package main

import (
	"fmt"
	"unsafe"
)

func main() {
	issue(101, 233) // both < 256
	issue(253, 886) // oldVal < 256
	issue(987, 254) // newVal < 256
	issue(998, 258) // both >= 256

	// Try this four calls, the output will be more strange.
	// issue(100, 255) // both < 256
	// issue(255, 999) // oldVal < 256
	// issue(999, 255) // newVal < 256
	// issue(999, 256) // both >= 256
}

// type eface struct {
// 	  _type *_type
// 	  data  unsafe.Pointer
// }
// data is *int (unsafe.Pointer(&i))
func issue(oldVal, newVal int) {
	fmt.Printf("\n==== running issue(%d, %d) ====\n", oldVal, newVal)

	var (
		i             = oldVal // int
		e interface{} = i
	)
	pdata := unsafe.Pointer(uintptr(unsafe.Pointer(&e)) + unsafe.Sizeof(&i))
	data := unsafe.Pointer(*(**int)(pdata))

	fmt.Printf("      i,   &i = %d, %p\n", i, &i)
	fmt.Printf("  *data, data = %d, %p\n", *((*int)(data)), data)

	// when 0 <= oldVal < 256, i will be changed, WHY???
	*((*int)(data)) = newVal
	fmt.Printf("\n  exec *((*int)(data)) = %d\n\n", newVal)

	fmt.Printf("      i,   &i = %d, %p\n", i, &i)
	fmt.Printf("  *data, data = %d, %p\n", *((*int)(data)), data)
}