This generates the tables and indexes which will be used by the HPACK
encoder. The headers are sorted by length, then by statistical frequency,
then by direction (preference for responses), then by name, then by index.
The purpose is to speed up their lookup.
This one was built by studying the HPACK Huffman table (RFC7541
appendix B). It creates 5 small tables (4*512 bytes, 1*64 bytes) to
map one byte at a time from the input stream based on the following
observations :
* rht_bit31_24[256] is indexed on bits 31..24 when < 0xfe
* rht_bit24_17[256] is indexed on bits 24..17 when 31..24 >= 0xfe
* rht_bit15_11_fe[32] is indexed on bits 15..11 when 24..17 == 0xfe
* rht_bit15_8[256] is indexed on bits 15..8 when 24..17 == 0xff
* rht_bit11_4[256] is indexed on bits 11..4 when 15..8 == 0xff
* when 11..4 == 0xff, 3..2 provide the following mapping :
* 00 => 0x0a, 01 => 0x0d, 10 => 0x16, 11 => EOS
Willy Tarreau