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Source file src/encoding/json/fold.go

  // 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.
  
  package json
  
  import (
  	"bytes"
  	"unicode/utf8"
  )
  
  const (
  	caseMask     = ^byte(0x20) // Mask to ignore case in ASCII.
  	kelvin       = '\u212a'
  	smallLongEss = '\u017f'
  )
  
  // foldFunc returns one of four different case folding equivalence
  // functions, from most general (and slow) to fastest:
  //
  // 1) bytes.EqualFold, if the key s contains any non-ASCII UTF-8
  // 2) equalFoldRight, if s contains special folding ASCII ('k', 'K', 's', 'S')
  // 3) asciiEqualFold, no special, but includes non-letters (including _)
  // 4) simpleLetterEqualFold, no specials, no non-letters.
  //
  // The letters S and K are special because they map to 3 runes, not just 2:
  //  * S maps to s and to U+017F 'ſ' Latin small letter long s
  //  * k maps to K and to U+212A 'K' Kelvin sign
  // See https://play.golang.org/p/tTxjOc0OGo
  //
  // The returned function is specialized for matching against s and
  // should only be given s. It's not curried for performance reasons.
  func foldFunc(s []byte) func(s, t []byte) bool {
  	nonLetter := false
  	special := false // special letter
  	for _, b := range s {
  		if b >= utf8.RuneSelf {
  			return bytes.EqualFold
  		}
  		upper := b & caseMask
  		if upper < 'A' || upper > 'Z' {
  			nonLetter = true
  		} else if upper == 'K' || upper == 'S' {
  			// See above for why these letters are special.
  			special = true
  		}
  	}
  	if special {
  		return equalFoldRight
  	}
  	if nonLetter {
  		return asciiEqualFold
  	}
  	return simpleLetterEqualFold
  }
  
  // equalFoldRight is a specialization of bytes.EqualFold when s is
  // known to be all ASCII (including punctuation), but contains an 's',
  // 'S', 'k', or 'K', requiring a Unicode fold on the bytes in t.
  // See comments on foldFunc.
  func equalFoldRight(s, t []byte) bool {
  	for _, sb := range s {
  		if len(t) == 0 {
  			return false
  		}
  		tb := t[0]
  		if tb < utf8.RuneSelf {
  			if sb != tb {
  				sbUpper := sb & caseMask
  				if 'A' <= sbUpper && sbUpper <= 'Z' {
  					if sbUpper != tb&caseMask {
  						return false
  					}
  				} else {
  					return false
  				}
  			}
  			t = t[1:]
  			continue
  		}
  		// sb is ASCII and t is not. t must be either kelvin
  		// sign or long s; sb must be s, S, k, or K.
  		tr, size := utf8.DecodeRune(t)
  		switch sb {
  		case 's', 'S':
  			if tr != smallLongEss {
  				return false
  			}
  		case 'k', 'K':
  			if tr != kelvin {
  				return false
  			}
  		default:
  			return false
  		}
  		t = t[size:]
  
  	}
  	if len(t) > 0 {
  		return false
  	}
  	return true
  }
  
  // asciiEqualFold is a specialization of bytes.EqualFold for use when
  // s is all ASCII (but may contain non-letters) and contains no
  // special-folding letters.
  // See comments on foldFunc.
  func asciiEqualFold(s, t []byte) bool {
  	if len(s) != len(t) {
  		return false
  	}
  	for i, sb := range s {
  		tb := t[i]
  		if sb == tb {
  			continue
  		}
  		if ('a' <= sb && sb <= 'z') || ('A' <= sb && sb <= 'Z') {
  			if sb&caseMask != tb&caseMask {
  				return false
  			}
  		} else {
  			return false
  		}
  	}
  	return true
  }
  
  // simpleLetterEqualFold is a specialization of bytes.EqualFold for
  // use when s is all ASCII letters (no underscores, etc) and also
  // doesn't contain 'k', 'K', 's', or 'S'.
  // See comments on foldFunc.
  func simpleLetterEqualFold(s, t []byte) bool {
  	if len(s) != len(t) {
  		return false
  	}
  	for i, b := range s {
  		if b&caseMask != t[i]&caseMask {
  			return false
  		}
  	}
  	return true
  }
  

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