PNG compression

gopng.go

package main

import (
	"fmt"
	"image/png"
	"os"
)

// DefaultCompression
// NoCompression
// BestSpeed
// BestCompression
const COMPRESSION_LEVEL = png.BestSpeed
const FILE_PERM = 0666

func main() {
	var inputPath, outputPath string
	argc := len(os.Args)
	if argc <= 1 || argc > 3 {
		fmt.Println("Usage: gopng <input.png> [output.png]")
		os.Exit(1)
	} else if argc == 3 {
		outputPath = os.Args[2]
	}
	inputPath = os.Args[1]
	err := Crush(inputPath, outputPath, COMPRESSION_LEVEL)
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}
}

func Crush(inputPath, outputPath string, compressionLevel png.CompressionLevel) error {
	var outputFile *os.File
	openFlags := os.O_RDONLY
	overwrite := outputPath == ""
	if overwrite {
		openFlags = os.O_RDWR
	}
	inputFile, err := os.OpenFile(inputPath, openFlags, FILE_PERM)
	if err != nil {
		return fmt.Errorf("Unable to open input file: %s", err)
	}
	defer inputFile.Close()

	image, err := png.Decode(inputFile)
	if err != nil {
		return fmt.Errorf("Unable to decode input file: %s", err)
	}

	if overwrite {
		outputFile = inputFile
		if ioOffset, err := outputFile.Seek(0, 0); ioOffset != 0 || err != nil {
			return fmt.Errorf("Unable to seek to start of output file (offset: %d): %s", ioOffset, err)
		}
		if err := outputFile.Truncate(0); err != nil {
			return fmt.Errorf("Unable to truncate output file: %s", err)
		}
	} else {
		outputFile, err = os.Create(outputPath)
		if err != nil {
			return fmt.Errorf("Unable to create output file: %s", err)
		}
		defer outputFile.Close()
	}

	encoder := png.Encoder{CompressionLevel: compressionLevel}
	err = encoder.Encode(outputFile, image)
	return err
}