| Running backref-crosses-blocks.deflate through script/print-bits.go and adding |
| commentary: |
| |
| offset xoffset ASCII hex binary |
| 000000 0x0000 . 0x00 0b_...._.000 Uncompressed block, non-final |
| 000001 0x0001 . 0x04 0b_0000_0100 Literal length: 0x0004 |
| 000002 0x0002 . 0x00 0b_0000_0000 |
| 000003 0x0003 . 0xFB 0b_1111_1011 Inverse: 0xFFFB |
| 000004 0x0004 . 0xFF 0b_1111_1111 |
| 000005 0x0005 a 0x61 0b_0110_0001 Literal "abcd" |
| 000006 0x0006 b 0x62 0b_0110_0010 |
| 000007 0x0007 c 0x63 0b_0110_0011 |
| 000008 0x0008 d 0x64 0b_0110_0100 |
| |
| The fixed Huffman block contains a back-reference (into the previous block) |
| then an end of block. The length=3, distance=2 back-reference decodes to "cdc", |
| so the overall decoding is "abcdcdc". |
| |
| 000009 0x0009 . 0x03 0b_...._.011 Fixed Huffman block, final |
| 000009 0x0009 . 0x03 0b_0000_0... lcode: 257 length: 3 |
| 000010 0x000A B 0x42 0b_...._..10 |
| 000010 0x000A B 0x42 0b_.100_00.. dcode: 1 distance: 2 |
| 000010 0x000A B 0x42 0b_0..._.... lcode: 256 end of block |
| 000011 0x000B . 0x00 0b_..00_0000 |
| |
| This back-reference is valid, even though there is no prior data in that block, |
| because there is prior data in previous blocks. RFC 1951 section 2 "Compressed |
| representation overview" says that "the LZ77 algorithm may use a reference to a |
| duplicated string occurring in a previous block". |
