.u8) base.status {
    var src0   : slice base.u8
    var src1   : slice base.u8
    var src2   : slice base.u8
    var src3   : slice base.u8
    var status : base.status

    if (this.components_workbuf_offsets[0] <= this.components_workbuf_offsets[1]) and
            (this.components_workbuf_offsets[1] <= args.workbuf.length()) {
        src0 = args.workbuf[this.components_workbuf_offsets[0] .. this.components_workbuf_offsets[1]]
    }

    if (this.components_workbuf_offsets[1] <= this.components_workbuf_offsets[2]) and
            (this.components_workbuf_offsets[2] <= args.workbuf.length()) {
        src1 = args.workbuf[this.components_workbuf_offsets[1] .. this.components_workbuf_offsets[2]]
    }

    if (this.components_workbuf_offsets[2] <= this.components_workbuf_offsets[3]) and
            (this.components_workbuf_offsets[3] <= args.workbuf.length()) {
        src2 = args.workbuf[this.components_workbuf_offsets[2] .. this.components_workbuf_offsets[3]]
    }

    if (this.components_workbuf_offsets[3] <= this.components_workbuf_offsets[4]) and
            (this.components_workbuf_offsets[4] <= args.workbuf.length()) {
        src3 = args.workbuf[this.components_workbuf_offsets[3] .. this.components_workbuf_offsets[4]]
    }

    status = this.swizzler.swizzle_ycck!(
            dst: args.dst,
            dst_palette: args.dst.palette_or_else(fallback: this.dst_palette[..]),
            width: this.width,
            height: this.height,
            src0: src0,
            src1: src1,
            src2: src2,
            src3: src3,
            width0: this.components_workbuf_widths[0],
            width1: this.components_workbuf_widths[1],
            width2: this.components_workbuf_widths[2],
            width3: this.components_workbuf_widths[3],
            height0: this.components_workbuf_heights[0],
            height1: this.components_workbuf_heights[1],
            height2: this.components_workbuf_heights[2],
            height3: this.components_workbuf_heights[3],
            stride0: this.components_workbuf_widths[0],
            stride1: this.components_workbuf_widths[1],
            stride2: this.components_workbuf_widths[2],
            stride3: this.components_workbuf_widths[3],
            h0: this.components_h[0],
            h1: this.components_h[1],
            h2: this.components_h[2],
            h3: this.components_h[3],
            v0: this.components_v[0],
            v1: this.components_v[1],
            v2: this.components_v[2],
            v3: this.components_v[3],
            triangle_filter_for_2to1: true,
            scratch_buffer_2k: this.bitstream_buffer[..])
    return status
}

pub func decoder.frame_dirty_rect() base.rect_ie_u32 {
    return this.util.make_rect_ie_u32(
            min_incl_x: 0,
            min_incl_y: 0,
            max_excl_x: this.width,
            max_excl_y: this.height)
}

pub func decoder.num_animation_loops() base.u32 {
    return 0
}

pub func decoder.num_decoded_frame_configs() base.u64 {
    if this.call_sequence > 0x20 {
        return 1
    }
    return 0
}

pub func decoder.num_decoded_frames() base.u64 {
    if this.call_sequence > 0x40 {
        return 1
    }
    return 0
}

pub func decoder.restart_frame!(index: base.u64, io_position: base.u64) base.status {
    var i : base.u32

    if this.call_sequence < 0x20 {
        return base."#bad call sequence"
    }
    if args.index <> 0 {
        return base."#bad argument"
    }
    this.call_sequence = 0x28
    this.frame_config_io_position = args.io_position
    this.restart_interval = this.saved_restart_interval
    while i < 8 {
        this.seen_dht[i] = false
        i += 1
    } endwhile
    return ok
}

pub func decoder.set_report_metadata!(fourcc: base.u32, report: base.bool) {
    // TODO: implement.
}

pub func decoder.tell_me_more?(dst: base.io_writer, minfo: nptr base.more_information, src: base.io_reader) {
    return base."#no more information"
}

pub func decoder.workbuf_len() base.range_ii_u64 {
    return this.util.make_range_ii_u64(
            min_incl: this.components_workbuf_offsets[4],
            max_incl: this.components_workbuf_offsets[4])
}