path: root/internal/flatex/huffman.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package flatex implements the DEFLATE compressed data format, described in
// RFC 1951.  The [compress/gzip] and [compress/zlib] packages implement access
// to DEFLATE-based file formats.
package flatex

import (
	"math/bits"
	"strconv"
	"sync"
)

const (
	// The special code used to mark the end of a block.
	endBlockMarker = 256
	maxCodeLen     = 16      // max length of Huffman code
	maxMatchOffset = 1 << 15 // The largest match offset
	// The next three numbers come from the RFC section 3.2.7, with the
	// additional proviso in section 3.2.5 which implies that distance codes
	// 30 and 31 should never occur in compressed data.
	maxNumLit  = 286
	maxNumDist = 30
	numCodes   = 19 // number of codes in Huffman meta-code
)

// A CorruptInputError reports the presence of corrupt input at a given offset.
type CorruptInputError int64

func (e CorruptInputError) Error() string {
	return "flate: corrupt input before offset " + strconv.FormatInt(int64(e), 10)
}

// The data structure for decoding Huffman tables is based on that of
// zlib. There is a lookup table of a fixed bit width (huffmanChunkBits),
// For codes smaller than the table width, there are multiple entries
// (each combination of trailing bits has the same value). For codes
// larger than the table width, the table contains a link to an overflow
// table. The width of each entry in the link table is the maximum code
// size minus the chunk width.
//
// Note that you can do a lookup in the table even without all bits
// filled. Since the extra bits are zero, and the DEFLATE Huffman codes
// have the property that shorter codes come before longer ones, the
// bit length estimate in the result is a lower bound on the actual
// number of bits.
//
// See the following:
//	https://github.com/madler/zlib/raw/master/doc/algorithm.txt

// chunk & 15 is number of bits
// chunk >> 4 is value, including table link

const (
	huffmanChunkBits  = 9
	huffmanNumChunks  = 1 << huffmanChunkBits
	huffmanCountMask  = 15
	huffmanValueShift = 4
)

type huffmanDecoder struct {
	min      int                      // the minimum code length
	chunks   [huffmanNumChunks]uint32 // chunks as described above
	links    [][]uint32               // overflow links
	linkMask uint32                   // mask the width of the link table
}

// Initialize Huffman decoding tables from array of code lengths.
// Following this function, h is guaranteed to be initialized into a complete
// tree (i.e., neither over-subscribed nor under-subscribed). The exception is a
// degenerate case where the tree has only a single symbol with length 1. Empty
// trees are permitted.
func (h *huffmanDecoder) init(lengths []int) bool {
	// Sanity enables additional runtime tests during Huffman
	// table construction. It's intended to be used during
	// development to supplement the currently ad-hoc unit tests.
	const sanity = false

	if h.min != 0 {
		*h = huffmanDecoder{}
	}

	// Count number of codes of each length,
	// compute min and max length.
	var count [maxCodeLen]int
	var min, max int
	for _, n := range lengths {
		if n == 0 {
			continue
		}
		if min == 0 || n < min {
			min = n
		}
		if n > max {
			max = n
		}
		count[n]++
	}

	// Empty tree. The decompressor.huffSym function will fail later if the tree
	// is used. Technically, an empty tree is only valid for the HDIST tree and
	// not the HCLEN and HLIT tree. However, a stream with an empty HCLEN tree
	// is guaranteed to fail since it will attempt to use the tree to decode the
	// codes for the HLIT and HDIST trees. Similarly, an empty HLIT tree is
	// guaranteed to fail later since the compressed data section must be
	// composed of at least one symbol (the end-of-block marker).
	if max == 0 {
		return true
	}

	code := 0
	var nextcode [maxCodeLen]int
	for i := min; i <= max; i++ {
		code <<= 1
		nextcode[i] = code
		code += count[i]
	}

	// Check that the coding is complete (i.e., that we've
	// assigned all 2-to-the-max possible bit sequences).
	// Exception: To be compatible with zlib, we also need to
	// accept degenerate single-code codings. See also
	// TestDegenerateHuffmanCoding.
	if code != 1<<uint(max) && (code != 1 || max != 1) {
		return false
	}

	h.min = min
	if max > huffmanChunkBits {
		numLinks := 1 << (uint(max) - huffmanChunkBits)
		h.linkMask = uint32(numLinks - 1)

		// create link tables
		link := nextcode[huffmanChunkBits+1] >> 1
		h.links = make([][]uint32, huffmanNumChunks-link)
		for j := uint(link); j < huffmanNumChunks; j++ {
			reverse := int(bits.Reverse16(uint16(j)))
			reverse >>= uint(16 - huffmanChunkBits)
			off := j - uint(link)
			if sanity && h.chunks[reverse] != 0 {
				panic("impossible: overwriting existing chunk")
			}
			h.chunks[reverse] = uint32(off<<huffmanValueShift | (huffmanChunkBits + 1))
			h.links[off] = make([]uint32, numLinks)
		}
	}

	for i, n := range lengths {
		if n == 0 {
			continue
		}
		code := nextcode[n]
		nextcode[n]++
		chunk := uint32(i<<huffmanValueShift | n)
		reverse := int(bits.Reverse16(uint16(code)))
		reverse >>= uint(16 - n)
		if n <= huffmanChunkBits {
			for off := reverse; off < len(h.chunks); off += 1 << uint(n) {
				// We should never need to overwrite
				// an existing chunk. Also, 0 is
				// never a valid chunk, because the
				// lower 4 "count" bits should be
				// between 1 and 15.
				if sanity && h.chunks[off] != 0 {
					panic("impossible: overwriting existing chunk")
				}
				h.chunks[off] = chunk
			}
		} else {
			j := reverse & (huffmanNumChunks - 1)
			if sanity && h.chunks[j]&huffmanCountMask != huffmanChunkBits+1 {
				// Longer codes should have been
				// associated with a link table above.
				panic("impossible: not an indirect chunk")
			}
			value := h.chunks[j] >> huffmanValueShift
			linktab := h.links[value]
			reverse >>= huffmanChunkBits
			for off := reverse; off < len(linktab); off += 1 << uint(n-huffmanChunkBits) {
				if sanity && linktab[off] != 0 {
					panic("impossible: overwriting existing chunk")
				}
				linktab[off] = chunk
			}
		}
	}

	if sanity {
		// Above we've sanity checked that we never overwrote
		// an existing entry. Here we additionally check that
		// we filled the tables completely.
		for i, chunk := range h.chunks {
			if chunk == 0 {
				// As an exception, in the degenerate
				// single-code case, we allow odd
				// chunks to be missing.
				if code == 1 && i%2 == 1 {
					continue
				}
				panic("impossible: missing chunk")
			}
		}
		for _, linktab := range h.links {
			for _, chunk := range linktab {
				if chunk == 0 {
					panic("impossible: missing chunk")
				}
			}
		}
	}

	return true
}

// RFC 1951 section 3.2.7.
// Compression with dynamic Huffman codes
var codeOrder = [...]int{16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15}

var (
	// Initialize the fixedHuffmanDecoder only once upon first use.
	fixedOnce           sync.Once
	fixedHuffmanDecoder huffmanDecoder
)

func fixedHuffmanDecoderInit() {
	fixedOnce.Do(func() {
		// These come from the RFC section 3.2.6.
		var bits [288]int
		for i := 0; i < 144; i++ {
			bits[i] = 8
		}
		for i := 144; i < 256; i++ {
			bits[i] = 9
		}
		for i := 256; i < 280; i++ {
			bits[i] = 7
		}
		for i := 280; i < 288; i++ {
			bits[i] = 8
		}
		fixedHuffmanDecoder.init(bits[:])
	})
}