mirror of
https://github.com/go-gitea/gitea
synced 2024-11-04 21:29:12 +01:00
d77176912b
* Migrate to go modules * make vendor * Update mvdan.cc/xurls * make vendor * Update code.gitea.io/git * make fmt-check * Update github.com/go-sql-driver/mysql * make vendor
238 lines
8 KiB
Go
238 lines
8 KiB
Go
// Copyright 2016 The Snappy-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.
|
|
|
|
// +build !amd64 appengine !gc noasm
|
|
|
|
package snappy
|
|
|
|
func load32(b []byte, i int) uint32 {
|
|
b = b[i : i+4 : len(b)] // Help the compiler eliminate bounds checks on the next line.
|
|
return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
|
|
}
|
|
|
|
func load64(b []byte, i int) uint64 {
|
|
b = b[i : i+8 : len(b)] // Help the compiler eliminate bounds checks on the next line.
|
|
return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24 |
|
|
uint64(b[4])<<32 | uint64(b[5])<<40 | uint64(b[6])<<48 | uint64(b[7])<<56
|
|
}
|
|
|
|
// emitLiteral writes a literal chunk and returns the number of bytes written.
|
|
//
|
|
// It assumes that:
|
|
// dst is long enough to hold the encoded bytes
|
|
// 1 <= len(lit) && len(lit) <= 65536
|
|
func emitLiteral(dst, lit []byte) int {
|
|
i, n := 0, uint(len(lit)-1)
|
|
switch {
|
|
case n < 60:
|
|
dst[0] = uint8(n)<<2 | tagLiteral
|
|
i = 1
|
|
case n < 1<<8:
|
|
dst[0] = 60<<2 | tagLiteral
|
|
dst[1] = uint8(n)
|
|
i = 2
|
|
default:
|
|
dst[0] = 61<<2 | tagLiteral
|
|
dst[1] = uint8(n)
|
|
dst[2] = uint8(n >> 8)
|
|
i = 3
|
|
}
|
|
return i + copy(dst[i:], lit)
|
|
}
|
|
|
|
// emitCopy writes a copy chunk and returns the number of bytes written.
|
|
//
|
|
// It assumes that:
|
|
// dst is long enough to hold the encoded bytes
|
|
// 1 <= offset && offset <= 65535
|
|
// 4 <= length && length <= 65535
|
|
func emitCopy(dst []byte, offset, length int) int {
|
|
i := 0
|
|
// The maximum length for a single tagCopy1 or tagCopy2 op is 64 bytes. The
|
|
// threshold for this loop is a little higher (at 68 = 64 + 4), and the
|
|
// length emitted down below is is a little lower (at 60 = 64 - 4), because
|
|
// it's shorter to encode a length 67 copy as a length 60 tagCopy2 followed
|
|
// by a length 7 tagCopy1 (which encodes as 3+2 bytes) than to encode it as
|
|
// a length 64 tagCopy2 followed by a length 3 tagCopy2 (which encodes as
|
|
// 3+3 bytes). The magic 4 in the 64±4 is because the minimum length for a
|
|
// tagCopy1 op is 4 bytes, which is why a length 3 copy has to be an
|
|
// encodes-as-3-bytes tagCopy2 instead of an encodes-as-2-bytes tagCopy1.
|
|
for length >= 68 {
|
|
// Emit a length 64 copy, encoded as 3 bytes.
|
|
dst[i+0] = 63<<2 | tagCopy2
|
|
dst[i+1] = uint8(offset)
|
|
dst[i+2] = uint8(offset >> 8)
|
|
i += 3
|
|
length -= 64
|
|
}
|
|
if length > 64 {
|
|
// Emit a length 60 copy, encoded as 3 bytes.
|
|
dst[i+0] = 59<<2 | tagCopy2
|
|
dst[i+1] = uint8(offset)
|
|
dst[i+2] = uint8(offset >> 8)
|
|
i += 3
|
|
length -= 60
|
|
}
|
|
if length >= 12 || offset >= 2048 {
|
|
// Emit the remaining copy, encoded as 3 bytes.
|
|
dst[i+0] = uint8(length-1)<<2 | tagCopy2
|
|
dst[i+1] = uint8(offset)
|
|
dst[i+2] = uint8(offset >> 8)
|
|
return i + 3
|
|
}
|
|
// Emit the remaining copy, encoded as 2 bytes.
|
|
dst[i+0] = uint8(offset>>8)<<5 | uint8(length-4)<<2 | tagCopy1
|
|
dst[i+1] = uint8(offset)
|
|
return i + 2
|
|
}
|
|
|
|
// extendMatch returns the largest k such that k <= len(src) and that
|
|
// src[i:i+k-j] and src[j:k] have the same contents.
|
|
//
|
|
// It assumes that:
|
|
// 0 <= i && i < j && j <= len(src)
|
|
func extendMatch(src []byte, i, j int) int {
|
|
for ; j < len(src) && src[i] == src[j]; i, j = i+1, j+1 {
|
|
}
|
|
return j
|
|
}
|
|
|
|
func hash(u, shift uint32) uint32 {
|
|
return (u * 0x1e35a7bd) >> shift
|
|
}
|
|
|
|
// encodeBlock encodes a non-empty src to a guaranteed-large-enough dst. It
|
|
// assumes that the varint-encoded length of the decompressed bytes has already
|
|
// been written.
|
|
//
|
|
// It also assumes that:
|
|
// len(dst) >= MaxEncodedLen(len(src)) &&
|
|
// minNonLiteralBlockSize <= len(src) && len(src) <= maxBlockSize
|
|
func encodeBlock(dst, src []byte) (d int) {
|
|
// Initialize the hash table. Its size ranges from 1<<8 to 1<<14 inclusive.
|
|
// The table element type is uint16, as s < sLimit and sLimit < len(src)
|
|
// and len(src) <= maxBlockSize and maxBlockSize == 65536.
|
|
const (
|
|
maxTableSize = 1 << 14
|
|
// tableMask is redundant, but helps the compiler eliminate bounds
|
|
// checks.
|
|
tableMask = maxTableSize - 1
|
|
)
|
|
shift := uint32(32 - 8)
|
|
for tableSize := 1 << 8; tableSize < maxTableSize && tableSize < len(src); tableSize *= 2 {
|
|
shift--
|
|
}
|
|
// In Go, all array elements are zero-initialized, so there is no advantage
|
|
// to a smaller tableSize per se. However, it matches the C++ algorithm,
|
|
// and in the asm versions of this code, we can get away with zeroing only
|
|
// the first tableSize elements.
|
|
var table [maxTableSize]uint16
|
|
|
|
// sLimit is when to stop looking for offset/length copies. The inputMargin
|
|
// lets us use a fast path for emitLiteral in the main loop, while we are
|
|
// looking for copies.
|
|
sLimit := len(src) - inputMargin
|
|
|
|
// nextEmit is where in src the next emitLiteral should start from.
|
|
nextEmit := 0
|
|
|
|
// The encoded form must start with a literal, as there are no previous
|
|
// bytes to copy, so we start looking for hash matches at s == 1.
|
|
s := 1
|
|
nextHash := hash(load32(src, s), shift)
|
|
|
|
for {
|
|
// Copied from the C++ snappy implementation:
|
|
//
|
|
// Heuristic match skipping: If 32 bytes are scanned with no matches
|
|
// found, start looking only at every other byte. If 32 more bytes are
|
|
// scanned (or skipped), look at every third byte, etc.. When a match
|
|
// is found, immediately go back to looking at every byte. This is a
|
|
// small loss (~5% performance, ~0.1% density) for compressible data
|
|
// due to more bookkeeping, but for non-compressible data (such as
|
|
// JPEG) it's a huge win since the compressor quickly "realizes" the
|
|
// data is incompressible and doesn't bother looking for matches
|
|
// everywhere.
|
|
//
|
|
// The "skip" variable keeps track of how many bytes there are since
|
|
// the last match; dividing it by 32 (ie. right-shifting by five) gives
|
|
// the number of bytes to move ahead for each iteration.
|
|
skip := 32
|
|
|
|
nextS := s
|
|
candidate := 0
|
|
for {
|
|
s = nextS
|
|
bytesBetweenHashLookups := skip >> 5
|
|
nextS = s + bytesBetweenHashLookups
|
|
skip += bytesBetweenHashLookups
|
|
if nextS > sLimit {
|
|
goto emitRemainder
|
|
}
|
|
candidate = int(table[nextHash&tableMask])
|
|
table[nextHash&tableMask] = uint16(s)
|
|
nextHash = hash(load32(src, nextS), shift)
|
|
if load32(src, s) == load32(src, candidate) {
|
|
break
|
|
}
|
|
}
|
|
|
|
// A 4-byte match has been found. We'll later see if more than 4 bytes
|
|
// match. But, prior to the match, src[nextEmit:s] are unmatched. Emit
|
|
// them as literal bytes.
|
|
d += emitLiteral(dst[d:], src[nextEmit:s])
|
|
|
|
// Call emitCopy, and then see if another emitCopy could be our next
|
|
// move. Repeat until we find no match for the input immediately after
|
|
// what was consumed by the last emitCopy call.
|
|
//
|
|
// If we exit this loop normally then we need to call emitLiteral next,
|
|
// though we don't yet know how big the literal will be. We handle that
|
|
// by proceeding to the next iteration of the main loop. We also can
|
|
// exit this loop via goto if we get close to exhausting the input.
|
|
for {
|
|
// Invariant: we have a 4-byte match at s, and no need to emit any
|
|
// literal bytes prior to s.
|
|
base := s
|
|
|
|
// Extend the 4-byte match as long as possible.
|
|
//
|
|
// This is an inlined version of:
|
|
// s = extendMatch(src, candidate+4, s+4)
|
|
s += 4
|
|
for i := candidate + 4; s < len(src) && src[i] == src[s]; i, s = i+1, s+1 {
|
|
}
|
|
|
|
d += emitCopy(dst[d:], base-candidate, s-base)
|
|
nextEmit = s
|
|
if s >= sLimit {
|
|
goto emitRemainder
|
|
}
|
|
|
|
// We could immediately start working at s now, but to improve
|
|
// compression we first update the hash table at s-1 and at s. If
|
|
// another emitCopy is not our next move, also calculate nextHash
|
|
// at s+1. At least on GOARCH=amd64, these three hash calculations
|
|
// are faster as one load64 call (with some shifts) instead of
|
|
// three load32 calls.
|
|
x := load64(src, s-1)
|
|
prevHash := hash(uint32(x>>0), shift)
|
|
table[prevHash&tableMask] = uint16(s - 1)
|
|
currHash := hash(uint32(x>>8), shift)
|
|
candidate = int(table[currHash&tableMask])
|
|
table[currHash&tableMask] = uint16(s)
|
|
if uint32(x>>8) != load32(src, candidate) {
|
|
nextHash = hash(uint32(x>>16), shift)
|
|
s++
|
|
break
|
|
}
|
|
}
|
|
}
|
|
|
|
emitRemainder:
|
|
if nextEmit < len(src) {
|
|
d += emitLiteral(dst[d:], src[nextEmit:])
|
|
}
|
|
return d
|
|
}
|