1 #ifndef _PARSE_TRIGRAMS_H
2 #define _PARSE_TRIGRAMS_H 1
11 // One or more trigrams, with an implicit OR between them. For case-sensitive searches,
12 // this is just e.g. “abc”, but for case-insensitive, it would be “abc OR abC or aBc ...” etc.
13 struct TrigramDisjunction {
14 unsigned index; // For debugging only.
16 // The alternatives as determined by parse_trigrams().
17 std::vector<uint32_t> trigram_alternatives;
19 // Like trigram_alternatives, but only the ones we've actually read from the
20 // hash table (the non-existent ones are filtered out). The second member is
21 // the length in bytes. Incomplete if remaining_trigrams_to_read > 0.
22 std::vector<std::pair<Trigram, size_t>> read_trigrams;
24 // Sum of num_docids in all trigrams. This is usually a fairly good indicator
25 // of the real number of docids, since there are few files that would have e.g.
26 // both abc and abC in them (but of course, with multiple files in the same
27 // docid block, it is far from unheard of).
28 uint32_t max_num_docids;
30 // While reading posting lists: Holds the union of the posting lists read
31 // so far. Once remaining_trigrams_to_read == 0 (all are read), will be taken
32 // out and used for intersections against the other disjunctions.
33 std::vector<uint32_t> docids;
35 // While looking up in the hash table (filling out read_trigrams): Number of
36 // lookups in the hash table remaining. While reading actual posting lists
37 // (filling out docids): Number of posting lists left to read.
38 unsigned remaining_trigrams_to_read;
41 // Take the given needle (search string) and break it down into a set of trigrams
42 // (or trigram alternatives; see TrigramDisjunction) that must be present for the
43 // string to match. (Note: They are not _sufficient_ for the string to match;
44 // false positives might very well occur and must be weeded out later.)
46 // For the case-sensitive case, this is straightforward; just take every trigram
47 // present in the needle and add them (e.g. abcd -> abc AND bcd).
48 // For case-insensitivity, it's trickier; see the comments in the function.
50 // Note that our trigrams are on the basis of bytes, not Unicode code points.
51 // This both simplifies table structure (everything is the same length), and
52 // guards us against trigram explosion (imagine every combination of CJK characters
53 // getting their own trigram).
54 void parse_trigrams(const std::string &needle, bool ignore_case, std::vector<TrigramDisjunction> *trigram_groups);
56 static constexpr uint32_t WILDCARD_UNIGRAM = 0xFF000000;
57 static constexpr uint32_t PREMATURE_END_UNIGRAM = 0xFF000001;
59 // Reads a unigram, taking into account escaping (\<foo> becomes <foo>).
60 // Returns WILDCARD_UNIGRAM if there's an invalid unigram, ie., we found
61 // a glob character (?, * or a [] group). Returns EOS_UNIGRAM if we went
62 // past the end of the string, e.g., a string that ends in a backslash.
63 // The second element is always the length.
64 std::pair<uint32_t, size_t> read_unigram(const std::string &s, size_t start);
66 // Reads a trigram, ie., three calls to read_unigram(). Needs to start on a valid unigram.
67 // Returns WILDCARD_UNIGRAM or PREMATURE_END_UNIGRAM of either of those occurred
68 // during reading of the string.
69 uint32_t read_trigram(const std::string &s, size_t start);
72 std::string print_td(const TrigramDisjunction &td);
73 std::string print_trigram(uint32_t trgm);
75 #endif // !defined(_PARSE_TRIGRAMS_H)