// Copyright 2022 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.

pri func decoder.decode_huffman_fast!(src: base.io_reader) base.status {
    var bits       : base.u32
    var n_bits     : base.u32[..= 31]
    var block_size : base.u32[..= 900000]
    var which      : base.u8[..= 5]
    var ticks      : base.u32[..= 50]
    var section    : base.u32
    var run_shift  : base.u32[..= 23]

    var table_entry : base.u16
    var child       : base.u16
    var child_ff    : base.u32[..= 255]
    var i           : base.u32
    var j           : base.u32
    var output      : base.u32[..= 255]
    var run         : base.u32[..= 12_582912]  // 12_582912 = (3 << 22)
    var mtft0       : base.u32[..= 255]

    bits = this.bits
    n_bits = this.n_bits
    block_size = this.block_size
    which = this.decode_huffman_which
    ticks = this.decode_huffman_ticks
    section = this.decode_huffman_section
    run_shift = this.decode_huffman_run_shift

    // Apply the Huffman trees.
    while.outer args.src.length() >= 4 {
        if ticks > 0 {
            ticks -= 1
        } else {
            ticks = 49
            section ~mod+= 1
            if section >= this.num_sections {
                return "#bad number of sections"
            }
            which = CLAMP_TO_5[this.huffman_selectors[section & 32767] & 7]
        }

        bits |= args.src.peek_u32be() >> n_bits
        args.src.skip_u32_fast!(actual: (31 - n_bits) >> 3, worst_case: 4)
        n_bits |= 24

        table_entry = this.huffman_tables[which][bits >> 24]
        bits ~mod<<= table_entry >> 12
        n_bits -= (table_entry >> 12) as base.u32
        child = table_entry & 0x3FF
        while child < 257 {
            child = this.huffman_trees[which][child][bits >> 31]
            bits ~mod<<= 1
            if n_bits <= 0 {
                return "#internal error: inconsistent Huffman decoder state"
            }
            n_bits -= 1
        } endwhile

        if child < 0x300 {  // mNN symbol.
            // Move to front.
            child_ff = (child & 0xFF) as base.u32
            output = this.mtft[child_ff] as base.u32
            this.mtft[1 .. 1 + child_ff].copy_from_slice!(
                    s: this.mtft[.. child_ff])
            this.mtft[0] = output as base.u8

            this.letter_counts[output] ~mod+= 1
            this.bwt[block_size] = output
            if block_size >= this.max_incl_block_size {
                return "#bad block length"
            }
            assert block_size < 900000 via "a < b: a < c; c <= b"(c: this.max_incl_block_size)
            block_size += 1
            run_shift = 0
            continue.outer
        } else if child == 0x300 {  // EOB symbol.
            this.decode_huffman_finished = true
            break.outer
        }

        // RUNA or RUNB symbol.
        if run_shift >= 23 {
            return "#bad block length"
        }
        run = ((child as base.u32) & 3) << run_shift
        run_shift += 1
        i = block_size
        j = run + block_size
        if j > this.max_incl_block_size {
            return "#bad block length"
        }
        assert j <= 900000 via "a <= b: a <= c; c <= b"(c: this.max_incl_block_size)
        block_size = j

        mtft0 = this.mtft[0] as base.u32
        this.letter_counts[mtft0] ~mod+= run
        while i < j,
                pre j <= 900000,
        {
            assert i < 900000 via "a < b: a < c; c <= b"(c: j)
            this.bwt[i] = mtft0
            i += 1
        } endwhile
    } endwhile.outer

    this.bits = bits
    this.n_bits = n_bits
    this.block_size = block_size
    this.decode_huffman_which = which
    this.decode_huffman_ticks = ticks
    this.decode_huffman_section = section
    this.decode_huffman_run_shift = run_shift

    return ok
}