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