kesavand | c71914f | 2022-03-25 11:19:03 +0530 | [diff] [blame^] | 1 | // Copyright 2016 The Snappy-Go Authors. All rights reserved. |
| 2 | // Use of this source code is governed by a BSD-style |
| 3 | // license that can be found in the LICENSE file. |
| 4 | |
| 5 | package snapref |
| 6 | |
| 7 | // decode writes the decoding of src to dst. It assumes that the varint-encoded |
| 8 | // length of the decompressed bytes has already been read, and that len(dst) |
| 9 | // equals that length. |
| 10 | // |
| 11 | // It returns 0 on success or a decodeErrCodeXxx error code on failure. |
| 12 | func decode(dst, src []byte) int { |
| 13 | var d, s, offset, length int |
| 14 | for s < len(src) { |
| 15 | switch src[s] & 0x03 { |
| 16 | case tagLiteral: |
| 17 | x := uint32(src[s] >> 2) |
| 18 | switch { |
| 19 | case x < 60: |
| 20 | s++ |
| 21 | case x == 60: |
| 22 | s += 2 |
| 23 | if uint(s) > uint(len(src)) { // The uint conversions catch overflow from the previous line. |
| 24 | return decodeErrCodeCorrupt |
| 25 | } |
| 26 | x = uint32(src[s-1]) |
| 27 | case x == 61: |
| 28 | s += 3 |
| 29 | if uint(s) > uint(len(src)) { // The uint conversions catch overflow from the previous line. |
| 30 | return decodeErrCodeCorrupt |
| 31 | } |
| 32 | x = uint32(src[s-2]) | uint32(src[s-1])<<8 |
| 33 | case x == 62: |
| 34 | s += 4 |
| 35 | if uint(s) > uint(len(src)) { // The uint conversions catch overflow from the previous line. |
| 36 | return decodeErrCodeCorrupt |
| 37 | } |
| 38 | x = uint32(src[s-3]) | uint32(src[s-2])<<8 | uint32(src[s-1])<<16 |
| 39 | case x == 63: |
| 40 | s += 5 |
| 41 | if uint(s) > uint(len(src)) { // The uint conversions catch overflow from the previous line. |
| 42 | return decodeErrCodeCorrupt |
| 43 | } |
| 44 | x = uint32(src[s-4]) | uint32(src[s-3])<<8 | uint32(src[s-2])<<16 | uint32(src[s-1])<<24 |
| 45 | } |
| 46 | length = int(x) + 1 |
| 47 | if length <= 0 { |
| 48 | return decodeErrCodeUnsupportedLiteralLength |
| 49 | } |
| 50 | if length > len(dst)-d || length > len(src)-s { |
| 51 | return decodeErrCodeCorrupt |
| 52 | } |
| 53 | copy(dst[d:], src[s:s+length]) |
| 54 | d += length |
| 55 | s += length |
| 56 | continue |
| 57 | |
| 58 | case tagCopy1: |
| 59 | s += 2 |
| 60 | if uint(s) > uint(len(src)) { // The uint conversions catch overflow from the previous line. |
| 61 | return decodeErrCodeCorrupt |
| 62 | } |
| 63 | length = 4 + int(src[s-2])>>2&0x7 |
| 64 | offset = int(uint32(src[s-2])&0xe0<<3 | uint32(src[s-1])) |
| 65 | |
| 66 | case tagCopy2: |
| 67 | s += 3 |
| 68 | if uint(s) > uint(len(src)) { // The uint conversions catch overflow from the previous line. |
| 69 | return decodeErrCodeCorrupt |
| 70 | } |
| 71 | length = 1 + int(src[s-3])>>2 |
| 72 | offset = int(uint32(src[s-2]) | uint32(src[s-1])<<8) |
| 73 | |
| 74 | case tagCopy4: |
| 75 | s += 5 |
| 76 | if uint(s) > uint(len(src)) { // The uint conversions catch overflow from the previous line. |
| 77 | return decodeErrCodeCorrupt |
| 78 | } |
| 79 | length = 1 + int(src[s-5])>>2 |
| 80 | offset = int(uint32(src[s-4]) | uint32(src[s-3])<<8 | uint32(src[s-2])<<16 | uint32(src[s-1])<<24) |
| 81 | } |
| 82 | |
| 83 | if offset <= 0 || d < offset || length > len(dst)-d { |
| 84 | return decodeErrCodeCorrupt |
| 85 | } |
| 86 | // Copy from an earlier sub-slice of dst to a later sub-slice. |
| 87 | // If no overlap, use the built-in copy: |
| 88 | if offset >= length { |
| 89 | copy(dst[d:d+length], dst[d-offset:]) |
| 90 | d += length |
| 91 | continue |
| 92 | } |
| 93 | |
| 94 | // Unlike the built-in copy function, this byte-by-byte copy always runs |
| 95 | // forwards, even if the slices overlap. Conceptually, this is: |
| 96 | // |
| 97 | // d += forwardCopy(dst[d:d+length], dst[d-offset:]) |
| 98 | // |
| 99 | // We align the slices into a and b and show the compiler they are the same size. |
| 100 | // This allows the loop to run without bounds checks. |
| 101 | a := dst[d : d+length] |
| 102 | b := dst[d-offset:] |
| 103 | b = b[:len(a)] |
| 104 | for i := range a { |
| 105 | a[i] = b[i] |
| 106 | } |
| 107 | d += length |
| 108 | } |
| 109 | if d != len(dst) { |
| 110 | return decodeErrCodeCorrupt |
| 111 | } |
| 112 | return 0 |
| 113 | } |