DEFLATE compresses by referring back to earlier bytes, and each reference starts with a length. Today you turn a length symbol plus its extra bits into an actual copy length.
Convert a literal/length symbol above 256 into a match length using the base-length and extra-bits tables.
DEFLATE’s real compression comes from back-references: “copy N bytes that appeared M bytes ago.” The N is encoded as a length symbol (257 through 285) plus a few extra bits. Rather than a code per possible length, the spec groups lengths into ranges: each symbol carries a base length, and a fixed number of extra bits select the exact value within its range. Symbol 257 is base 3 with no extra bits; 265 is base 11 with 1 extra bit, so it covers lengths 11 and 12; 285 is the single largest, exactly 258.
The extra bits are read with the low-first ReadBits from earlier, not the Huffman order - length symbols come through the Huffman decoder, but their trailing extra bits are ordinary fixed-width values. Build the two small tables (base and extra-bit count, indexed by symbol - 257) and the conversion is one addition. Next you will do the same for the distance half of a reference, and then actually perform the copy.
// symbol 257 -> base 3; 258->4 ... 264->10 (all 0 extra). Then extra bits// grow: 265..268 have 1 extra, 269..272 have 2, and so on; 285 -> 258 exactly.// length = base[sym] + ReadBits(extra[sym])var lenBase [29]int; var lenExtra [29]int // indexed by sym-257