| // Copyright 2022 The Wuffs Authors. |
| // |
| // Licensed under the Apache License, Version 2.0 (the "License"); |
| // you may not use this file except in compliance with the License. |
| // You may obtain a copy of the License at |
| // |
| // https://www.apache.org/licenses/LICENSE-2.0 |
| // |
| // Unless required by applicable law or agreed to in writing, software |
| // distributed under the License is distributed on an "AS IS" BASIS, |
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| // See the License for the specific language governing permissions and |
| // limitations under the License. |
| |
| // ---------------- |
| |
| // Package litonlylzma provides a decoder and encoder for Literal Only LZMA, a |
| // subset of the LZMA compressed file format. |
| // |
| // Subset means that the Encode method's output is a valid LZMA file and can be |
| // decompressed by the tools available at https://www.7-zip.org/sdk.html (and |
| // available as /usr/bin/lzma on a Debian system). |
| // |
| // Subset also means that this codec's compression ratios are not as good as |
| // full-LZMA (although compression times are much faster). Moderate compression |
| // is still achieved, through range coding, but there are no Lempel Ziv |
| // back-references. The main benefit, compared to full-LZMA, is that this |
| // implementation is much simpler and hence easier to study. It is only a few |
| // hundred lines of code. |
| // |
| // Example compression numbers on a small English text file (at |
| // https://github.com/google/wuffs/blob/main/test/data/romeo.txt): |
| // - romeo.txt is 942 bytes (100%). |
| // - romeo.txt.litonlylzma is 659 bytes (70%). |
| // - romeo.txt.xz is 644 bytes (68%). |
| // - romeo.txt.lzma is 598 bytes (63%). |
| // - romeo.txt.bz2 is 568 bytes (60%). |
| // - romeo.txt.zst is 559 bytes (59%). |
| // - romeo.txt.gz is 558 bytes (59%). |
| // |
| // Example compression numbers on a large archive of source code (at |
| // https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.0.1.tar.xz): |
| // - linux-5.0.1.tar is 863313920 bytes (100%). |
| // - linux-5.0.1.tar.litonlylzma is 449726070 bytes (52%). |
| // - linux-5.0.1.tar.gz is 164575959 bytes (19%). |
| // - linux-5.0.1.tar.zst is 156959897 bytes (18%). |
| // - linux-5.0.1.tar.bz2 is 125873134 bytes (15%). |
| // - linux-5.0.1.tar.xz is 108233572 bytes (13%). |
| // - linux-5.0.1.tar.lzma is 108216601 bytes (13%). |
| // |
| // The various tools (/usr/bin/gzip, /usr/bin/xz, etc) were all ran with their |
| // default settings, not their "maximum compression" settings. This is why the |
| // linux-5.0.1.tar.xz file size above differs from what cdn.kernel.org served. |
| package litonlylzma |
| |
| import ( |
| "errors" |
| ) |
| |
| var ( |
| // ErrUnsupportedLZMAData is potentially returned by Decode. |
| ErrUnsupportedLZMAData = errors.New("litonlylzma: unsupported LZMA data") |
| |
| errInvalidLZMAData = errors.New("litonlylzma: invalid LZMA data") |
| errUnexpectedEOF = errors.New("litonlylzma: unexpected EOF") |
| errUnsupportedFileFormat = errors.New("litonlylzma: unsupported FileFormat") |
| ) |
| |
| const ( |
| // LZMA has a few configuration knobs but our subset-of-LZMA hard codes |
| // these to LZMA's default values: |
| // https://github.com/jljusten/LZMA-SDK/blob/781863cdf592da3e97420f50de5dac056ad352a5/C/LzmaEnc.h#L19-L21 |
| // https://github.com/jljusten/LZMA-SDK/blob/781863cdf592da3e97420f50de5dac056ad352a5/DOC/lzma-specification.txt#L43-L45 |
| lc = 3 // The number of Literal Context bits. |
| lp = 0 // The number of Literal Position bits. |
| pb = 2 // The number of Position Bits. |
| |
| lpMask = (1 << lp) - 1 |
| pbMask = (1 << pb) - 1 |
| ) |
| |
| // lzmaHeader5 is the first 5 bytes of the LZMA header using the hard-coded |
| // configuration for the subset-of-LZMA that this package supports. 0x5D |
| // encodes the (lc, lp, pb) triple and the next four bytes hold the u32le |
| // dictionary size. 0x1000 is LZMA's minimum dictionary size, although our |
| // subset-of-LZMA does not use a dictionary (also called a "sliding window"). |
| const lzmaHeader5 = "\x5D\x00\x10\x00\x00" |
| |
| // rangeDecoder and rangeEncoder hold the state for range coding (also known as |
| // arithmetic coding, roughly speaking). "range" is a keyword in Go, so in this |
| // implementation, what's traditionally called the "range" fields are called |
| // "width" instead. |
| // |
| // Conceptually, the encoded bitstream holds a high precision number (a binary |
| // fraction) in the range from zero to one. High precision could mean thousands |
| // or millions of bits, but when decoding, only 32 of these are 'paged in' at a |
| // time - this is the rangeDecoder.bits field. The encoder similarly works on |
| // only a small, fixed number of bits. |
| // |
| // When encoding, that high precision *number* isn't fully known until the end |
| // of the input is reached. *During* the encoding process, what's known is a |
| // high precision *interval* that gets narrowed down as the input bytes are |
| // consumed. That interval is represented by the rangeEncoder.low and |
| // rangeEncoder.width fields (the width is the difference between the |
| // interval's upper and lower bound). Both low and width are, conceptually, |
| // 32-bit values - they operate at the same scale - but low is an uint64 |
| // because of potential overflow. We'll revisit that below. |
| // |
| // Width is initialized to 0xFFFF_FFFF and shrinks every time a bit is decoded |
| // or encoded. Similarly, low can only increase or stay the same on each bit |
| // step. When width drops below (1 << 24), the rangeDecoder or rangeEncoder |
| // 'zooms in', left-shifting (with truncation) by 8 the bits, low and width |
| // fields and reading a src byte or writing a dst byte of compressed data. |
| // |
| // Writing a byte may be delayed because the encoding interval could span |
| // multiple possible digits. To use a decimal fraction analogy, suppose that |
| // our interval is [0.7138872, 0.7146055] and the low field nominally holds 4 |
| // digits. We could emit '7' and '1' but we have to hold back on emitting |
| // either '3' or '4' until we can narrow down the interval further. That '3' |
| // digit is stashed as the pendingHead field. low is set to 8872, width is set |
| // to (46055 - 38872) = 7183 and we progress until the width drops below 1000, |
| // causing a 'zoom in' event. There are three cases: |
| // - if low is less than 9000 (but starts with an '8'), we can emit the '3' |
| // with confidence (and then stash '8' in the pendingHead field). |
| // - if low overflows 9999, we can emit the '4' with confidence (and then stash |
| // low's 'thousands digit' in the pendingHead field). |
| // - if low is in 9000 ..= 9999 then we could still be unsure. Keep the '3' |
| // pending but extend it so that there are two pending digits: '39'. If this |
| // occurs again: '399', and so on. |
| // |
| // In all three cases, 'zooming in' causes us to shift out the most significant |
| // digit of the low field. This method is therefore named shiftLow here, the |
| // same name used in other range coding implementations, such as |
| // https://github.com/jljusten/LZMA-SDK/blob/781863cdf592da3e97420f50de5dac056ad352a5/C/LzmaEnc.c#L572-L602 |
| // |
| // The pendingEtc fields combine to hold one or more digits ('0' ..= '9' |
| // decimal digits in our analogy, 0x00 ..= 0xFF bytes in our actual |
| // implementation). There are (1 + pendingExtra) digits in total and while the |
| // head digit might be any value, the extra digits must be all '9' (in the |
| // decimal analogy) or 0xFF (in our actual implementation). |
| // |
| // Some other implementations use the name "cache" instead of "pending", and |
| // store a "cacheSize" field equivalent to our (1 + pendingExtra). |
| // |
| // As above, the low field is conceptually a uint32, but it is a uint64 because |
| // of this potential overflow. Its high 32 bits are usually zero, rarely one |
| // and never anything higher. The high 32 bits essentially hold an overflow |
| // carry bit. If set, it flips "3999etc" to "4000etc". |
| // |
| // "Low is conceptually 4 bytes; pendingHead holds another byte" is why the |
| // shortest LZMA payload is 5 bytes even though the first byte is always 0x00 |
| // and seems redundant. |
| |
| type rangeDecoder struct { |
| src []byte |
| bits uint32 |
| width uint32 |
| } |
| |
| type rangeEncoder struct { |
| dst []byte |
| low uint64 |
| width uint32 |
| pendingHead uint8 |
| pendingExtra uint64 |
| } |
| |
| // Precondition: (rEnc.width < (1 << 24)) or we are at the end of our input |
| // (and are flushing the rangeEncoder). |
| func (rEnc *rangeEncoder) shiftLow() { |
| if rEnc.low < 0x0_FF00_0000 { |
| rEnc.dst = append(rEnc.dst, rEnc.pendingHead+0x00) |
| for ; rEnc.pendingExtra > 0; rEnc.pendingExtra-- { |
| rEnc.dst = append(rEnc.dst, 0xFF) |
| } |
| rEnc.pendingHead = uint8(rEnc.low >> 24) |
| rEnc.pendingExtra = 0 |
| rEnc.low = (rEnc.low << 8) & 0xFFFF_FFFF |
| |
| } else if rEnc.low < 0x1_0000_0000 { |
| rEnc.pendingExtra++ |
| rEnc.low = (rEnc.low << 8) & 0xFFFF_FFFF |
| |
| } else { |
| rEnc.dst = append(rEnc.dst, rEnc.pendingHead+0x01) |
| for ; rEnc.pendingExtra > 0; rEnc.pendingExtra-- { |
| rEnc.dst = append(rEnc.dst, 0x00) |
| } |
| rEnc.pendingHead = uint8(rEnc.low >> 24) |
| rEnc.pendingExtra = 0 |
| rEnc.low = (rEnc.low << 8) & 0xFFFF_FFFF |
| } |
| } |
| |
| // prob is a probability (out of 100%) whose uint16 value ranges from 0 to |
| // (1 << 11) = 2048 inclusive. For example, prob(1024) means a 50% probability |
| // and prob(384) means a (384 / 2048) = 18.75% probability. |
| // |
| // Specifically, it is the probability that the next bit is 0 (instead of 1). |
| type prob uint16 |
| |
| const ( |
| // probBits is such that prob(1 << probBits) means a 100% probability. |
| probBits = 11 |
| minProb = 0 |
| maxProb = 1 << probBits |
| ) |
| |
| func setProbsToOneHalf(p []prob) { |
| for i := range p { |
| p[i] = 1 << (probBits - 1) |
| } |
| } |
| |
| // adaptShift scales how much to adjust "the probability that the next bit is |
| // 0" after seeing the current bit. The probability is adaptive in that the |
| // current bit being 0 (or 1) increases (or decreases) the probability that the |
| // next bit is 0. The larger adaptShift is, the slower the adaptation. |
| const adaptShift = 5 |
| |
| func (p *prob) decodeBit(rDec *rangeDecoder) (bitValue uint32, retErr error) { |
| threshold := (rDec.width >> probBits) * uint32(*p) |
| if rDec.bits < threshold { |
| bitValue = 0 |
| rDec.width = threshold |
| *p += (maxProb - *p) >> adaptShift |
| } else { |
| bitValue = 1 |
| rDec.bits -= threshold |
| rDec.width -= threshold |
| *p -= (*p - minProb) >> adaptShift |
| } |
| |
| if rDec.width < (1 << 24) { |
| if len(rDec.src) <= 0 { |
| return 0, errUnexpectedEOF |
| } |
| rDec.bits = rDec.bits<<8 | uint32(rDec.src[0]) |
| rDec.width <<= 8 |
| rDec.src = rDec.src[1:] |
| } |
| return bitValue, retErr |
| } |
| |
| func (p *prob) encodeBit(rEnc *rangeEncoder, bitValue uint32) { |
| threshold := (rEnc.width >> probBits) * uint32(*p) |
| if bitValue == 0 { |
| rEnc.width = threshold |
| *p += (maxProb - *p) >> adaptShift |
| } else { |
| rEnc.low += uint64(threshold) |
| rEnc.width -= threshold |
| *p -= (*p - minProb) >> adaptShift |
| } |
| if rEnc.width < (1 << 24) { |
| rEnc.width <<= 8 |
| rEnc.shiftLow() |
| } |
| } |
| |
| // byteProbs hold probabilities for coding 8 bits (1 byte) of data. It is a 256 |
| // element array such that: |
| // - The 0th element is unused. |
| // - The 1st element holds the probability that the 7th bit (the highest, most |
| // significant bit) is 0. |
| // - The 2nd element holds the probability that the 6th bit is 0, conditional |
| // on the high bit being 0. |
| // - The 3rd element holds the probability that the 6th bit is 0, conditional |
| // on the high bit being 1. |
| // - The 4th element holds the probability that the 5th bit is 0, conditional |
| // on the high two bits being 00. |
| // - The 5th element holds the probability that the 5th bit is 0, conditional |
| // on the high two bits being 01. |
| // - The 6th element holds the probability that the 5th bit is 0, conditional |
| // on the high two bits being 10. |
| // - The 7th element holds the probability that the 5th bit is 0, conditional |
| // on the high two bits being 11. |
| // - The 8th element holds the probability that the 4th bit is 0, conditional |
| // on the high three bits being 000. |
| // - etc |
| // - The 255th element holds the probability that the 0th bit (the lowest, |
| // least significant bit) is 0, conditional on the high seven bits being |
| // 1111111. |
| // |
| // Put another way, the 256 elements' value of N, as in "it's a probability for |
| // the Nth bit", looks like this (when arranged in 8 rows of 32 elements): |
| // |
| // u7665555444444443333333333333333 |
| // 22222222222222222222222222222222 |
| // 11111111111111111111111111111111 |
| // 11111111111111111111111111111111 |
| // 00000000000000000000000000000000 |
| // 00000000000000000000000000000000 |
| // 00000000000000000000000000000000 |
| // 00000000000000000000000000000000 |
| // |
| // The 'u' means that the 0th element is unused. |
| type byteProbs [0x100]prob |
| |
| func (p *byteProbs) decodeByte(rDec *rangeDecoder) (byteValue byte, retErr error) { |
| index := uint32(1) |
| for index < 0x100 { |
| if bitValue, err := p[index].decodeBit(rDec); err != nil { |
| return 0, err |
| } else { |
| index = index<<1 | bitValue |
| } |
| } |
| return byte(index), nil |
| } |
| |
| func (p *byteProbs) encodeByte(rEnc *rangeEncoder, byteValue byte) { |
| b := uint32(byteValue) |
| index := uint32(1) |
| for i := 7; i >= 0; i-- { |
| bitValue := (b >> uint(i)) & 1 |
| p[index].encodeBit(rEnc, bitValue) |
| index = index<<1 | bitValue |
| } |
| } |
| |
| // FileFormat is a compressed file format, either the original LZMA format |
| // itself or something that builds on or varies that. |
| // |
| // Each valid FileFormatXxx value does not distinguish between the full file |
| // format (as spoken by numerous Xxx software tools) and the subset-of-Xxx |
| // implemented by this package. Specifically, FileFormatLZMA.Encode will |
| // produce compressed data that is valid full-LZMA (and is also valid |
| // subset-of-LZMA). FileFormatLZMA.Decode can decode subset-of-LZMA (but not |
| // full-LZMA). |
| // |
| // This package currently defines only one valid FileFormatXxx value: |
| // FileFormatLZMA. Future versions of this package could define e.g. |
| // FileFormatLZMA2 or FileFormatXz such that FileFormatXz.Encode would produce |
| // valid full-XZ data: a valid XZ file (that could be decoded by official XZ |
| // tools) that used the LZMA compression filter, but elected not to use the |
| // full capabilities of that filter (using LZ literals only, not matches). |
| type FileFormat uint32 |
| |
| const ( |
| FileFormatInvalid = FileFormat(0) |
| FileFormatLZMA = FileFormat(1) |
| ) |
| |
| // Decode converts src from the Literal Only LZMA compressed file format, |
| // appending the decoding to dst. |
| // |
| // f should be FileFormatLZMA but future versions of this package may support |
| // other file formats. |
| // |
| // It is valid to pass a nil dst, like the way it's valid to pass nil to the |
| // built-in append function. |
| // |
| // Decode also returns the suffix of src that was not used during decoding. |
| // |
| // It can also return partial results. The appendedDst and remainingSrc return |
| // values may be non-zero even if retErr is non-zero. |
| // |
| // It returns ErrUnsupportedLZMAData if the source bytes look like LZMA |
| // formatted data that is outside the subset-of-LZMA that this package |
| // implements. |
| func (f FileFormat) Decode(dst []byte, src []byte) (appendedDst []byte, remainingSrc []byte, retErr error) { |
| if f != FileFormatLZMA { |
| return dst, nil, errUnsupportedFileFormat |
| } |
| |
| // LZMA consists of a 13 byte header and then at least a 5 byte payload |
| // (range coded data). The 13 byte header is: |
| // - 1 byte for the (lc, lp, pb) triple with max values (8, 4, 4), |
| // - 4 bytes u32le dictionary size and |
| // - 8 bytes i64le uncompressed size. |
| if (len(src) < 18) || (src[0] >= (9 * 5 * 5)) { |
| return dst, nil, errInvalidLZMAData |
| } else if (src[0] != lzmaHeader5[0]) || |
| (src[1] != lzmaHeader5[1]) || |
| (src[2] != lzmaHeader5[2]) || |
| (src[3] != lzmaHeader5[3]) || |
| (src[4] != lzmaHeader5[4]) || |
| (src[13] != 0x00) { |
| return dst, nil, ErrUnsupportedLZMAData |
| } |
| |
| size := uint64(0) |
| for i := 0; i < 8; i++ { |
| size |= uint64(src[5+i]) << uint(8*i) |
| } |
| if int64(size) < -1 { |
| return dst, nil, errInvalidLZMAData |
| } else if int64(size) == -1 { |
| return dst, nil, ErrUnsupportedLZMAData |
| } |
| |
| rDec := rangeDecoder{ |
| src: src[18:], |
| bits: (uint32(src[14]) << 24) | |
| (uint32(src[15]) << 16) | |
| (uint32(src[16]) << 8) | |
| (uint32(src[17]) << 0), |
| width: 0xFFFF_FFFF, |
| } |
| |
| posProbs := [1 << pb]prob{} |
| setProbsToOneHalf(posProbs[:]) |
| |
| litProbs := [1 << (lc + lp)]byteProbs{} |
| for ij := range litProbs { |
| setProbsToOneHalf(litProbs[ij][:]) |
| } |
| |
| pos := uint32(0) |
| prev := byte(0) |
| curr := byte(0) |
| for ; size > 0; size-- { |
| if bitValue, err := posProbs[pos&pbMask].decodeBit(&rDec); err != nil { |
| return dst, nil, err |
| } else if bitValue != 0 { |
| // A full-LZMA decoder would implement Lempel Ziv matches (or the |
| // End of Stream marker) here. Our simpler subset-of-LZMA decoder |
| // only uses literals. |
| return dst, rDec.src, ErrUnsupportedLZMAData |
| } |
| |
| i := (pos & lpMask) << lc |
| j := uint32(prev) >> (8 - lc) |
| if curr, retErr = litProbs[i|j].decodeByte(&rDec); retErr != nil { |
| return dst, rDec.src, retErr |
| } |
| dst = append(dst, curr) |
| pos++ |
| prev = curr |
| } |
| |
| return dst, rDec.src, nil |
| } |
| |
| // Encode converts src to the Literal Only LZMA compressed file format, |
| // appending the encoding to dst. |
| // |
| // f should be FileFormatLZMA but future versions of this package may support |
| // other file formats. |
| // |
| // It is valid to pass a nil dst, like the way it's valid to pass nil to the |
| // built-in append function. |
| func (f FileFormat) Encode(dst []byte, src []byte) (appendedDst []byte, retErr error) { |
| if f != FileFormatLZMA { |
| return nil, errUnsupportedFileFormat |
| } |
| |
| dst = append(dst, lzmaHeader5...) |
| size := len(src) |
| for i := 0; i < 8; i++ { |
| dst = append(dst, byte(size)) |
| size >>= 8 |
| } |
| |
| rEnc := rangeEncoder{ |
| dst: dst, |
| width: 0xFFFF_FFFF, |
| } |
| |
| posProbs := [1 << pb]prob{} |
| setProbsToOneHalf(posProbs[:]) |
| |
| litProbs := [1 << (lc + lp)]byteProbs{} |
| for ij := range litProbs { |
| setProbsToOneHalf(litProbs[ij][:]) |
| } |
| |
| pos := uint32(0) |
| prev := byte(0) |
| for _, curr := range src { |
| // A full-LZMA encoder would choose between Lempel Ziv literals and |
| // matches. Our simpler subset-of-LZMA encoder only uses literals, and |
| // it is not a streaming encoder (it does not need to emit an End of |
| // Stream marker), so we always encode a 0 bit here. |
| posProbs[pos&pbMask].encodeBit(&rEnc, 0) |
| |
| i := (pos & lpMask) << lc |
| j := uint32(prev) >> (8 - lc) |
| litProbs[i|j].encodeByte(&rEnc, curr) |
| pos++ |
| prev = curr |
| } |
| |
| // Flush the rangeEncoder. |
| for i := 0; i < 5; i++ { |
| rEnc.shiftLow() |
| } |
| |
| return rEnc.dst, nil |
| } |