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Source file src/bytes/bytes_amd64.go

  // Copyright 2016 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 bytes
  
  //go:noescape
  
  // indexShortStr returns the index of the first instance of c in s, or -1 if c is not present in s.
  // indexShortStr requires 2 <= len(c) <= shortStringLen
  func indexShortStr(s, c []byte) int // ../runtime/asm_$GOARCH.s
  func supportAVX2() bool             // ../runtime/asm_$GOARCH.s
  
  var shortStringLen int
  
  func init() {
  	if supportAVX2() {
  		shortStringLen = 63
  	} else {
  		shortStringLen = 31
  	}
  }
  
  // Index returns the index of the first instance of sep in s, or -1 if sep is not present in s.
  func Index(s, sep []byte) int {
  	n := len(sep)
  	switch {
  	case n == 0:
  		return 0
  	case n == 1:
  		return IndexByte(s, sep[0])
  	case n == len(s):
  		if Equal(sep, s) {
  			return 0
  		}
  		return -1
  	case n > len(s):
  		return -1
  	case n <= shortStringLen:
  		// Use brute force when s and sep both are small
  		if len(s) <= 64 {
  			return indexShortStr(s, sep)
  		}
  		c := sep[0]
  		i := 0
  		t := s[:len(s)-n+1]
  		fails := 0
  		for i < len(t) {
  			if t[i] != c {
  				// IndexByte skips 16/32 bytes per iteration,
  				// so it's faster than indexShortStr.
  				o := IndexByte(t[i:], c)
  				if o < 0 {
  					return -1
  				}
  				i += o
  			}
  			if Equal(s[i:i+n], sep) {
  				return i
  			}
  			fails++
  			i++
  			// Switch to indexShortStr when IndexByte produces too many false positives.
  			// Too many means more that 1 error per 8 characters.
  			// Allow some errors in the beginning.
  			if fails > (i+16)/8 {
  				r := indexShortStr(s[i:], sep)
  				if r >= 0 {
  					return r + i
  				}
  				return -1
  			}
  		}
  		return -1
  	}
  	// Rabin-Karp search
  	hashsep, pow := hashStr(sep)
  	var h uint32
  	for i := 0; i < n; i++ {
  		h = h*primeRK + uint32(s[i])
  	}
  	if h == hashsep && Equal(s[:n], sep) {
  		return 0
  	}
  	for i := n; i < len(s); {
  		h *= primeRK
  		h += uint32(s[i])
  		h -= pow * uint32(s[i-n])
  		i++
  		if h == hashsep && Equal(s[i-n:i], sep) {
  			return i - n
  		}
  	}
  	return -1
  }
  
  // primeRK is the prime base used in Rabin-Karp algorithm.
  const primeRK = 16777619
  
  // hashStr returns the hash and the appropriate multiplicative
  // factor for use in Rabin-Karp algorithm.
  func hashStr(sep []byte) (uint32, uint32) {
  	hash := uint32(0)
  	for i := 0; i < len(sep); i++ {
  		hash = hash*primeRK + uint32(sep[i])
  	}
  	var pow, sq uint32 = 1, primeRK
  	for i := len(sep); i > 0; i >>= 1 {
  		if i&1 != 0 {
  			pow *= sq
  		}
  		sq *= sq
  	}
  	return hash, pow
  }
  

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