std/jpeg/decode_mcu_progressive_ac_high_bits.wuffs

// Copyright 2023 The Wuffs Authors.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//
// SPDX-License-Identifier: Apache-2.0 OR MIT

pri func decoder.decode_mcu_progressive_ac_high_bits!(workbuf: slice base.u8, mx: base.u32[..= 0x1FFF], my: base.u32[..= 0x1FFF]) base.u32 {
    var ret : base.u32

    var bits   : base.u64
    var n_bits : base.u32

    var r   : base.io_reader
    var pos : base.u32

    var ac_huff_table_fast : nptr roarray[256] base.u16

    var ac_h       : base.u8[..= 7]
    var ac_symbol  : base.u32[..= 0xFF]
    var ac_ht_fast : base.u32
    var ac_bl      : base.u32
    var ac_code    : base.u32
    var ac_blm1    : base.u32[..= 15]
    var ac_ht_slow : base.u32
    var ac_value   : base.u16
    var ac_extend  : base.u16

    var ac_rrrr : base.u32[..= 15]
    var ac_ssss : base.u32[..= 15]

    var z : base.u32[..= 79]  // 79 = 63 + 15 + 1.

    if this.eob_run > 0 {
        this.eob_run -= 1
        return 0
    }

    bits = this.bitstream_bits
    n_bits = this.bitstream_n_bits

    if this.bitstream_ri > this.bitstream_wi {
        return 2  // Internal error.
    }
    io_bind (io: r, data: this.bitstream_buffer[this.bitstream_ri .. this.bitstream_wi], history_position: this.bitstream_ri as base.u64) {
        while.goto_done true {{

        while.block true {
            // Ensure that we have enough bits for this iteration of the
            // while.block loop body. Worst case, there are 64 components and
            // each one needs (16 + 15) bits (round that up to 4 bytes), so we
            // need (64 * 4) = 256 bytes available. 8 more bytes of slack means
            // that we can always call peek_u64be.
            if r.length() < 264 {
                ret = 1  // Request another fill_bitstream call.
                break.goto_done
            }

            // Ensure the informal "the lower bound on the number of loaded
            // bits is at least 16" loop pre-condition below.
            if n_bits < 16 {
                bits |= r.peek_u64be() >> (n_bits & 63)
                // Skip these lines that are normally part of "load more bits".
                // It's unnecessary, just for the informal (NB > 16) below, and
                // skipping it avoids a small but negative performance impact.
                //
                // r.skip_u32_fast!( etc )
                // n_bits |= 56
            }

            z = this.mcu_zig_index
            this.mcu_zig_index = 0
            ac_h = this.mcu_blocks_ac_hselector[0]
            ac_huff_table_fast = this.huff_tables_fast[ac_h][..] as ptr array[256] base.u16
            while.ac_components z <= (this.scan_se as base.u32),
                    inv ac_huff_table_fast <> nullptr,
            {
                assert z < 64 via "a < b: a <= c; c < b"(c: (this.scan_se as base.u32))

                // Use the high bits of the bits variable to look up the
                // Huffman table. Conceptually, we should do this *after*
                // "Load at least 56 bits" below (at "LOOK-UP" further below),
                // but it is noticably faster to do it beforehand:
                // https://github.com/google/wuffs/commit/d9c7740ea6c4a450401c393787838436895b87bb
                //
                // Earlier look-up is OK because there is a informal loop
                // pre-condition (informal means not enforced by the Wuffs
                // compiler) that (NB >= 16) here, where NB is "the number of
                // loaded bits in the bits variable". It is literally n_bits
                // for almost all loop iterations, but it can be higher on the
                // first iteration, set up by "Ensure the informal" above.
                //
                // For subsequent iterations, the "Load at least 56 bits" below
                // does what it says. The Huffman code consumes up to 16 bits
                // and processing the Huffman symbol's ssss nibble consumes up
                // to 15 bits. (56 - 16 - 15) = 25 >= 16 so at the end of the
                // loop body, (n_bits >= 16).
                //
                // This 16 bound has some slack, as (NB >= 8) is all we need
                // when ac_huff_table_fast has (1 << 8) entries, but a bound of
                // 16 (the longest JPEG Huffman code bit length) won't need to
                // change even if, in the future, the look-up table size grows.
                ac_ht_fast = ac_huff_table_fast[bits >> 56] as base.u32

                // Load at least 56 bits.
                //
                // TODO: eliminate this bounds check entirely.
                if r.length() < 8 {
                    ret = 2  // Internal error.
                    break.goto_done
                }
                bits |= r.peek_u64be() >> (n_bits & 63)
                r.skip_u32_fast!(actual: (63 - (n_bits & 63)) >> 3, worst_case: 8)
                n_bits |= 56

                // Read the Huffman-encoded ac_symbol, up to 16 bits long.
                //
                // LOOK-UP: "ac_ht_fast = etc" conceptually happens here, just
                // before deriving ac_bl from ac_ht_fast.
                ac_bl = ac_ht_fast >> 8
                if n_bits >= ac_bl {
                    ac_symbol = 0xFF & ac_ht_fast
                    bits ~mod<<= (ac_bl & 63)
                    n_bits -= ac_bl
                } else {
                    ac_code = (bits >> 55) as base.u32
                    ac_blm1 = 8
                    bits ~mod<<= 9
                    n_bits ~mod-= 9
                    while true,
                            inv ac_huff_table_fast <> nullptr,
                            inv z < 64,
                    {
                        ac_ht_slow = this.huff_tables_slow[ac_h][ac_blm1]
                        if ac_code < (ac_ht_slow >> 8) {
                            ac_symbol = this.huff_tables_symbols[ac_h][0xFF & (ac_code ~mod+ ac_ht_slow)] as base.u32
                            break
                        }
                        ac_code = (ac_code ~mod<< 1) | ((bits >> 63) as base.u32)
                        bits ~mod<<= 1
                        n_bits ~mod-= 1
                        ac_blm1 = (ac_blm1 + 1) & 15
                        if ac_blm1 == 0 {
                            ac_symbol = 0
                            break
                        }
                    } endwhile
                }

                // Split the 8-bit ac_symbol into two 4-bit halves, per section
                // F.2.2.2 "Decoding procedure for AC coefficients".
                ac_rrrr = ac_symbol >> 4
                z += ac_rrrr + 1
                ac_ssss = ac_symbol & 15
                ac_extend = EXTEND[ac_ssss]

                // Process the ac_value in the next ac_ssss (up to 15) bits or
                // the eob_run in the next ac_rrrr (up to 15) bits.
                if ac_ssss > 0 {
                    ac_value = ((bits >> (64 - ac_ssss)) & 0xFFFF) as base.u16
                    ac_value ~mod+= ac_extend &
                            (((this.util.sign_extend_rshift_u64(a: bits, n: 63) & 0xFFFF) as base.u16) ^ 0xFFFF)
                    bits ~mod<<= ac_ssss
                    n_bits ~mod-= ac_ssss
                    this.mcu_blocks[0][UNZIG[z]] =
                            ((ac_value ~mod<< this.scan_al) & 0xFFFF) as base.u16
                } else if ac_rrrr < 15 {
                    this.eob_run = (((1 as base.u16) << ac_rrrr) - 1) as base.u16
                    if ac_rrrr > 0 {
                        this.eob_run ~mod+= ((bits >> (64 - ac_rrrr)) & 0xFFFF) as base.u16
                        bits ~mod<<= ac_rrrr
                        n_bits ~mod-= ac_rrrr
                    }
                    break.ac_components
                }
            } endwhile.ac_components

            break.block
        } endwhile.block

        break.goto_done
        }} endwhile.goto_done

        pos = (r.position() & 0xFFFF_FFFF) as base.u32
        if pos > this.bitstream_wi {
            ret = 2  // Internal error.
        } else {
            assert pos <= 0x800 via "a <= b: a <= c; c <= b"(c: this.bitstream_wi)
            this.bitstream_ri = pos
        }
    }

    this.bitstream_bits = bits
    this.bitstream_n_bits = n_bits
    return ret
}