Working though the Knuth-Morris-Pratt algorithm.

KMP

package main

import "fmt"

//naive approach
func inStr(s, w string) int {
	pos, i := 0, 0
	for pos+i < len(s) {
		//fmt.Printf("%x: s[%x+%x] == w[%x] / %c == %c\n", pos, pos, i, i, s[pos+i], w[i])
		if s[pos+i] == w[i] {
			if i == len(w)-1 {
				return pos
			}
			i++
		} else {
			pos++
			i = 0
		}
	}
	return -1
}

func kmp(s, w string) int {
	t := preprocess(w)
	pos, i := 0, 0
	for pos+i < len(s) {
		//fmt.Printf("%x: s[%x+%x] == w[%x] / %c == %c\n", pos, pos, i, i, s[pos+i], w[i])
		if s[pos+i] == w[i] {
			if i == len(w)-1 {
				return pos
			}
			i++
		} else {
			pos = pos + i - t[i]
			if t[i] > 0 {
				i = t[i]
			} else {
				i = 0
			}
		}
	}
	return -1
}

func preprocess(w string) []int {
	t := make([]int, len(w))
	if len(w) > 0 {
		t[0] = -1
	}
	pos, cnd := 2, 0
	for pos < len(w) {
		//fmt.Printf("%x: w[%x-1] == w[%x] / %c == %c\n", pos, pos, cnd, w[pos-1], w[cnd])
		if w[pos-1] == w[cnd] {
			cnd++
			t[pos] = cnd
			pos++
		} else if cnd > 0 {
			cnd = t[cnd]
		} else {
			t[pos] = 0
			pos++
		}
	}
	return t
}

func main() {
	//fmt.Println(preprocess(""))
	//fmt.Println(preprocess("abcdefghij"))
	//fmt.Println(preprocess("a"))
	//fmt.Println(preprocess("aaaf"))
	//fmt.Println(preprocess("ababccabaabcdef"))
	//fmt.Println(preprocess("abcdefabcdef"))
	//fmt.Println(preprocess("aaaaaaa"))
	//fmt.Println(inStr("aaaaaaaf", "aaaf"))
	//fmt.Println(kmp("aaaaaaaf", "aaaf"))
	//fmt.Println(kmp("abcdef", "efg"))
	//fmt.Println(kmp("abcdef", "cd"))
	fmt.Println(kmp("ABC ABCDAB ABCDABCDABDE", "ABCDABD"))
}