1# Hyb (hyphenation pattern binary) file format 2 3The hyb file format is how hyphenation patterns are stored in the system image. 4 5Goals include: 6 7* Concise (system image space is at a premium) 8* Usable when mmap'ed, so it doesn't take significant physical RAM 9* Fast to compute 10* Simple 11 12It is _not_ intended as an interchange format, so there is no attempt to make the format 13extensible or facilitate backward and forward compatibility. 14 15Further, at some point we will probably pack patterns for multiple languages into a single 16physical file, to reduce number of open mmap'ed files. This document doesn't cover that. 17 18## Theoretical basis 19 20At heart, the file contains packed tries with suffix compression, actually quite similar 21to the implementation in TeX. 22 23The file contains three sections. The first section represents the "alphabet," including 24case folding. It is effectively a map from Unicode code point to a small integer. 25 26The second section contains the trie in packed form. It is an array of 3-tuples, packed 27into a 32 bit integer. Each (suffix-compressed) trie node has a unique index within this 28array, and the pattern field in the tuple is the pattern for that node. Further, each edge 29in the trie has an entry in the array, and the character and link fields in the tuple 30represent the label and destination of the edge. The packing strategy is as in 31[Word Hy-phen-a-tion by Com-put-er](http://www.tug.org/docs/liang/liang-thesis.pdf) by 32Franklin Mark Liang. 33 34The trie representation is similar but not identical to the "double-array trie". 35The fundamental operation of lookup of the edge from `s` to `t` with label `c` is 36to compare `c == character[s + c]`, and if so, `t = link[s + c]`. 37 38The third section contains the pattern strings. This section is in two parts: first, 39an array with a 3-tuple for each pattern (length, number of trailing 0's, and offset 40into the string pool); and second, the string pool. Each pattern is encoded as a byte 41(packing 2 per byte would be possible but the space savings would not be signficant). 42 43As much as possible of the file is represented as 32 bit integers, as that is especially 44efficent to access. All are little-endian (this could be revised if the code ever needs 45to be ported to big-endian systems). 46 47## Header 48 49``` 50uint32_t magic == 0x62ad7968 51uint32_t version = 0 52uint32_t alphabet_offset (in bytes) 53uint32_t trie_offset (in bytes) 54uint32_t pattern_offset (in bytes) 55uint32_t file_size (in bytes) 56``` 57 58Offsets are from the front of the file, and in bytes. 59 60## Alphabet 61 62The alphabet table comes in two versions. The first is well suited to dense Unicode 63ranges and is limited to 256. The second is more general, but lookups will be slower. 64 65### Alphabet, direct version 66 67``` 68uint32_t version = 0 69uint32_t min_codepoint 70uint32_t max_codepoint (exclusive) 71uint8_t[] data 72``` 73 74The size of the data array is max_codepoint - min_codepoint. 0 represents an unmapped 75character. Note that, in the current implementation, automatic hyphenation is disabled 76for any word containing an unmapped character. 77 78In general, pad bytes follow this table, aligning the next table to a 4-byte boundary. 79 80### Alphabet, general version 81 82``` 83uint32_t version = 1 84uint32_t n_entries 85uint32_t[n_entries] data 86``` 87 88Each element in the data table is `(codepoint << 11) | value`. Note that this is 89restricted to 11 bits (2048 possible values). The largest known current value is 483 90(for Sanskrit). 91 92The entries are sorted by codepoint, to facilitate binary search. Another reasonable 93implementation for consumers of the data would be to build a hash table at load time. 94 95## Trie 96 97``` 98uint32_t version = 0 99uint32_t char_mask 100uint32_t link_shift 101uint32_t link_mask 102uint32_t pattern_shift 103uint32_t n_entries 104uint32_t[n_entries] data 105``` 106 107Each element in the data table is `(pattern << pattern_shift) | (link << link_shift) | char`. 108 109All known pattern tables fit in 32 bits total. If this is exceeded, there is a fairly 110straightforward tweak, where each node occupies a slot by itself (as opposed to sharing 111it with edge slots), which would require very minimal changes to the implementation (TODO 112present in more detail). 113 114## Pattern 115 116``` 117uint32_t version = 0 118uint32_t n_entries 119uint32_t pattern_offset (in bytes) 120uint32_t pattern_size (in bytes) 121uint32_t[n_entries] data 122uint8_t[] pattern_buf 123``` 124 125Each element in data table is `(len << 26) | (shift << 20) | offset`, where an offset of 0 126points to the first byte of pattern_buf. 127 128Generally pattern_offset is `16 + 4 * n_entries`. 129 130For example, 'a4m5ato' would be represented as `[4, 5, 0, 0, 0]`, then len = 2, shift = 3, and 131offset points to [4, 5] in the pattern buffer. 132 133Future extension: additional data representing nonstandard hyphenation. See 134[Automatic non-standard hyphenation in OpenOffice.org](https://www.tug.org/TUGboat/tb27-1/tb86nemeth.pdf) 135for more information about that issue. 136