good good study, day day up!

vendor/gopl.io/ch8/thumbnail/thumbnail_test.go

@@ -0,0 +1,148 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// This file is just a place to put example code from the book.
+// It does not actually run any code in gopl.io/ch8/thumbnail.
+
+package thumbnail_test
+
+import (
+	"log"
+	"os"
+	"sync"
+
+	"gopl.io/ch8/thumbnail"
+)
+
+//!+1
+// makeThumbnails makes thumbnails of the specified files.
+func makeThumbnails(filenames []string) {
+	for _, f := range filenames {
+		if _, err := thumbnail.ImageFile(f); err != nil {
+			log.Println(err)
+		}
+	}
+}
+
+//!-1
+
+//!+2
+// NOTE: incorrect!
+func makeThumbnails2(filenames []string) {
+	for _, f := range filenames {
+		go thumbnail.ImageFile(f) // NOTE: ignoring errors
+	}
+}
+
+//!-2
+
+//!+3
+// makeThumbnails3 makes thumbnails of the specified files in parallel.
+func makeThumbnails3(filenames []string) {
+	ch := make(chan struct{})
+	for _, f := range filenames {
+		go func(f string) {
+			thumbnail.ImageFile(f) // NOTE: ignoring errors
+			ch <- struct{}{}
+		}(f)
+	}
+
+	// Wait for goroutines to complete.
+	for range filenames {
+		<-ch
+	}
+}
+
+//!-3
+
+//!+4
+// makeThumbnails4 makes thumbnails for the specified files in parallel.
+// It returns an error if any step failed.
+func makeThumbnails4(filenames []string) error {
+	errors := make(chan error)
+
+	for _, f := range filenames {
+		go func(f string) {
+			_, err := thumbnail.ImageFile(f)
+			errors <- err
+		}(f)
+	}
+
+	for range filenames {
+		if err := <-errors; err != nil {
+			return err // NOTE: incorrect: goroutine leak!
+		}
+	}
+
+	return nil
+}
+
+//!-4
+
+//!+5
+// makeThumbnails5 makes thumbnails for the specified files in parallel.
+// It returns the generated file names in an arbitrary order,
+// or an error if any step failed.
+func makeThumbnails5(filenames []string) (thumbfiles []string, err error) {
+	type item struct {
+		thumbfile string
+		err       error
+	}
+
+	ch := make(chan item, len(filenames))
+	for _, f := range filenames {
+		go func(f string) {
+			var it item
+			it.thumbfile, it.err = thumbnail.ImageFile(f)
+			ch <- it
+		}(f)
+	}
+
+	for range filenames {
+		it := <-ch
+		if it.err != nil {
+			return nil, it.err
+		}
+		thumbfiles = append(thumbfiles, it.thumbfile)
+	}
+
+	return thumbfiles, nil
+}
+
+//!-5
+
+//!+6
+// makeThumbnails6 makes thumbnails for each file received from the channel.
+// It returns the number of bytes occupied by the files it creates.
+func makeThumbnails6(filenames <-chan string) int64 {
+	sizes := make(chan int64)
+	var wg sync.WaitGroup // number of working goroutines
+	for f := range filenames {
+		wg.Add(1)
+		// worker
+		go func(f string) {
+			defer wg.Done()
+			thumb, err := thumbnail.ImageFile(f)
+			if err != nil {
+				log.Println(err)
+				return
+			}
+			info, _ := os.Stat(thumb) // OK to ignore error
+			sizes <- info.Size()
+		}(f)
+	}
+
+	// closer
+	go func() {
+		wg.Wait()
+		close(sizes)
+	}()
+
+	var total int64
+	for size := range sizes {
+		total += size
+	}
+	return total
+}
+
+//!-6