zlib-rs 0.3.0

#![forbid(unsafe_code)]

use crate::{Code, ENOUGH_DISTS, ENOUGH_LENS};

pub(crate) enum CodeType {
    Codes,
    Lens,
    Dists,
}

const MAX_BITS: usize = 15;

fn min_max<const N: usize>(count: [u16; N]) -> (usize, usize) {
    let mut max = MAX_BITS;
    while max >= 1 {
        if count[max] != 0 {
            break;
        }
        max -= 1;
    }

    let mut min = 1;
    while min < max {
        if count[min] != 0 {
            break;
        }
        min += 1;
    }

    (min, max)
}

/// Length codes 257..285 base
const LBASE: [u16; 31] = [
    3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 15, 17, 19, 23, 27, 31, 35, 43, 51, 59, 67, 83, 99, 115, 131,
    163, 195, 227, 258, 0, 0,
];
/// Length codes 257..285 extra
const LEXT: [u16; 31] = [
    16, 16, 16, 16, 16, 16, 16, 16, 17, 17, 17, 17, 18, 18, 18, 18, 19, 19, 19, 19, 20, 20, 20, 20,
    21, 21, 21, 21, 16, 77, 202,
];
/// Distance codes 0..29 base
const DBASE: [u16; 32] = [
    1, 2, 3, 4, 5, 7, 9, 13, 17, 25, 33, 49, 65, 97, 129, 193, 257, 385, 513, 769, 1025, 1537,
    2049, 3073, 4097, 6145, 8193, 12289, 16385, 24577, 0, 0,
];
/// Distance codes 0..29 extra
const DEXT: [u16; 32] = [
    16, 16, 16, 16, 17, 17, 18, 18, 19, 19, 20, 20, 21, 21, 22, 22, 23, 23, 24, 24, 25, 25, 26, 26,
    27, 27, 28, 28, 29, 29, 64, 64,
];

#[repr(i32)]
#[derive(Debug, PartialEq, Eq)]
pub(crate) enum InflateTable {
    EnoughIsNotEnough = 1,
    Success(usize) = 0,
    InvalidCode = -1,
}

pub(crate) fn inflate_table(
    codetype: CodeType,
    lens: &[u16],
    codes: usize,
    table: &mut [Code],
    bits: usize,
    work: &mut [u16],
) -> InflateTable {
    // number of codes of each length
    let mut count = [0u16; MAX_BITS + 1];

    for len in lens[0..codes].iter().copied() {
        count[len as usize] += 1;
    }

    let mut root = bits;

    let (min, max) = min_max(count);
    root = Ord::min(root, max);
    root = Ord::max(root, min);

    if max == 0 {
        // no symbols to code at all
        let code = Code {
            op: 64,
            bits: 1,
            val: 0,
        };

        table[0] = code;
        table[1] = code;

        return InflateTable::Success(1);
    }

    /* check for an over-subscribed or incomplete set of lengths */
    let mut left = 1i32;
    let mut len = 1;
    while len <= MAX_BITS {
        left <<= 1;
        left -= count[len] as i32;
        if left < 0 {
            // over-subscribed
            return InflateTable::InvalidCode;
        }
        len += 1;
    }

    if left > 0 && (matches!(codetype, CodeType::Codes) || max != 1) {
        // incomplete set
        return InflateTable::InvalidCode;
    }

    /* generate offsets into symbol table for each length for sorting */

    // offsets in table for each length
    let mut offs = [0u16; MAX_BITS + 1];
    for len in 1..MAX_BITS {
        offs[len + 1] = offs[len] + count[len];
    }

    /* sort symbols by length, by symbol order within each length */
    for (sym, len) in lens[0..codes].iter().copied().enumerate() {
        if len != 0 {
            let offset = offs[len as usize];
            offs[len as usize] += 1;
            work[offset as usize] = sym as u16;
        }
    }

    let (base, extra, match_) = match codetype {
        CodeType::Codes => (&[] as &[_], &[] as &[_], 20),
        CodeType::Lens => (&LBASE[..], &LEXT[..], 257),
        CodeType::Dists => (&DBASE[..], &DEXT[..], 0),
    };

    let used = 1 << root;

    /* check available table space */
    if matches!(codetype, CodeType::Lens) && used > ENOUGH_LENS {
        return InflateTable::EnoughIsNotEnough;
    }

    if matches!(codetype, CodeType::Dists) && used > ENOUGH_DISTS {
        return InflateTable::EnoughIsNotEnough;
    }

    let mut huff = 0; // starting code
    let mut reversed_huff = 0u32; // starting code, reversed
    let mut sym = 0;
    let mut len = min;
    let mut next = 0usize; // index into `table`
    let mut curr = root;
    let mut drop_ = 0;
    let mut low = usize::MAX; // trigger new subtable when len > root
    let mut used = 1 << root;
    let mask = used - 1; /* mask for comparing low */

    // process all codes and make table entries
    'outer: loop {
        // create table entry
        let here = if work[sym] >= match_ {
            Code {
                bits: (len - drop_) as u8,
                op: extra[(work[sym] - match_) as usize] as u8,
                val: base[(work[sym] - match_) as usize],
            }
        } else if work[sym] + 1 < match_ {
            Code {
                bits: (len - drop_) as u8,
                op: 0,
                val: work[sym],
            }
        } else {
            Code {
                bits: (len - drop_) as u8,
                op: 0b01100000,
                val: 0,
            }
        };

        // replicate for those indices with low len bits equal to huff
        let incr = 1 << (len - drop_);
        let min = 1 << curr; // also has the name 'fill' in the C code

        let base = &mut table[next + (huff >> drop_)..];
        for fill in (0..min).step_by(incr) {
            base[fill] = here;
        }

        // backwards increment the len-bit code huff
        reversed_huff = reversed_huff.wrapping_add(0x80000000u32 >> (len - 1));
        huff = reversed_huff.reverse_bits() as usize;

        // go to next symbol, update count, len
        sym += 1;
        count[len] -= 1;
        if count[len] == 0 {
            if len == max {
                break 'outer;
            }
            len = lens[work[sym] as usize] as usize;
        }

        // create new sub-table if needed
        if len > root && (huff & mask) != low {
            /* if first time, transition to sub-tables */
            if drop_ == 0 {
                drop_ = root;
            }

            /* increment past last table */
            next += min; /* here min is 1 << curr */

            /* determine length of next table */
            curr = len - drop_;
            let mut left = 1 << curr;
            while curr + drop_ < max {
                left -= count[curr + drop_] as i32;
                if left <= 0 {
                    break;
                }
                curr += 1;
                left <<= 1;
            }

            /* check for enough space */
            used += 1usize << curr;

            if matches!(codetype, CodeType::Lens) && used > ENOUGH_LENS {
                return InflateTable::EnoughIsNotEnough;
            }

            if matches!(codetype, CodeType::Dists) && used > ENOUGH_DISTS {
                return InflateTable::EnoughIsNotEnough;
            }

            /* point entry in root table to sub-table */
            low = huff & mask;
            table[low] = Code {
                op: curr as u8,
                bits: root as u8,
                val: next as u16,
            };
        }
    }

    /* fill in remaining table entry if code is incomplete (guaranteed to have
    at most one remaining entry, since if the code is incomplete, the
    maximum code length that was allowed to get this far is one bit) */
    if huff != 0 {
        let here = Code {
            op: 64,
            bits: (len - drop_) as u8,
            val: 0,
        };

        table[next..][huff] = here;
    }

    /* set return parameters */
    InflateTable::Success(root)
}

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

    #[test]
    fn not_enough_errors() {
        // we need to call inflate_table() directly in order to manifest
        // not-enough errors, since zlib insures that enough is always enough

        let table = [Code::default(); crate::ENOUGH_DISTS];

        let mut work = [0; 16];
        let mut lens: [_; 16] = core::array::from_fn(|i| (i + 1) as u16);
        lens[15] = 15;

        let mut next = table;
        let bits = 15;
        let ret = inflate_table(CodeType::Dists, &lens, 16, &mut next, bits, &mut work);
        assert_eq!(ret, InflateTable::EnoughIsNotEnough);

        let mut next = table;
        let bits = 1;
        let ret = inflate_table(CodeType::Dists, &lens, 16, &mut next, bits, &mut work);
        assert_eq!(ret, InflateTable::EnoughIsNotEnough);
    }

    fn build_fixed_length_table(work: &mut [u16]) -> [Code; 512] {
        let mut lens = [0; 288];

        // literal/length table
        let mut sym = 0usize;
        while sym < 144 {
            lens[sym] = 8;
            sym += 1;
        }
        while sym < 256 {
            lens[sym] = 9;
            sym += 1;
        }
        while sym < 280 {
            lens[sym] = 7;
            sym += 1;
        }
        while sym < 288 {
            lens[sym] = 8;
            sym += 1;
        }

        let mut next = [Code::default(); 512];
        let bits = 9;
        inflate_table(CodeType::Lens, &lens, 288, &mut next, bits, work);

        core::array::from_fn(|i| {
            let mut code = next[i];

            code.op = if i & 0b0111_1111 == 99 { 64 } else { code.op };

            code
        })
    }

    #[test]
    fn generate_fixed_length_table() {
        let mut work = [0; 512];
        let generated = build_fixed_length_table(&mut work);

        assert_eq!(generated, crate::inflate::inffixed_tbl::LENFIX);
    }

    fn build_fixed_distance_table(work: &mut [u16]) -> [Code; 32] {
        let mut lens = [0; 288];

        let mut sym = 0;
        while sym < 32 {
            lens[sym] = 5;
            sym += 1;
        }

        let mut next = [Code::default(); 32];
        let bits = 5;
        inflate_table(CodeType::Dists, &lens, 32, &mut next, bits, work);

        next
    }

    #[test]
    fn generate_fixed_distance_table() {
        let mut work = [0; 512];
        let generated = build_fixed_distance_table(&mut work);

        assert_eq!(generated, crate::inflate::inffixed_tbl::DISTFIX);
    }
}