good good study, day day up!

vendor/gopl.io/ch10/cross/main.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 295.
+
+// The cross command prints the values of GOOS and GOARCH for this target.
+package main
+
+import (
+	"fmt"
+	"runtime"
+)
+
+//!+
+func main() {
+	fmt.Println(runtime.GOOS, runtime.GOARCH)
+}
+
+//!-

vendor/gopl.io/ch10/jpeg/main.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 287.
+
+//!+main
+
+// The jpeg command reads a PNG image from the standard input
+// and writes it as a JPEG image to the standard output.
+package main
+
+import (
+	"fmt"
+	"image"
+	"image/jpeg"
+	_ "image/png" // register PNG decoder
+	"io"
+	"os"
+)
+
+func main() {
+	if err := toJPEG(os.Stdin, os.Stdout); err != nil {
+		fmt.Fprintf(os.Stderr, "jpeg: %v\n", err)
+		os.Exit(1)
+	}
+}
+
+func toJPEG(in io.Reader, out io.Writer) error {
+	img, kind, err := image.Decode(in)
+	if err != nil {
+		return err
+	}
+	fmt.Fprintln(os.Stderr, "Input format =", kind)
+	return jpeg.Encode(out, img, &jpeg.Options{Quality: 95})
+}
+
+//!-main
+
+/*
+//!+with
+$ go build gopl.io/ch3/mandelbrot
+$ go build gopl.io/ch10/jpeg
+$ ./mandelbrot | ./jpeg >mandelbrot.jpg
+Input format = png
+//!-with
+
+//!+without
+$ go build gopl.io/ch10/jpeg
+$ ./mandelbrot | ./jpeg >mandelbrot.jpg
+jpeg: image: unknown format
+//!-without
+*/