github.com/panjjo/go@v0.0.0-20161104043856-d62b31386338/src/strings/strings_amd64.go (about)

     1  // Copyright 2015 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package strings
     6  
     7  //go:noescape
     8  
     9  // indexShortStr returns the index of the first instance of c in s, or -1 if c is not present in s.
    10  // indexShortStr requires 2 <= len(c) <= shortStringLen
    11  func indexShortStr(s, c string) int // ../runtime/asm_$GOARCH.s
    12  func supportAVX2() bool             // ../runtime/asm_$GOARCH.s
    13  
    14  var shortStringLen int
    15  
    16  func init() {
    17  	if supportAVX2() {
    18  		shortStringLen = 63
    19  	} else {
    20  		shortStringLen = 31
    21  	}
    22  }
    23  
    24  // Index returns the index of the first instance of sep in s, or -1 if sep is not present in s.
    25  func Index(s, sep string) int {
    26  	n := len(sep)
    27  	switch {
    28  	case n == 0:
    29  		return 0
    30  	case n == 1:
    31  		return IndexByte(s, sep[0])
    32  	case n == len(s):
    33  		if sep == s {
    34  			return 0
    35  		}
    36  		return -1
    37  	case n > len(s):
    38  		return -1
    39  	case n <= shortStringLen:
    40  		// Use brute force when s and sep both are small
    41  		if len(s) <= 64 {
    42  			return indexShortStr(s, sep)
    43  		}
    44  		c := sep[0]
    45  		i := 0
    46  		t := s[:len(s)-n+1]
    47  		fails := 0
    48  		for i < len(t) {
    49  			if t[i] != c {
    50  				// IndexByte skips 16/32 bytes per iteration,
    51  				// so it's faster than indexShortStr.
    52  				o := IndexByte(t[i:], c)
    53  				if o < 0 {
    54  					return -1
    55  				}
    56  				i += o
    57  			}
    58  			if s[i:i+n] == sep {
    59  				return i
    60  			}
    61  			fails++
    62  			i++
    63  			// Switch to indexShortStr when IndexByte produces too many false positives.
    64  			// Too many means more that 1 error per 8 characters.
    65  			// Allow some errors in the beginning.
    66  			if fails > (i+16)/8 {
    67  				r := indexShortStr(s[i:], sep)
    68  				if r >= 0 {
    69  					return r + i
    70  				}
    71  				return -1
    72  			}
    73  		}
    74  		return -1
    75  	}
    76  	// Rabin-Karp search
    77  	hashsep, pow := hashStr(sep)
    78  	var h uint32
    79  	for i := 0; i < n; i++ {
    80  		h = h*primeRK + uint32(s[i])
    81  	}
    82  	if h == hashsep && s[:n] == sep {
    83  		return 0
    84  	}
    85  	for i := n; i < len(s); {
    86  		h *= primeRK
    87  		h += uint32(s[i])
    88  		h -= pow * uint32(s[i-n])
    89  		i++
    90  		if h == hashsep && s[i-n:i] == sep {
    91  			return i - n
    92  		}
    93  	}
    94  	return -1
    95  }