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// Copyright 2011 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.
package snappy
import (
"encoding/binary"
)
// We limit how far copy back-references can go, the same as the C++ code.
const maxOffset = 1 << 15
// emitLiteral writes a literal chunk and returns the number of bytes written.
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
case n < 1<<16:
dst[0] = 61<<2 | tagLiteral
dst[1] = uint8(n)
dst[2] = uint8(n >> 8)
i = 3
case n < 1<<24:
dst[0] = 62<<2 | tagLiteral
dst[1] = uint8(n)
dst[2] = uint8(n >> 8)
dst[3] = uint8(n >> 16)
i = 4
case int64(n) < 1<<32:
dst[0] = 63<<2 | tagLiteral
dst[1] = uint8(n)
dst[2] = uint8(n >> 8)
dst[3] = uint8(n >> 16)
dst[4] = uint8(n >> 24)
i = 5
default:
panic("snappy: source buffer is too long")
}
if copy(dst[i:], lit) != len(lit) {
panic("snappy: destination buffer is too short")
}
return i + len(lit)
}
// emitCopy writes a copy chunk and returns the number of bytes written.
func emitCopy(dst []byte, offset, length int) int {
i := 0
for length > 0 {
x := length - 4
if 0 <= x && x < 1<<3 && offset < 1<<11 {
dst[i+0] = uint8(offset>>8)&0x07<<5 | uint8(x)<<2 | tagCopy1
dst[i+1] = uint8(offset)
i += 2
break
}
x = length
if x > 1<<6 {
x = 1 << 6
}
dst[i+0] = uint8(x-1)<<2 | tagCopy2
dst[i+1] = uint8(offset)
dst[i+2] = uint8(offset >> 8)
i += 3
length -= x
}
return i
}
// Encode returns the encoded form of src. The returned slice may be a sub-
// slice of dst if dst was large enough to hold the entire encoded block.
// Otherwise, a newly allocated slice will be returned.
// It is valid to pass a nil dst.
func Encode(dst, src []byte) ([]byte, error) {
if n := MaxEncodedLen(len(src)); len(dst) < n {
dst = make([]byte, n)
}
// The block starts with the varint-encoded length of the decompressed bytes.
d := binary.PutUvarint(dst, uint64(len(src)))
// Return early if src is short.
if len(src) <= 4 {
if len(src) != 0 {
d += emitLiteral(dst[d:], src)
}
return dst[:d], nil
}
// Initialize the hash table. Its size ranges from 1<<8 to 1<<14 inclusive.
const maxTableSize = 1 << 14
shift, tableSize := uint(32-8), 1<<8
for tableSize < maxTableSize && tableSize < len(src) {
shift--
tableSize *= 2
}
var table [maxTableSize]int
// Iterate over the source bytes.
var (
s int // The iterator position.
t int // The last position with the same hash as s.
lit int // The start position of any pending literal bytes.
)
for s+3 < len(src) {
// Update the hash table.
b0, b1, b2, b3 := src[s], src[s+1], src[s+2], src[s+3]
h := uint32(b0) | uint32(b1)<<8 | uint32(b2)<<16 | uint32(b3)<<24
p := &table[(h*0x1e35a7bd)>>shift]
// We need to to store values in [-1, inf) in table. To save
// some initialization time, (re)use the table's zero value
// and shift the values against this zero: add 1 on writes,
// subtract 1 on reads.
t, *p = *p-1, s+1
// If t is invalid or src[s:s+4] differs from src[t:t+4], accumulate a literal byte.
if t < 0 || s-t >= maxOffset || b0 != src[t] || b1 != src[t+1] || b2 != src[t+2] || b3 != src[t+3] {
s++
continue
}
// Otherwise, we have a match. First, emit any pending literal bytes.
if lit != s {
d += emitLiteral(dst[d:], src[lit:s])
}
// Extend the match to be as long as possible.
s0 := s
s, t = s+4, t+4
for s < len(src) && src[s] == src[t] {
s++
t++
}
// Emit the copied bytes.
d += emitCopy(dst[d:], s-t, s-s0)
lit = s
}
// Emit any final pending literal bytes and return.
if lit != len(src) {
d += emitLiteral(dst[d:], src[lit:])
}
return dst[:d], nil
}
// MaxEncodedLen returns the maximum length of a snappy block, given its
// uncompressed length.
func MaxEncodedLen(srcLen int) int {
// Compressed data can be defined as:
// compressed := item* literal*
// item := literal* copy
//
// The trailing literal sequence has a space blowup of at most 62/60
// since a literal of length 60 needs one tag byte + one extra byte
// for length information.
//
// Item blowup is trickier to measure. Suppose the "copy" op copies
// 4 bytes of data. Because of a special check in the encoding code,
// we produce a 4-byte copy only if the offset is < 65536. Therefore
// the copy op takes 3 bytes to encode, and this type of item leads
// to at most the 62/60 blowup for representing literals.
//
// Suppose the "copy" op copies 5 bytes of data. If the offset is big
// enough, it will take 5 bytes to encode the copy op. Therefore the
// worst case here is a one-byte literal followed by a five-byte copy.
// That is, 6 bytes of input turn into 7 bytes of "compressed" data.
//
// This last factor dominates the blowup, so the final estimate is:
return 32 + srcLen + srcLen/6
}
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