main.go

package main

import (
	"encoding/binary"
	"fmt"
	"io/ioutil"
	"os"
)

const pngHeader = "\x89PNG\r\n\x1a\n"

func main() {

	buffer, err := ioutil.ReadAll(os.Stdin)
	if err != nil {
		panic(err)
	}

	if string(buffer[0:len(pngHeader)]) != pngHeader {
		panic("invalid png")
	}

	buffer = buffer[len(pngHeader):]

	for len(buffer) > 0 {
		length := binary.BigEndian.Uint32(buffer[:4])
		tag := buffer[4:8]
		fmt.Println(string(tag), length)
		if string(tag) == "pHYs" {
			data := buffer[8 : length+8]

			fmt.Println("pHYs", binary.BigEndian.Uint32(data[:4]), binary.BigEndian.Uint32(data[4:8]), uint(data[8]))

		}
		buffer = buffer[length+12:]

	}

}