Unexpected code point conversion

Why is one byte converted to rune with value 65533 instead of 132 in the following application?

I have ascii code conversion table (old ascii code -> new ascii code) that I should implement, so I need the correct ascii values (132 in this case) in converter.

Sample program:

package main

import (
    "io/ioutil"
    "flag"
    "bytes"
    "fmt"
)

func converter(r rune) rune {
    fmt.Printf("%v ", int(r))
    return r
}

func main() {

    // parse the command line
    var infile string
    flag.StringVar(&infile, "in", "", "input file")
    flag.Parse()

    // read the whole file at once
    b, err := ioutil.ReadFile(infile)
    if err != nil {
        panic(err)
    }

    fmt.Printf("%v\n", b)

    // convert charset
    converted := bytes.Map(converter, b)

    fmt.Printf("\n%v\n", converted)
}

Sample input file (in hex):

4A 84 6C 6B 0D 0A

Sample output from the application:

[74 132 108 107 13 10]
74 65533 108 107 13 10
[74 239 191 189 108 107 13 10]

Rune is a Unicode value, not ASCII. So your bytes are interpreted as UTF8.

If we look at the function that you are using:
https://golang.org/src/bytes/bytes.go?s=9029:9081#L344

We can see that for every byte in slice it is converted to Unicode rune.

r := rune(s[i])

What it does is a conversion of bytes, starting from s[i] to UTF8 letter.

In UTF8 one letter can
occupy more that one byte. This is opposite to ASCII encoding where
one letter always takes one byte.

You can read more about UTF8 here https://en.wikipedia.org/wiki/UTF-8

This is the reason you have the wrong result.

To fix it, you should iterate over your bytes using for range loop and save the output to new slice.

func converter(b byte) byte {
    fmt.Printf("%v ", int(r))
    return b
}

...

converted := make([]byte, len(b))

for i, v := range b {
   // v is your byte value - convert it here
   converted[i] = converter(v)
}

Read the bytes from the text, and then you can use something on these lines - the last column in the output will be comparable to the ASCII value.

package main

import (
	"encoding/hex"
	"fmt"
	"unicode/utf8"
)

func main() {
	//s := "Hello, 世界"
	//Assuming the following is the hex you have read in from the file..
	b, err := hex.DecodeString("48656c6c6f2c20e4b896e7958c")
	if err != nil {
		fmt.Println(err)
	}
	fmt.Println(b)
	s := string(b)
	for i := 0; i < len(s); {
		r, size := utf8.DecodeRuneInString(s[i:])
		fmt.Printf("%d\t%c\t%d\n", i, r, r)
		i += size
	}
	anotherWay(s)

}
func anotherWay(s string) {
	fmt.Println("\nAnother way")
	for i, r := range s {
		fmt.Printf("%d\t%c\t%d\n", i, r, r)
	}
}

On playground : https://play.golang.org/p/9WusGxWv8w