How to take addresses of the bytes in a string?

Use unsafe.StringData

package main

import (
	"unsafe"
)

var h = "Hello "
var w = "world!"

func commonUnderlyingBytes(x, y string) bool {
	return &[]byte(x)[4] == &[]byte(y)[4]
}

func main() {
	var s = h + w
	//var p = &s[4] // error: cannot take address of s[4]
	var p = unsafe.StringData(s[4:])
	println(p, string(*p)) // 0xc00004470c o
}

Use reflect.Value.UnsafePointer

package main

import (
	"reflect"
)

var h = "Hello "
var w = "world!"

func commonUnderlyingBytes(x, y string) bool {
	return &[]byte(x)[4] == &[]byte(y)[4]
}

func main() {
	var s = h + w
	//var p = &s[4] // error: cannot take address of s[4]
	var p = (*byte)(reflect.ValueOf(s[4:]).UnsafePointer())
	println(p, string(*p)) // 0xc000104eec o
}

Convert string to a byte slice, then take addresses of the byte slice

package main

var h = "Hello "
var w = "world!"

func takeAddressOfFirstByte(s string) *byte {
	return &[]byte(s)[0]
}

func commonUnderlyingBytes(x, y string) bool {
	return len(x) != 0 && len(y) != 0 &&
		takeAddressOfFirstByte(x) == takeAddressOfFirstByte(y)
}

func main() {
	var s = h + w

	var p1 = &[]byte(s)[4]
	println(p1, string(*p1))
	
	var p2 = &[]byte(s)[4]
	println(p2, string(*p2))

	println(p1 == p2)
	
	var s2 = s
	println(commonUnderlyingBytes(s[4:], s2[4:]))
}

$ gotv 1.21. run aa.go
[Run]: $HOME/.cache/gotv/tag_go1.21.12/bin/go run demo1.go
0xc00003c6c4 o
0xc00003c6a4 o
false
false
$ gotv 1.22. run aa.go
[Run]: $HOME/.cache/gotv/tag_go1.22.5/bin/go run demo1.go
0xc0000426ec o
0xc0000426ec o
true
true

package main

var h = "Hello "
var w = "world!"

func main() {
	var s = h + w

	var p1 = &[]byte(s)[4]
	println(p1, string(*p1))
	
	var p2 = &[]byte(s)[4]
	*p2 = 'X' // modify the byte
	println(p2, string(*p2))

	println(p1 == p2)
}

$ gotv 1.21. run aa.go
[Run]: $HOME/.cache/gotv/tag_go1.21.12/bin/go run demo2.go
0xc00003c6d4 o
0xc00003c6b4 X
false
$ gotv 1.22. run aa.go
[Run]: $HOME/.cache/gotv/tag_go1.22.5/bin/go run demo2.go
0xc0000426f0 o
0xc0000426cc X
false

func CommonStringPrefix1(x, y string) int {
	if len(x) == 0 || len(y) == 0 {
		return 0
	}
	if &[]byte(x)[0] == &[]byte(y)[0] {
		if len(x) <= len(y) {
			return len(x)
		} else {
			return len(y)
		}
	}
	return CommonStringPrefix2(x, y)
}

func CommonStringPrefix2(x, y string) int {
	var min = len(x)
	if len(y) < len(x) {
		min = len(y)
	}
	x2, y2 := x[:min], y[:min]
	if len(x2) == len(y2) {
		for i := 0; i < len(x2); i++ {
			if x2[i] != y2[i] {
				return i
			}
		}
	}
	return min
}

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