hpvcd 0.3.2

/*
 * // Copyright (c) Radzivon Bartoshyk 6/2026. All rights reserved.
 * //
 * // Redistribution and use in source and binary forms, with or without modification,
 * // are permitted provided that the following conditions are met:
 * //
 * // 1.  Redistributions of source code must retain the above copyright notice, this
 * // list of conditions and the following disclaimer.
 * //
 * // 2.  Redistributions in binary form must reproduce the above copyright notice,
 * // this list of conditions and the following disclaimer in the documentation
 * // and/or other materials provided with the distribution.
 * //
 * // 3.  Neither the name of the copyright holder nor the names of its
 * // contributors may be used to endorse or promote products derived from
 * // this software without specific prior written permission.
 * //
 * // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
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 * // FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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 * // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

pub(crate) use crate::cabac::contexts::CtxModel;

use std::borrow::Cow;

#[rustfmt::skip]
static RANGE_TAB_LPS: [[u8; 4]; 64] = [
    [128,176,208,240],[128,167,197,227],[128,158,187,216],[123,150,178,205],
    [116,142,169,195],[111,135,160,185],[105,128,152,175],[100,122,144,166],
    [95,116,137,158],[90,110,130,150],[85,104,123,142],[81,99,117,135],
    [77,94,111,128],[73,89,105,122],[69,85,100,116],[66,80,95,110],
    [62,76,90,104],[59,72,86,99],[56,69,81,94],[53,65,77,89],
    [51,62,73,85],[48,59,69,80],[46,56,66,76],[43,53,63,72],
    [41,50,59,69],[39,48,56,65],[37,45,54,62],[35,43,51,59],
    [33,41,48,56],[32,39,46,53],[30,37,43,50],[29,35,41,48],
    [27,33,39,45],[26,31,37,43],[24,30,35,41],[23,28,33,39],
    [22,27,32,37],[21,26,30,35],[20,24,29,33],[19,23,27,31],
    [18,22,26,30],[17,21,25,28],[16,20,23,27],[15,19,22,25],
    [14,18,21,24],[14,17,20,23],[13,16,19,22],[12,15,18,21],
    [12,14,17,20],[11,14,16,19],[11,13,15,18],[10,12,15,17],
    [10,12,14,16],[9,11,13,15],[9,11,12,14],[8,10,12,14],
    [8,9,11,13],[7,9,11,12],[7,9,10,12],[7,8,10,11],
    [6,8,9,11],[6,7,9,10],[6,7,8,9],[2,2,2,2],
];

#[rustfmt::skip]
static TRANS_IDX_MPS: [u8; 64] = [
     1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14,15,16,
    17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,
    33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,
    49,50,51,52,53,54,55,56,57,58,59,60,61,62,62,63,
];

#[rustfmt::skip]
static TRANS_IDX_LPS: [u8; 64] = [
     0, 0, 1, 2, 2, 4, 4, 5, 6, 7, 8, 9, 9,11,11,12,
    13,13,15,15,16,16,18,18,19,19,21,21,22,22,23,24,
    24,25,26,26,27,27,28,29,29,30,30,30,31,32,32,33,
    33,33,34,34,35,35,35,36,36,36,37,37,37,38,38,63,
];

// One load supplies rangeLPS and both next packed context states.  The table is
// indexed by the complete packed context byte (pStateIdx + valMPS) and the
// current range class.  Keeping this generated from the normative tables makes
// the hot decision path smaller without duplicating hand-maintained constants.
const fn build_decision_table() -> [[u32; 4]; 128] {
    let mut table = [[0u32; 4]; 128];
    let mut packed_state = 0usize;
    while packed_state < table.len() {
        let state = packed_state & 63;
        let mps = ((packed_state >> 6) & 1) as u8;
        let next_mps = TRANS_IDX_MPS[state] | (mps << 6);
        let lps_mps = if state == 0 { mps ^ 1 } else { mps };
        let next_lps = TRANS_IDX_LPS[state] | (lps_mps << 6);

        let mut range_class = 0usize;
        while range_class < 4 {
            table[packed_state][range_class] = RANGE_TAB_LPS[state][range_class] as u32
                | ((next_mps as u32) << 8)
                | ((next_lps as u32) << 16);
            range_class += 1;
        }
        packed_state += 1;
    }
    table
}

static DECISION_TABLE: [[u32; 4]; 128] = build_decision_table();

/// CABAC decoder. Feed it EBSP-unescaped slice payload bytes.
pub(crate) struct CabacDecoder<'a> {
    pub(crate) range: u32,
    pub(crate) offset: u32,
    data: Cow<'a, [u8]>,
    /// Index of the next byte in `data` not yet loaded into `bitbuf`.
    pub(crate) byte_pos: usize,
    /// Bit reservoir, left-aligned: the next bit to consume is bit 63.
    bitbuf: u64,
    /// Number of valid (unconsumed) bits currently in `bitbuf`.
    bitcnt: u32,
}

impl CabacDecoder<'static> {
    pub(crate) fn new(data: &[u8]) -> Result<Self, crate::error::DecodeError> {
        Self::from_cow(Cow::Owned(data.to_vec()))
    }
}

impl<'a> CabacDecoder<'a> {
    pub(crate) fn new_borrowed(data: &'a [u8]) -> Result<Self, crate::error::DecodeError> {
        CabacDecoder::from_cow(Cow::Borrowed(data))
    }

    fn from_cow(data: Cow<'a, [u8]>) -> Result<Self, crate::error::DecodeError> {
        if data.len() < 2 {
            return Err(crate::error::DecodeError::Bitstream(
                "CABAC payload too short to initialise".into(),
            ));
        }
        let mut dec = CabacDecoder {
            range: 510,
            offset: 0,
            data,
            byte_pos: 0,
            bitbuf: 0,
            bitcnt: 0,
        };
        dec.offset = dec.read_bits(9);
        Ok(dec)
    }

    /// Reset the engine onto a new owned byte buffer (e.g. the next slice
    /// segment's CABAC data) and prime it, reusing this decoder instance.
    pub(crate) fn reset_with(&mut self, data: &[u8]) -> Result<(), crate::error::DecodeError> {
        if data.len() < 2 {
            return Err(crate::error::DecodeError::Bitstream(
                "CABAC payload too short to initialise".into(),
            ));
        }
        self.data = Cow::Owned(data.to_vec());
        self.range = 510;
        self.offset = 0;
        self.byte_pos = 0;
        self.bitbuf = 0;
        self.bitcnt = 0;
        self.offset = self.read_bits(9);
        Ok(())
    }

    /// Refill the reservoir with whole bytes until at least 56 bits are buffered
    /// (or the input is exhausted).
    #[inline(always)]
    fn refill(&mut self) {
        while self.bitcnt <= 56 && self.byte_pos < self.data.len() {
            self.bitbuf |= (self.data[self.byte_pos] as u64) << (56 - self.bitcnt);
            self.bitcnt += 8;
            self.byte_pos += 1;
        }
    }

    /// Read the next input bit (MSB-first). Returns stuffed 1s past end-of-input.
    #[inline(always)]
    fn next_bit(&mut self) -> u32 {
        if self.bitcnt == 0 {
            self.refill();
            if self.bitcnt == 0 {
                return 1; // past end of input: bitstream stuffing
            }
        }
        let bit = (self.bitbuf >> 63) as u32;
        self.bitbuf <<= 1;
        self.bitcnt -= 1;
        bit
    }

    /// Read `n` (≤ 32) bits MSB-first, padding past end-of-input with 1s.
    #[inline]
    fn read_bits(&mut self, n: u32) -> u32 {
        if self.bitcnt < n {
            self.refill();
        }
        if self.bitcnt >= n {
            // Fast path: all n bits are in the reservoir.
            let v = (self.bitbuf >> (64 - n)) as u32;
            self.bitbuf <<= n;
            self.bitcnt -= n;
            v
        } else {
            // Slow path near end of input: take what we have, stuff the rest.
            let mut v = 0u32;
            for _ in 0..n {
                v = (v << 1) | self.next_bit();
            }
            v
        }
    }

    #[inline]
    fn renorm(&mut self) {
        // range ∈ [1, 510]; shift left until bit 8 (value 256) is set, then pull
        // the same number of fresh bits into offset in one batched read.
        if self.range < 256 {
            let shift = self.range.leading_zeros() - 23; // = 8 - floor(log2(range))
            self.range <<= shift;
            self.offset = (self.offset << shift) | self.read_bits(shift);
        }
    }

    /// Decode one context-coded bin (HEVC Table 9-15 DecodeDecision).
    #[inline(always)]
    pub(crate) fn decode_bin(&mut self, ctx: &mut CtxModel) -> u8 {
        let packed_state = ctx.state;
        let mps = packed_state >> 6;
        let decision = DECISION_TABLE[packed_state as usize][(self.range >> 6) as usize & 3];
        let lps = decision & 0xff;
        let next_mps = (decision >> 8) as u8;
        let next_lps = (decision >> 16) as u8;

        self.range -= lps;
        if self.offset >= self.range {
            self.offset -= self.range;
            self.range = lps;
            ctx.state = next_lps;
            self.renorm();
            mps ^ 1
        } else {
            ctx.state = next_mps;
            self.renorm();
            mps
        }
    }

    /// Align the arithmetic engine before coefficient bypass syntax when
    /// `cabac_bypass_alignment_enabled_flag` and `escapeDataPresent` are set.
    /// HEVC §9.3.4.3.6 changes only the current range; the offset and input
    /// position remain untouched.
    #[inline(always)]
    pub(crate) fn align_bypass(&mut self) {
        self.range = 256;
    }

    /// Decode one bypass bin (HEVC §9.3.4.2 DecodeBypass).
    #[inline(always)]
    pub(crate) fn decode_bypass(&mut self) -> u8 {
        self.offset = (self.offset << 1) | self.next_bit();
        if self.offset >= self.range {
            self.offset -= self.range;
            1
        } else {
            0
        }
    }

    /// Decode `count` consecutive bypass bins and return them MSB-first.
    ///
    /// The arithmetic dependency remains serial, but the raw input bits are
    /// fetched in one reservoir operation and `offset` is kept local throughout
    /// the run.  This is particularly useful for coefficient signs and fixed
    /// Rice/last-position suffixes.
    #[inline(always)]
    pub(crate) fn decode_bypass_bits(&mut self, count: u32) -> u32 {
        debug_assert!(count <= 32);
        if count == 0 {
            return 0;
        }

        let raw = self.read_bits(count);
        let range = self.range;
        let mut offset = self.offset;
        let mut bins = 0u32;
        for shift in (0..count).rev() {
            offset = (offset << 1) | ((raw >> shift) & 1);
            let bin = (offset >= range) as u32;
            offset -= range * bin;
            bins = (bins << 1) | bin;
        }
        self.offset = offset;
        bins
    }

    /// Decode terminate bin (end_of_slice / end_of_sub_stream).
    #[inline]
    pub(crate) fn decode_terminate(&mut self) -> u8 {
        self.range -= 2;
        if self.offset >= self.range {
            1
        } else {
            self.renorm();
            0
        }
    }
}

impl<'a> CabacDecoder<'a> {
    /// Byte-align: discard buffered bits so the next read starts on a byte
    /// boundary (WPP). The raw bit-reader position (bits consumed from `data`)
    /// is `byte_pos*8 - bitcnt`; rounding that up to the next byte boundary
    /// gives `byte_pos - bitcnt/8` regardless of whether we are mid-byte.
    pub(crate) fn byte_align(&mut self) {
        self.byte_pos -= (self.bitcnt / 8) as usize;
        self.bitbuf = 0;
        self.bitcnt = 0;
    }

    /// Re-prime the engine from the current byte position (WPP row start).
    pub(crate) fn reinit_engine(&mut self) {
        self.range = 510;
        self.offset = 0;
        self.bitbuf = 0;
        self.bitcnt = 0;
        self.offset = self.read_bits(9);
    }

    /// Re-prime CABAC after byte-aligned PCM samples. Fixed-length reads may
    /// have prefetched whole bytes, which must be returned before the reservoir
    /// is cleared by arithmetic-engine initialization.
    pub(crate) fn reinit_after_pcm(&mut self) {
        self.byte_align();
        self.reinit_engine();
    }

    /// Read `n` (≤ 32) raw bits from the stream, MSB-first. Used for PCM sample
    /// data (§7.3.8.5 / §9.3.1), which is coded as fixed-length uncompressed bits
    /// between an engine byte-alignment and a subsequent re-initialization.
    pub(crate) fn read_pcm_bits(&mut self, n: u32) -> u32 {
        if n == 0 {
            return 0;
        }
        self.read_bits(n)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn pcm_reinit_preserves_prefetched_cabac_bytes() {
        let data = [0x80, 0x00, 0x12, 0x34, 0xab, 0xcd, 0xef, 0x42];
        let mut dec = CabacDecoder::new_borrowed(&data).unwrap();

        // A terminated CABAC segment aligns to byte 2; consume two bytes of
        // raw PCM while refill deliberately prefetches beyond them.
        dec.byte_align();
        assert_eq!(dec.read_pcm_bits(16), 0x1234);
        dec.reinit_after_pcm();

        // Reinitialization must begin at byte 4, not at the end of the refill.
        assert_eq!(
            dec.offset,
            (u32::from(data[4]) << 1) | u32::from(data[5] >> 7)
        );
    }

    #[test]
    fn packed_decision_table_matches_normative_tables() {
        for packed_state in 0u8..128 {
            let state = usize::from(packed_state & 63);
            let mps = packed_state >> 6;
            let expected_mps = TRANS_IDX_MPS[state] | (mps << 6);
            let lps_mps = if state == 0 { mps ^ 1 } else { mps };
            let expected_lps = TRANS_IDX_LPS[state] | (lps_mps << 6);

            for range_class in 0..4 {
                let decision = DECISION_TABLE[usize::from(packed_state)][range_class];
                assert_eq!(
                    decision & 0xff,
                    u32::from(RANGE_TAB_LPS[state][range_class])
                );
                assert_eq!((decision >> 8) as u8, expected_mps);
                assert_eq!((decision >> 16) as u8, expected_lps);
            }
        }
    }

    #[test]
    fn batched_bypass_matches_single_bin_decoding() {
        let data = [
            0x55, 0xaa, 0x13, 0x7c, 0xe1, 0x09, 0xb6, 0x42, 0xfd, 0x18, 0x83, 0x6d, 0x24, 0xc7,
            0x5a, 0x91,
        ];

        for warmup in 0..16 {
            for count in 0..=32 {
                let mut scalar = CabacDecoder::new_borrowed(&data).unwrap();
                let mut batched = CabacDecoder::new_borrowed(&data).unwrap();

                for _ in 0..warmup {
                    assert_eq!(scalar.decode_bypass(), batched.decode_bypass());
                }

                let mut expected = 0u32;
                for _ in 0..count {
                    expected = (expected << 1) | u32::from(scalar.decode_bypass());
                }
                assert_eq!(batched.decode_bypass_bits(count), expected);
                assert_eq!(batched.range, scalar.range);
                assert_eq!(batched.offset, scalar.offset);
                assert_eq!(
                    batched.byte_pos * 8 - batched.bitcnt as usize,
                    scalar.byte_pos * 8 - scalar.bitcnt as usize
                );

                // Batched reading may prefetch a different number of whole
                // bytes, but subsequent arithmetic bins and byte alignment
                // must remain identical.
                for _ in 0..64 {
                    assert_eq!(batched.decode_bypass(), scalar.decode_bypass());
                }
                assert_eq!(batched.offset, scalar.offset);
                batched.byte_align();
                scalar.byte_align();
                assert_eq!(batched.byte_pos, scalar.byte_pos);
            }
        }
    }

    #[test]
    fn batched_bypass_matches_after_context_bins() {
        let data = [
            0x8d, 0x27, 0xf4, 0x19, 0xa6, 0x53, 0x0b, 0xdc, 0x71, 0x3e, 0x95, 0x42, 0xe8, 0x16,
            0xbf, 0x64, 0x29, 0xd3, 0x7a, 0x05, 0xc1, 0x9e, 0x38, 0x57,
        ];

        for decision_count in 0..32 {
            for bypass_count in 0..=32 {
                let mut scalar = CabacDecoder::new_borrowed(&data).unwrap();
                let mut batched = CabacDecoder::new_borrowed(&data).unwrap();
                let mut scalar_ctx = CtxModel::new(17, 0);
                let mut batched_ctx = scalar_ctx;

                for i in 0..decision_count {
                    assert_eq!(
                        scalar.decode_bin(&mut scalar_ctx),
                        batched.decode_bin(&mut batched_ctx)
                    );
                    // Exercise the exact residual pattern: context bins with
                    // occasional single bypass bins before a batched sign/Rice
                    // suffix. This catches offset-only drift that a range-only
                    // lockstep trace cannot see.
                    if i % 3 == 2 {
                        assert_eq!(scalar.decode_bypass(), batched.decode_bypass());
                    }
                }

                let mut expected = 0u32;
                for _ in 0..bypass_count {
                    expected = (expected << 1) | u32::from(scalar.decode_bypass());
                }
                assert_eq!(batched.decode_bypass_bits(bypass_count), expected);
                assert_eq!(batched.range, scalar.range);
                assert_eq!(batched.offset, scalar.offset);
                assert_eq!(batched_ctx.state, scalar_ctx.state);
                assert_eq!(
                    batched.byte_pos * 8 - batched.bitcnt as usize,
                    scalar.byte_pos * 8 - scalar.bitcnt as usize
                );
            }
        }
    }

    #[test]
    fn bypass_alignment_changes_only_range() {
        let data = [0x96, 0x35, 0xca, 0x71, 0x0f, 0xe4, 0x58, 0xa2];
        let mut cab = CabacDecoder::new_borrowed(&data).unwrap();
        let mut ctx = CtxModel::new(23, 1);

        // Put the engine into a non-trivial state first; alignment must not
        // consume input, renormalize, or alter the arithmetic offset.
        for _ in 0..5 {
            let _ = cab.decode_bin(&mut ctx);
        }
        let offset = cab.offset;
        let byte_pos = cab.byte_pos;
        let bitbuf = cab.bitbuf;
        let bitcnt = cab.bitcnt;

        cab.range = 319;
        cab.align_bypass();

        assert_eq!(cab.range, 256);
        assert_eq!(cab.offset, offset);
        assert_eq!(cab.byte_pos, byte_pos);
        assert_eq!(cab.bitbuf, bitbuf);
        assert_eq!(cab.bitcnt, bitcnt);
    }
}