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// Copyright 2013 The Go Authors. All rights reserved.
//
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file or at
// https://developers.google.com/open-source/licenses/bsd.
// This file implements the Paice/Husk stemming algorithm.
// http://www.comp.lancs.ac.uk/computing/research/stemming/Links/paice.htm
package database
import (
"bytes"
"regexp"
"strconv"
)
const stemRuleText = `
ai*2. a*1.
bb1.
city3s. ci2> cn1t>
dd1. dei3y> deec2ss. dee1. de2> dooh4>
e1>
feil1v. fi2>
gni3> gai3y. ga2> gg1.
ht*2. hsiug5ct. hsi3>
i*1. i1y>
ji1d. juf1s. ju1d. jo1d. jeh1r. jrev1t. jsim2t. jn1d. j1s.
lbaifi6. lbai4y. lba3> lbi3. lib2l> lc1. lufi4y. luf3> lu2. lai3> lau3> la2> ll1.
mui3. mu*2. msi3> mm1.
nois4j> noix4ct. noi3> nai3> na2> nee0. ne2> nn1.
pihs4> pp1.
re2> rae0. ra2. ro2> ru2> rr1. rt1> rei3y>
sei3y> sis2. si2> ssen4> ss0. suo3> su*2. s*1> s0.
tacilp4y. ta2> tnem4> tne3> tna3> tpir2b. tpro2b. tcud1. tpmus2. tpec2iv. tulo2v. tsis0. tsi3> tt1.
uqi3. ugo1.
vis3j> vie0. vi2>
ylb1> yli3y> ylp0. yl2> ygo1. yhp1. ymo1. ypo1. yti3> yte3> ytl2. yrtsi5. yra3> yro3> yfi3. ycn2t> yca3>
zi2> zy1s.
`
type stemRule struct {
text string
suffix []byte
intact bool
remove int
append []byte
more bool
}
func parseStemRules() map[byte][]*stemRule {
rules := make(map[byte][]*stemRule)
for _, m := range regexp.MustCompile(`(?m)(?:^| )([a-zA-Z]*)(\*?)([0-9])([a-zA-z]*)([.>])`).FindAllStringSubmatch(stemRuleText, -1) {
suffix := []byte(m[1])
for i := 0; i < len(suffix)/2; i++ {
j := len(suffix) - 1 - i
suffix[i], suffix[j] = suffix[j], suffix[i]
}
remove, _ := strconv.Atoi(m[3])
r := &stemRule{
text: m[0],
suffix: suffix,
intact: m[2] == "*",
remove: remove,
append: []byte(m[4]),
more: m[5] == ">",
}
c := suffix[len(suffix)-1]
rules[c] = append(rules[c], r)
}
return rules
}
var stemRules = parseStemRules()
func firstVowel(offset int, p []byte) int {
for i, b := range p {
switch b {
case 'a', 'e', 'i', 'o', 'u':
return offset + i
case 'y':
if offset+i > 0 {
return offset + i
}
}
}
return -1
}
func acceptableStem(a, b []byte) bool {
i := firstVowel(0, a)
if i < 0 {
i = firstVowel(len(a), b)
}
l := len(a) + len(b)
if i == 0 {
return l > 1
}
return i >= 0 && l > 2
}
func stem(s string) string {
stem := bytes.ToLower([]byte(s))
intact := true
run := acceptableStem(stem, []byte{})
for run {
run = false
for _, rule := range stemRules[stem[len(stem)-1]] {
if bytes.HasSuffix(stem, rule.suffix) &&
(intact || !rule.intact) &&
acceptableStem(stem[:len(stem)-rule.remove], rule.append) {
stem = append(stem[:len(stem)-rule.remove], rule.append...)
intact = false
run = rule.more
break
}
}
}
return string(stem)
}