parallel.go

package main

import (
	"flag"
	"fmt"
	"os"
	"path/filepath"
	"sync"
	"time"
)

var maxConcurrency int

func main() {

	flag.IntVar(&maxConcurrency, "p", 1, "")
	flag.Parse()

	roots := flag.Args()
	if len(roots) == 0 {
		roots = []string{"."}
	}

	now := time.Now()

	fileSizes := make(chan int64)
	var wg sync.WaitGroup

	var lock = make(chan struct{}, maxConcurrency)

	for _, root := range roots {
		wg.Add(1)
		go walkDirectory(root, &wg, fileSizes, lock)
	}

	go func() {
		wg.Wait()
		close(fileSizes)
	}()

	var nFiles, nBytes int64

	for size := range fileSizes {
		nFiles++
		nBytes += size
	}

	fmt.Println("total time: ", time.Since(now))
	printUsage(nFiles, nBytes)
}

func printUsage(files, bytes int64) {
	fmt.Printf("%d files %.1f GB\n", files, float64(bytes)/1e9)
}

func walkDirectory(dir string, wg *sync.WaitGroup, fileSize chan<- int64, lock chan struct{}) {
	defer wg.Done()
	for _, entry := range getEntries(dir, lock) {
		if entry.IsDir() {
			wg.Add(1)
			subdir := filepath.Join(dir, entry.Name())
			go walkDirectory(subdir, wg, fileSize, lock)
		} else {
			info, _ := entry.Info()
			fileSize <- info.Size()
		}
	}
}

func getEntries(dir string, lock chan struct{}) []os.DirEntry {
	lock <- struct{}{}
	defer func() { <-lock }()
	entries, err := os.ReadDir(dir)
	if err != nil {
		panic(err)
	}
	return entries
}

serial.go

package main

import (
	"flag"
	"fmt"
	"os"
	"path/filepath"
	"time"
)

func main() {
	flag.Parse()

	roots := flag.Args()
	if len(roots) == 0 {
		roots = []string{"."}
	}

	now := time.Now()

	var totalFiles, totalBytes int64

	for _, root := range roots {
		files, bytes := walkDirectory(root)
		totalFiles += files
		totalBytes += bytes
	}

	fmt.Println("total time: ", time.Since(now))
	printUsage(totalFiles, totalBytes)
}

func printUsage(files, bytes int64) {
	fmt.Printf("%d files %.1f GB\n", files, float64(bytes)/1e9)
}

func walkDirectory(dir string) (noOfFiles, totalSize int64) {
	for _, entry := range getEntries(dir) {
		if entry.IsDir() {
			subDir := filepath.Join(dir, entry.Name())
			files, bytes := walkDirectory(subDir)
			noOfFiles += files
			totalSize += bytes
		} else {
			noOfFiles++
			e, err := entry.Info()
			if err != nil {
				panic(err)
			}
			totalSize += e.Size()
		}
	}
	return
}

func getEntries(dir string) []os.DirEntry {
	entries, err := os.ReadDir(dir)
	if err != nil {
		return nil
	}
	return entries
}