How to compare strings in golang?

I want to make a function that calculates the length of the common segment (starting from the beginning) in two strings. For example:

foo:="Makan"
bar:="Makon"

The result should be 3.

foo:="Indah"
bar:="Ihkasyandehlo"

The result should be 1.

It's not clear what you are asking because you limited your test cases to ASCII characters.
I've added a Unicode test case and I've included answers for bytes, runes, or both.

<kbd>play.golang.org</kbd>:

package main

import (
	"fmt"
	"unicode/utf8"
)

func commonBytes(s, t string) (bytes int) {
	if len(s) > len(t) {
		s, t = t, s
	}
	i := 0
	for ; i < len(s); i++ {
		if s[i] != t[i] {
			break
		}
	}
	return i
}

func commonRunes(s, t string) (runes int) {
	if len(s) > len(t) {
		s, t = t, s
	}
	i := 0
	for ; i < len(s); i++ {
		if s[i] != t[i] {
			break
		}
	}
	return utf8.RuneCountInString(s[:i])
}

func commonBytesRunes(s, t string) (bytes, runes int) {
	if len(s) > len(t) {
		s, t = t, s
	}
	i := 0
	for ; i < len(s); i++ {
		if s[i] != t[i] {
			break
		}
	}
	return i, utf8.RuneCountInString(s[:i])
}

func main() {
	Tests := []struct {
		word1, word2 string
	}{
		{"Makan", "Makon"},
		{"Indah", "Ihkasyandehlo"},
		{"日本語", "日本語"},
	}
	for _, test := range Tests {
		fmt.Println("Words:        ", test.word1, test.word2)
		fmt.Println("Bytes:        ", commonBytes(test.word1, test.word2))
		fmt.Println("Runes:        ", commonRunes(test.word1, test.word2))
		fmt.Print("Bytes & Runes: ")
		fmt.Println(commonBytesRunes(test.word1, test.word2))
	}
}

Output:

<pre>
Words: Makan Makon
Bytes: 3
Runes: 3
Bytes & Runes: 3 3
Words: Indah Ihkasyandehlo
Bytes: 1
Runes: 1
Bytes & Runes: 1 1
Words: 日本語 日本語
Bytes: 9
Runes: 3
Bytes & Runes: 9 3
</pre>

Note that if you were working with Unicode characters, the result could be quite different.
Try for instance using utf8.DecodeRuneInString().

See this example:

package main

import "fmt"
import "unicode/utf8"

func index(s1, s2 string) int {
	res := 0
	for i, w := 0, 0; i < len(s2); i += w {
		if i >= len(s1) {
			return res
		}
		runeValue1, width := utf8.DecodeRuneInString(s1[i:])
		runeValue2, width := utf8.DecodeRuneInString(s2[i:])
		if runeValue1 != runeValue2 {
			return res
		}
		if runeValue1 == utf8.RuneError || runeValue2 == utf8.RuneError {
			return res
		}
		w = width
		res = i + w
	}
	return res
}

func main() {
	foo := "日本本a語"
	bar := "日本本b語"
	fmt.Println(index(foo, bar))
	foo = "日本語"
	bar = "日otest"
	fmt.Println(index(foo, bar))
	foo = "\xF0"
	bar = "\xFF"
	fmt.Println(index(foo, bar))
}

Here, the result would be:

• 9 (3 common runes of width '3')

• 3 (1 rune of width '3')

• 0 (invalid rune, meaning utf8.RuneError)

You mean like this. Please note, this will not handle UTF 8, only ascii.

package main

import (
	"fmt"
)

func equal(s1, s2 string) int {
	eq := 0
	if len(s1) > len(s2) {
		s1, s2 = s2, s1
	}
	for key, _ := range s1 {
		if s1[key] == s2[key] {
			eq++
		} else {
			break
		}
	}
	return eq
}

func main() {
	fmt.Println(equal("buzzfizz", "buzz"))
	fmt.Println(equal("Makan", "Makon"))
	fmt.Println(equal("Indah", "Ihkasyandehlo"))
}