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// SPDX-License-Identifier: BSD-3-Clause
// Copyright (c) 2026 Fernando Sahmkow
#include "lzw.h"
#include <array>
#include <cstring>
#include <whiteout/common_types.h>
namespace whiteout::textures::tiff {
namespace {
constexpr u32 kClearCode = 256;
constexpr u32 kEoiCode = 257;
constexpr u32 kFirstFreeCode = 258;
constexpr u32 kMaxCode = 1u << 12; // 12-bit codes max
constexpr int kMinCodeSize = 9;
constexpr int kMaxCodeSize = 12;
/// MSB-first bit reader over a contiguous byte stream.
struct MsbBitReader {
const u8* data;
size_t size;
size_t bytePos = 0;
u32 bitBuf = 0;
int bitsAvail = 0;
MsbBitReader(std::span<const u8> s) : data(s.data()), size(s.size()) {}
void refill() {
while (bitsAvail <= 24 && bytePos < size) {
bitBuf |= static_cast<u32>(data[bytePos++]) << (24 - bitsAvail);
bitsAvail += 8;
}
}
/// Returns -1 on EOF (insufficient bits remaining).
int readBits(int n) {
if (bitsAvail < n)
refill();
if (bitsAvail < n)
return -1;
const u32 v = (bitBuf >> (32 - n)) & ((1u << n) - 1u);
bitBuf <<= n;
bitsAvail -= n;
return static_cast<int>(v);
}
};
/// String table entry — fixed-capacity ring of (prefix, suffix) pairs.
/// During decode we walk the prefix chain to reconstruct the full string.
struct Entry {
u16 prefix; // index of the previous entry (or 0xFFFF for root literals)
u8 suffix;
u16 length; // total decoded length of this entry (>= 1)
};
constexpr u16 kNoPrefix = 0xFFFF;
/// Reconstruct the byte sequence for `code` by walking the prefix chain,
/// writing into `out` in reverse, then returning the start byte. Returns
/// the number of bytes written, or 0 if the code is invalid.
u16 emitCodeBytes(const std::array<Entry, kMaxCode>& table, u32 code,
u8* out, size_t outCap, size_t writePos) {
// First locate length and validate.
if (code >= kMaxCode)
return 0;
u32 cur = code;
u16 len = table[cur].length;
if (len == 0)
return 0;
if (writePos + len > outCap)
return 0;
// Walk backwards filling out[writePos + len - 1] down to writePos.
u8* dst = out + writePos + len;
while (cur != kNoPrefix && len > 0) {
*(--dst) = table[cur].suffix;
cur = table[cur].prefix;
--len;
}
return table[code].length;
}
} // namespace
std::vector<u8> lzwDecompress(std::span<const u8> src, size_t expectedSize) {
if (src.empty())
return {};
std::vector<u8> out;
if (expectedSize > 0)
out.reserve(expectedSize);
std::array<Entry, kMaxCode> table{};
// Initialize the first 256 entries as single-byte literals.
for (u32 i = 0; i < 256; ++i)
table[i] = Entry{kNoPrefix, static_cast<u8>(i), 1};
// Codes 256 (CLEAR) and 257 (EOI) carry no data.
MsbBitReader br(src);
int codeSize = kMinCodeSize;
u32 nextCode = kFirstFreeCode;
u32 prevCode = static_cast<u32>(-1);
while (true) {
int code = br.readBits(codeSize);
if (code < 0) {
// Some encoders (libtiff in certain configurations) terminate
// strips by exhausting the bitstream rather than by emitting an
// explicit EOI code. If we've already produced the caller's
// expected output size, accept the stream as complete.
if (expectedSize > 0 && out.size() == expectedSize)
return out;
return {};
}
const u32 c = static_cast<u32>(code);
if (c == kEoiCode)
break;
if (c == kClearCode) {
codeSize = kMinCodeSize;
nextCode = kFirstFreeCode;
prevCode = static_cast<u32>(-1);
continue;
}
// Two valid cases: existing code (c < nextCode) or KwKwK (c == nextCode).
if (c > nextCode)
return {}; // invalid code
// Reserve space then walk-emit.
const u16 writePos16 = 0; // unused; we use the index into `out`.
(void)writePos16;
if (c < nextCode) {
// Existing entry — emit its string.
const u16 len = table[c].length;
const size_t before = out.size();
out.resize(before + len);
if (emitCodeBytes(table, c, out.data(), out.size(), before) == 0)
return {};
if (prevCode != static_cast<u32>(-1) && nextCode < kMaxCode) {
// Add new table entry: prev + first byte of current.
const u8 firstByte = out[before];
Entry& e = table[nextCode];
e.prefix = static_cast<u16>(prevCode);
e.suffix = firstByte;
e.length = static_cast<u16>(table[prevCode].length + 1);
++nextCode;
// TIFF early-change: bump when next_code == (1 << codeSize) - 1.
if (codeSize < kMaxCodeSize &&
nextCode == ((1u << codeSize) - 1))
++codeSize;
}
} else {
// KwKwK: c == nextCode, emit prev + prev[0].
if (prevCode == static_cast<u32>(-1))
return {}; // invalid: KwKwK without a previous code
const u16 prevLen = table[prevCode].length;
const size_t before = out.size();
const size_t newLen = static_cast<size_t>(prevLen) + 1;
out.resize(before + newLen);
// Reconstruct prev, then append prev[0].
if (emitCodeBytes(table, prevCode, out.data(), out.size(), before) == 0)
return {};
const u8 firstByte = out[before];
out[before + prevLen] = firstByte;
if (nextCode < kMaxCode) {
Entry& e = table[nextCode];
e.prefix = static_cast<u16>(prevCode);
e.suffix = firstByte;
e.length = static_cast<u16>(prevLen + 1);
++nextCode;
if (codeSize < kMaxCodeSize &&
nextCode == ((1u << codeSize) - 1))
++codeSize;
}
}
prevCode = c;
if (expectedSize > 0 && out.size() > expectedSize)
return {};
}
return out;
}
} // namespace whiteout::textures::tiff