strings.go包实现了一个Rabin-Karp算法.有点意思.
关于这个算法:
图灵社区有一篇:
图说Rabin-Karp字符串查找算法
关于Go源码实现:
网友GoLove已写一个篇非常详细的说明了. http://www.cnblogs.com/golove/p/3234673.html
GoLove那个已经分析的非常清楚了,只是前面那一串说明太长了.我把他的说明替换成代码形式.
直接跑起来,这样更能看得清楚些.
package main
import (
"fmt"
"unicode/utf8"
)
func main(){
count := Count("9876520210520","520")
fmt.Println("count==",count)
}
// primeRK is the prime base used in Rabin-Karp algorithm.
//primeRK相当于进制
//本例中,只用到0-9这10个数字,即所有字符的总个数为10,所以定为10
//源码中是16777619,即相当于16777619进制
//The magic is in the interesting relationship between the special prime
//16777619 (2^24 + 403) and 2^32 and 2^8.
const primeRK = 10 // 16777619
// hashStr returns the hash and the appropriate multiplicative
// factor for use in Rabin-Karp algorithm.
func hashStr(sep string) (uint32, uint32) {
hash := uint32(0)
charcode := [...]uint32{5,2,0}
for i := 0; i < len(sep); i++ {
//hash = hash*primeRK + uint32(sep[i])
hash = hash*primeRK + charcode[i]
}
//即相当于千位->百位->十位,得到乘数因子(pow),本例中的520,得到的pow是1000
var pow, sq uint32 = 1, primeRK
for i := len(sep); i > 0; i >>= 1 { //len(sep)=3 i>>{1,0} sq:{10,100}
if i&1 != 0 {
pow *= sq
}
sq *= sq
}
/*
var pow uint32 = 1
for i := len(sep); i > 0; i-- {
pow *= primeRK
}
*/
fmt.Println("hashStr() sep:",sep," hash:",hash," pow:",pow)
return hash, pow
}
// Count counts the number of non-overlapping instances of sep in s.
func Count(s, sep string) int {
fmt.Println("Count() s:",s," sep:",sep)
n := 0
// special cases
switch {
case len(sep) == 0: //seq为空,返回总数加1
return utf8.RuneCountInString(s) + 1
case len(sep) == 1: //seq为单个字符,直接遍历比较即可
// special case worth making fast
c := sep[0]
for i := 0; i < len(s); i++ {
if s[i] == c {
n++
}
}
return n
case len(sep) > len(s):
return 0
case len(sep) == len(s):
if sep == s {
return 1
}
return 0
}
// Rabin-Karp search
hashsep, pow := hashStr(sep)
lastmatch := 0 //最后一次匹配的位置
charcode := [...]uint32{9,8,7,6,5,2,0,2,1,0,5,2,0} //对应字符串"9876520210520"
//验证s字符串 0 - len(sep)是不是匹配的
h := uint32(0)
for i := 0; i < len(sep); i++ {
//h = h*primeRK + uint32(s[i])
h = h*primeRK + charcode[i]
}
//如初始s的len(seq)内容是匹配的,n++, lastmatch指向len(seq)位置
if h == hashsep && s[:len(sep)] == sep {
n++
lastmatch = len(sep)
}
for i := len(sep); i < len(s); {
fmt.Println("\na h ==",h )
h *= primeRK
//加上新的
//h += uint32(s[i])
h += charcode[i]
fmt.Println("b h ==",h )
// 去掉旧的
//h -= pow * uint32(s[i-len(sep)])
h -= pow * charcode[i-len(sep)]
fmt.Println("c h ==",h )
i++
if h == hashsep && lastmatch <= i-len(sep) && s[i-len(sep):i] == sep {
n++
lastmatch = i
fmt.Println("found n==",n ," lastmatch==",lastmatch)
}
}
return n
}这样替换后,可以很清楚的看到运行过程是如何做的:
Count() s: 9876520210520 sep: 520 hashStr() sep: 520 hash: 520 pow: 1000 a h == 987 b h == 9876 c h == 876 a h == 876 b h == 8765 c h == 765 a h == 765 b h == 7652 c h == 652 a h == 652 b h == 6520 c h == 520 found n== 1 lastmatch== 7 a h == 520 b h == 5202 c h == 202 a h == 202 b h == 2021 c h == 21 a h == 21 b h == 210 c h == 210 a h == 210 b h == 2105 c h == 105 a h == 105 b h == 1052 c h == 52 a h == 52 b h == 520 c h == 520 found n== 2 lastmatch== 13 count== 2
另外,对于" if h == hashsep && lastmatch <= i-len(sep) && s[i-len(sep):i] == sep {"这段,可以这样理解:
//防止计算出的hash相等,但实际串不同的情况
if h == hashsep && s[i-len(sep):i] == sep {
//比如Count("1111","11")这种,1111只能算2次,而不是3次
if lastmatch <= i-len(sep) {
n++
lastmatch = i
}
}
所以才要加上lastmatch.
再补上一个,为什么是16777619? 可以看看
网友Bryce写的这篇:http://blog.cyeam.com/golang/2015/01/15/go_index/
MAIL: xcl_168@aliyun.com
BLOG:http://blog.csdn.net/xcl168
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