Binary Encoding/Decoding File in Golang Gives Different Checksum - go

Question

I'm working on encoding and decoding files in golang. I specifically do need the 2D array that I'm using, this is just test code to show the point. I'm not entirely sure what I'm doing wrong, I'm attempting to convert the file into a list of uint32 numbers and then take those numbers and convert them back to a file. The problem is that when I do it the file looks fine but the checksum doesn't line up. I suspect that I'm doing something wrong in the conversion to uint32. I have to do the switch/case because I have no way of knowing how many bytes I'll read for sure at the end of a given file.
package main
import (
"bufio"
"encoding/binary"
"fmt"
"io"
"os"
)
const (
headerSeq = 8
body = 24
)
type part struct {
Seq int
Data uint32
}
func main() {
f, err := os.Open("speech.pdf")
if err != nil {
panic(err)
}
defer f.Close()
reader := bufio.NewReader(f)
b := make([]byte, 4)
o := make([][]byte, 0)
var value uint32
for {
n, err := reader.Read(b)
if err != nil {
if err != io.EOF {
panic(err)
}
}
if n == 0 {
break
}
fmt.Printf("len array %d\n", len(b))
fmt.Printf("len n %d\n", n)
switch n {
case 1:
value = uint32(b[0])
case 2:
value = uint32(uint32(b[1]) | uint32(b[0])<<8)
case 3:
value = uint32(uint32(b[2]) | uint32(b[1])<<8 | uint32(b[0])<<16)
case 4:
value = uint32(uint32(b[3]) | uint32(b[2])<<8 | uint32(b[1])<<16 | uint32(b[0])<<24)
}
fmt.Println(value)
bs := make([]byte, 4)
binary.BigEndian.PutUint32(bs, value)
o = append(o, bs)
}
fo, err := os.OpenFile("test.pdf", os.O_APPEND|os.O_WRONLY|os.O_CREATE, 0600)
if err != nil {
panic(err)
}
defer fo.Close()
for _, ba := range o {
_, err := fo.Write(ba)
if err != nil {
panic(err)
}
}
}

Answer 1

So, you want to write and read arrays of varying length in a file.
import "encoding/binary"
// You need a consistent byte order for reading and writing multi-byte data types
const order = binary.LittleEndian
var dataToWrite = []byte{ ... ... ... }
var err error
// To write a recoverable array of varying length
var w io.Writer
// First, encode the length of data that will be written
err = binary.Write(w, order, int64(len(dataToWrite)))
// Check error
err = binary.Write(w, order, dataToWrite)
// Check error
// To read a variable length array
var r io.Reader
var dataLen int64
// First, we need to know the length of data to be read
err = binary.Read(r, order, &dataLen)
// Check error
// Allocate a slice to hold the expected amount of data
dataReadIn := make([]byte, dataLen)
err = binary.Read(r, order, dataReadIn)
// Check error
This pattern works not just with byte, but any other fixed size data type. See binary.Write for specifics about the encoding.
If the size of encoded data is a big concern, you can save some bytes by storing the array length as a varint with binary.PutVarint and binary.ReadVarint