little endian / big endian things

README.md

little endian conversion

I was working on gpt package in go and I didn't realize that the author had handled conversion to little endian when writing header.

I was under the impression that I had to create the header with the little endian value.

For example, the Size of a standard header is 92 bytes. I didn't want to deal with writing:

Header {
    Size: binary.LittleEndian.Uint32([]byte{0x5C, 0x00, 0x00, 0x00}),
}

I wanted to just type type 92 to improve readability.

So I wrote these functions such that I could do:

Header {
    Size: leUint32(92)
}

In the end, it was a waste of my time.

le_test.go

package main

import (
	"bytes"
	"encoding/binary"
	"testing"
)

const standardHeaderSize = 92
const standardPartitionEntrySize = 128

func leUint32(n uint32) uint32 {
	b := make([]byte, 4)
	binary.LittleEndian.PutUint32(b, n)
	return binary.LittleEndian.Uint32(b)
}

func leUint64(n uint64) uint64 {
	b := make([]byte, 8)
	binary.LittleEndian.PutUint64(b, n)
	return binary.LittleEndian.Uint64(b)
}

func TestLeUint32(t *testing.T) {
	for _, d := range []struct {
		input    uint32
		expected []byte
	}{
		{0, []byte{0x00, 0x00, 0x00, 0x00}},
		{standardHeaderSize, []byte{0x5c, 0x00, 0x00, 0x00}},
		{standardPartitionEntrySize, []byte{0x80, 0x00, 0x00, 0x00}},
		{65536, []byte{0x00, 0x00, 0x01, 0x00}},
		{1953525134, []byte{0x8e, 0x6d, 0x70, 0x74}},
	} {
		asUint32 := leUint32(d.input)
		buf := make([]byte, 4)
		binary.LittleEndian.PutUint32(buf, asUint32)
		if bytes.Compare(buf, d.expected) != 0 {
			t.Errorf("leUint32(%d) found bytes as %v expected %v",
				d.input, buf, d.expected)
		}
	}
}

// TestLeUint64 -
func TestLeUint64(t *testing.T) {
	for _, d := range []struct {
		input    uint64
		expected []byte
	}{
		{0, []byte{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}},
		{standardHeaderSize, []byte{0x5c, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}},
		{standardPartitionEntrySize, []byte{0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}},
		{65536, []byte{0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00}},
		{uint64(4294967296), []byte{0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00}},
	} {
		asUint64 := leUint64(d.input)
		buf := make([]byte, 8)
		binary.LittleEndian.PutUint64(buf, asUint64)
		if bytes.Compare(buf, d.expected) != 0 {
			t.Errorf("leUint64(%d) found bytes as %v expected %v",
				d.input, buf, d.expected)
		}
	}
}