mtsatellite/common/image.go

// Copyright 2014 by Sascha L. Teichmann
// Use of this source code is governed by the MIT license
// that can be found in the LICENSE file.

package common

import (
	"bufio"
	"errors"
	"image"
	"image/png"
	"log"
	"os"
	"strconv"
	"sync"
	"time"
)

var rand uint32
var randmu sync.Mutex

func reseed() uint32 {
	return uint32(time.Now().UnixNano() + int64(os.Getpid()))
}

func nextSuffix() string {
	randmu.Lock()
	r := rand
	if r == 0 {
		r = reseed()
	}
	r = r*1664525 + 1013904223 // constants from Numerical Recipes
	rand = r
	randmu.Unlock()
	return strconv.Itoa(int(1e9 + r%1e9))[1:]
}

func SaveAsPNG(path string, img image.Image) (err error) {
	var file *os.File
	if file, err = os.Create(path); err != nil {
		return
	}
	writer := bufio.NewWriter(file)
	err = png.Encode(writer, img)
	writer.Flush()
	file.Close()
	return
}

func tmpName(tmpl string) (string, error) {
	tmpPre := tmpl + ".tmp"
	nconflict := 0
	for i := 0; i < 10000; i++ {
		tmp := tmpPre + nextSuffix()
		if _, err := os.Stat(tmp); err != nil {
			if os.IsNotExist(err) {
				return tmp, nil
			}
			return "", err
		}
		if nconflict++; nconflict > 10 {
			nconflict = 0
			rand = reseed()
		}
	}
	return "", errors.New("Cannot create temp name")
}

func SaveAsPNGAtomic(path string, img image.Image) (err error) {

	var tmpPath string
	if tmpPath, err = tmpName(path); err != nil {
		return
	}
	// Still a bit racy
	if err = SaveAsPNG(tmpPath, img); err != nil {
		return
	}
	return os.Rename(tmpPath, path)
}

func LoadPNG(path string) image.Image {
	var err error
	var file *os.File
	if file, err = os.Open(path); err != nil {
		//log.Printf("WARN: %s", err)
		return image.White
	}
	defer file.Close()
	reader := bufio.NewReader(file)
	var img image.Image
	if img, err = png.Decode(reader); err != nil {
		log.Printf("WARN: %s", err)
		return image.White
	}
	return img
}