Scott Baker | 8487c5d | 2019-10-18 12:49:46 -0700 | [diff] [blame^] | 1 | // +build !amd64 appengine !gc noasm |
| 2 | |
| 3 | package lz4 |
| 4 | |
| 5 | func decodeBlock(dst, src []byte) (ret int) { |
| 6 | const hasError = -2 |
| 7 | defer func() { |
| 8 | if recover() != nil { |
| 9 | ret = hasError |
| 10 | } |
| 11 | }() |
| 12 | |
| 13 | var si, di int |
| 14 | for { |
| 15 | // Literals and match lengths (token). |
| 16 | b := int(src[si]) |
| 17 | si++ |
| 18 | |
| 19 | // Literals. |
| 20 | if lLen := b >> 4; lLen > 0 { |
| 21 | switch { |
| 22 | case lLen < 0xF && si+16 < len(src): |
| 23 | // Shortcut 1 |
| 24 | // if we have enough room in src and dst, and the literals length |
| 25 | // is small enough (0..14) then copy all 16 bytes, even if not all |
| 26 | // are part of the literals. |
| 27 | copy(dst[di:], src[si:si+16]) |
| 28 | si += lLen |
| 29 | di += lLen |
| 30 | if mLen := b & 0xF; mLen < 0xF { |
| 31 | // Shortcut 2 |
| 32 | // if the match length (4..18) fits within the literals, then copy |
| 33 | // all 18 bytes, even if not all are part of the literals. |
| 34 | mLen += 4 |
| 35 | if offset := int(src[si]) | int(src[si+1])<<8; mLen <= offset { |
| 36 | i := di - offset |
| 37 | end := i + 18 |
| 38 | if end > len(dst) { |
| 39 | // The remaining buffer may not hold 18 bytes. |
| 40 | // See https://github.com/pierrec/lz4/issues/51. |
| 41 | end = len(dst) |
| 42 | } |
| 43 | copy(dst[di:], dst[i:end]) |
| 44 | si += 2 |
| 45 | di += mLen |
| 46 | continue |
| 47 | } |
| 48 | } |
| 49 | case lLen == 0xF: |
| 50 | for src[si] == 0xFF { |
| 51 | lLen += 0xFF |
| 52 | si++ |
| 53 | } |
| 54 | lLen += int(src[si]) |
| 55 | si++ |
| 56 | fallthrough |
| 57 | default: |
| 58 | copy(dst[di:di+lLen], src[si:si+lLen]) |
| 59 | si += lLen |
| 60 | di += lLen |
| 61 | } |
| 62 | } |
| 63 | if si >= len(src) { |
| 64 | return di |
| 65 | } |
| 66 | |
| 67 | offset := int(src[si]) | int(src[si+1])<<8 |
| 68 | if offset == 0 { |
| 69 | return hasError |
| 70 | } |
| 71 | si += 2 |
| 72 | |
| 73 | // Match. |
| 74 | mLen := b & 0xF |
| 75 | if mLen == 0xF { |
| 76 | for src[si] == 0xFF { |
| 77 | mLen += 0xFF |
| 78 | si++ |
| 79 | } |
| 80 | mLen += int(src[si]) |
| 81 | si++ |
| 82 | } |
| 83 | mLen += minMatch |
| 84 | |
| 85 | // Copy the match. |
| 86 | expanded := dst[di-offset:] |
| 87 | if mLen > offset { |
| 88 | // Efficiently copy the match dst[di-offset:di] into the dst slice. |
| 89 | bytesToCopy := offset * (mLen / offset) |
| 90 | for n := offset; n <= bytesToCopy+offset; n *= 2 { |
| 91 | copy(expanded[n:], expanded[:n]) |
| 92 | } |
| 93 | di += bytesToCopy |
| 94 | mLen -= bytesToCopy |
| 95 | } |
| 96 | di += copy(dst[di:di+mLen], expanded[:mLen]) |
| 97 | } |
| 98 | } |