Merge pull request #2 from ZaparooProject/feature/chd-support

feat: add CHD disc image support

Author: wizzomafizzo

chd/bitstream.go

@@ -0,0 +1,190 @@
+// Copyright (c) 2025 Niema Moshiri and The Zaparoo Project.
+// SPDX-License-Identifier: GPL-3.0-or-later
+//
+// This file is part of go-gameid.
+//
+// go-gameid is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// go-gameid is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with go-gameid.  If not, see <https://www.gnu.org/licenses/>.
+
+package chd
+
+// bitReader reads bits from a byte slice.
+type bitReader struct {
+	data   []byte
+	offset int  // bit offset
+	bits   uint // accumulated bits
+	avail  int  // bits available in accumulator
+}
+
+// newBitReader creates a new bit reader.
+func newBitReader(data []byte) *bitReader {
+	return &bitReader{data: data}
+}
+
+// read reads count bits from the stream.
+func (br *bitReader) read(count int) uint32 {
+	// Fill accumulator as needed
+	for br.avail < count {
+		byteOff := br.offset / 8
+		if byteOff >= len(br.data) {
+			br.bits <<= 8
+			br.avail += 8
+			continue
+		}
+		br.bits = (br.bits << 8) | uint(br.data[byteOff])
+		br.avail += 8
+		br.offset += 8
+	}
+
+	// Extract the bits
+	br.avail -= count
+	//nolint:gosec // Safe: bits accumulator is bounded by count which is at most 32
+	result := uint32((br.bits >> br.avail) & ((1 << count) - 1))
+	return result
+}
+
+// huffmanDecoder decodes Huffman-encoded data for CHD V5 maps.
+type huffmanDecoder struct {
+	lookup   []uint32
+	nodeBits []uint8
+	numCodes int
+	maxBits  int
+}
+
+// newHuffmanDecoder creates a Huffman decoder for the given parameters.
+func newHuffmanDecoder(numCodes, maxBits int) *huffmanDecoder {
+	return &huffmanDecoder{
+		numCodes: numCodes,
+		maxBits:  maxBits,
+		nodeBits: make([]uint8, numCodes),
+		lookup:   make([]uint32, 1<<maxBits),
+	}
+}
+
+// importTreeRLE imports a Huffman tree encoded with RLE.
+func (hd *huffmanDecoder) importTreeRLE(br *bitReader) error {
+	// Determine number of bits to read for each node
+	var numBits int
+	switch {
+	case hd.maxBits >= 16:
+		numBits = 5
+	case hd.maxBits >= 8:
+		numBits = 4
+	default:
+		numBits = 3
+	}
+
+	// Read the tree with RLE decoding
+	for curNode := 0; curNode < hd.numCodes; {
+		nodeBits := br.read(numBits)
+		if nodeBits != 1 {
+			//nolint:gosec // Safe: nodeBits from Huffman tree is bounded to 0-32
+			hd.nodeBits[curNode] = uint8(nodeBits)
+			curNode++
+			continue
+		}
+		// RLE encoding: read actual value
+		nodeBits = br.read(numBits)
+		if nodeBits == 1 {
+			// Literal 1
+			hd.nodeBits[curNode] = 1
+			curNode++
+			continue
+		}
+		// Repeat count follows
+		repCount := int(br.read(numBits)) + 3
+		//nolint:gosec // Safe: nodeBits from Huffman tree is bounded to 0-32
+		curNode = hd.fillNodeBits(curNode, uint8(nodeBits), repCount)
+	}
+
+	// Build lookup table
+	return hd.buildLookup()
+}
+
+// fillNodeBits fills nodeBits with a repeated value, returning the new curNode.
+func (hd *huffmanDecoder) fillNodeBits(curNode int, value uint8, repCount int) int {
+	for i := 0; i < repCount && curNode < hd.numCodes; i++ {
+		hd.nodeBits[curNode] = value
+		curNode++
+	}
+	return curNode
+}
+
+// buildLookup builds the lookup table from node bits.
+// This follows MAME's canonical code assignment which processes from highest to lowest bit length.
+func (hd *huffmanDecoder) buildLookup() error {
+	// Build histogram of bit lengths
+	bithisto := make([]uint32, 33)
+	for i := range hd.numCodes {
+		if hd.nodeBits[i] <= 32 {
+			bithisto[hd.nodeBits[i]]++
+		}
+	}
+
+	// For each code length, determine the starting code number
+	// Process from highest to lowest bit length (MAME convention)
+	var curstart uint32
+	for codelen := 32; codelen > 0; codelen-- {
+		nextstart := (curstart + bithisto[codelen]) >> 1
+		bithisto[codelen] = curstart
+		curstart = nextstart
+	}
+
+	// Now assign canonical codes and build lookup table
+	// nodeBits stores the assigned code for each symbol
+	nodeCodes := make([]uint32, hd.numCodes)
+	for i := range hd.numCodes {
+		bits := hd.nodeBits[i]
+		if bits > 0 {
+			nodeCodes[i] = bithisto[bits]
+			bithisto[bits]++
+		}
+	}
+
+	// Build lookup table
+	for i := range hd.numCodes {
+		bits := int(hd.nodeBits[i])
+		if bits > 0 {
+			// Set up the entry: (symbol << 5) | numbits
+			//nolint:gosec // Safe: i bounded by numCodes (16), bits bounded by maxBits (8)
+			value := uint32((i << 5) | bits)
+
+			// Fill all matching entries
+			shift := hd.maxBits - bits
+			base := int(nodeCodes[i]) << shift
+			end := int(nodeCodes[i]+1)<<shift - 1
+			for j := base; j <= end; j++ {
+				hd.lookup[j] = value
+			}
+		}
+	}
+
+	return nil
+}
+
+// decode decodes a single symbol from the bit stream.
+func (hd *huffmanDecoder) decode(br *bitReader) uint8 {
+	// Peek maxBits bits
+	peek := br.read(hd.maxBits)
+	entry := hd.lookup[peek]
+	//nolint:gosec // Safe: entry stores symbol in upper bits, bounded by numCodes (16)
+	symbol := uint8(entry >> 5)
+	bits := int(entry & 0x1f)
+
+	// Put back unused bits by adjusting the bit reader
+	if bits < hd.maxBits {
+		br.avail += hd.maxBits - bits
+	}
+
+	return symbol
+}