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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"
"errors"
)
// ErrCorrupt reports that the input is invalid.
var ErrCorrupt = errors.New("snappy: corrupt input")
// DecodedLen returns the length of the decoded block.
func DecodedLen(src []byte) (int, error) {
v, _, err := decodedLen(src)
return v, err
}
// decodedLen returns the length of the decoded block and the number of bytes
// that the length header occupied.
func decodedLen(src []byte) (blockLen, headerLen int, err error) {
v, n := binary.Uvarint(src)
if n == 0 {
return 0, 0, ErrCorrupt
}
if uint64(int(v)) != v {
return 0, 0, errors.New("snappy: decoded block is too large")
}
return int(v), n, nil
}
// Decode returns the decoded form of src. The returned slice may be a sub-
// slice of dst if dst was large enough to hold the entire decoded block.
// Otherwise, a newly allocated slice will be returned.
// It is valid to pass a nil dst.
func Decode(dst, src []byte) ([]byte, error) {
dLen, s, err := decodedLen(src)
if err != nil {
return nil, err
}
if len(dst) < dLen {
dst = make([]byte, dLen)
}
var d, offset, length int
for s < len(src) {
switch src[s] & 0x03 {
case tagLiteral:
x := uint(src[s] >> 2)
switch {
case x < 60:
s += 1
case x == 60:
s += 2
if s > len(src) {
return nil, ErrCorrupt
}
x = uint(src[s-1])
case x == 61:
s += 3
if s > len(src) {
return nil, ErrCorrupt
}
x = uint(src[s-2]) | uint(src[s-1])<<8
case x == 62:
s += 4
if s > len(src) {
return nil, ErrCorrupt
}
x = uint(src[s-3]) | uint(src[s-2])<<8 | uint(src[s-1])<<16
case x == 63:
s += 5
if s > len(src) {
return nil, ErrCorrupt
}
x = uint(src[s-4]) | uint(src[s-3])<<8 | uint(src[s-2])<<16 | uint(src[s-1])<<24
}
length = int(x + 1)
if length <= 0 {
return nil, errors.New("snappy: unsupported literal length")
}
if length > len(dst)-d || length > len(src)-s {
return nil, ErrCorrupt
}
copy(dst[d:], src[s:s+length])
d += length
s += length
continue
case tagCopy1:
s += 2
if s > len(src) {
return nil, ErrCorrupt
}
length = 4 + int(src[s-2])>>2&0x7
offset = int(src[s-2])&0xe0<<3 | int(src[s-1])
case tagCopy2:
s += 3
if s > len(src) {
return nil, ErrCorrupt
}
length = 1 + int(src[s-3])>>2
offset = int(src[s-2]) | int(src[s-1])<<8
case tagCopy4:
return nil, errors.New("snappy: unsupported COPY_4 tag")
}
end := d + length
if offset > d || end > len(dst) {
return nil, ErrCorrupt
}
for ; d < end; d++ {
dst[d] = dst[d-offset]
}
}
if d != dLen {
return nil, ErrCorrupt
}
return dst[:d], nil
}
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