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// Copyright (c) 2014 Couchbase, Inc.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// Package lowercase implements a TokenFilter which converts
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// tokens to lower case according to unicode rules.
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package lowercase
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import (
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"bytes"
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"unicode"
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"unicode/utf8"
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"github.com/blevesearch/bleve/v2/analysis"
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"github.com/blevesearch/bleve/v2/registry"
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)
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// Name is the name used to register LowerCaseFilter in the bleve registry
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const Name = "to_lower"
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type LowerCaseFilter struct {
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}
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func NewLowerCaseFilter() *LowerCaseFilter {
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return &LowerCaseFilter{}
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}
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func (f *LowerCaseFilter) Filter(input analysis.TokenStream) analysis.TokenStream {
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for _, token := range input {
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token.Term = toLowerDeferredCopy(token.Term)
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}
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return input
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}
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func LowerCaseFilterConstructor(config map[string]interface{}, cache *registry.Cache) (analysis.TokenFilter, error) {
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return NewLowerCaseFilter(), nil
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}
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func init() {
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registry.RegisterTokenFilter(Name, LowerCaseFilterConstructor)
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}
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// toLowerDeferredCopy will function exactly like
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// bytes.ToLower() only it will reuse (overwrite)
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// the original byte array when possible
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// NOTE: because its possible that the lower-case
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// form of a rune has a different utf-8 encoded
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// length, in these cases a new byte array is allocated
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func toLowerDeferredCopy(s []byte) []byte {
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j := 0
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for i := 0; i < len(s); {
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wid := 1
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r := rune(s[i])
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if r >= utf8.RuneSelf {
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r, wid = utf8.DecodeRune(s[i:])
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}
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l := unicode.ToLower(r)
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// If the rune is already lowercased, just move to the
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// next rune.
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if l == r {
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i += wid
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j += wid
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continue
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}
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// Handles the Unicode edge-case where the last
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// rune in a word on the greek Σ needs to be converted
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// differently.
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if l == 'σ' && i+2 == len(s) {
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l = 'ς'
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}
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lwid := utf8.RuneLen(l)
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if lwid > wid {
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// utf-8 encoded replacement is wider
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// for now, punt and defer
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// to bytes.ToLower() for the remainder
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// only known to happen with chars
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// Rune Ⱥ(570) width 2 - Lower ⱥ(11365) width 3
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// Rune Ⱦ(574) width 2 - Lower ⱦ(11366) width 3
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rest := bytes.ToLower(s[i:])
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rv := make([]byte, j+len(rest))
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copy(rv[:j], s[:j])
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copy(rv[j:], rest)
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return rv
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} else {
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utf8.EncodeRune(s[j:], l)
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}
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i += wid
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j += lwid
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}
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return s[:j]
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}
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