Add example to README

README.md

@@ -11,7 +11,100 @@ go get github.com/invisiblefunnel/egr
 ## Example
 
 ```go
-# TODO
+package main
+
+import (
+	"context"
+	"crypto/md5"
+	"fmt"
+	"log"
+	"os"
+	"path/filepath"
+
+	"github.com/invisiblefunnel/egr"
+)
+
+// Let's rewrite the pipeline example from the errgroup docs.
+// https://pkg.go.dev/golang.org/x/[email protected]/errgroup#example-Group-Pipeline
+func main() {
+	m, err := MD5All(context.Background(), ".")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	for k, sum := range m {
+		fmt.Printf("%s:\t%x\n", k, sum)
+	}
+}
+
+type result struct {
+	path string
+	sum  [md5.Size]byte
+}
+
+// MD5All reads all the files in the file tree rooted at root and returns a map
+// from file path to the MD5 sum of the file's contents. If the directory walk
+// fails or any read operation fails, MD5All returns an error.
+func MD5All(ctx context.Context, root string) (map[string][md5.Size]byte, error) {
+	queueSize := 1 // choose based on workload
+	numDigesters := 20
+
+	digesters, ctx := egr.WithContext[string](ctx, queueSize)
+	collector, ctx := egr.WithContext[result](ctx, queueSize)
+	collector.SetLimit(1)
+
+	for i := 0; i < numDigesters; i++ {
+		digesters.Go(func(queue <-chan string) error {
+			for path := range queue {
+				data, err := os.ReadFile(path)
+				if err != nil {
+					return err
+				}
+				err = collector.Push(ctx, result{path, md5.Sum(data)})
+				if err != nil {
+					return err
+				}
+			}
+			return nil
+		})
+	}
+
+	m := make(map[string][md5.Size]byte)
+
+	// Only launch one collector goroutine to update the map
+	collector.Go(func(queue <-chan result) error {
+		for r := range queue {
+			m[r.path] = r.sum
+		}
+		return nil
+	})
+
+	// Walk the directory and Push paths to the digesters queue
+	err := filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+		if !info.Mode().IsRegular() {
+			return nil
+		}
+		return digesters.Push(ctx, path)
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	// Wait for digesters to finish
+	if err := digesters.Wait(); err != nil {
+		return nil, err
+	}
+
+	// Wait for collector to finish
+	if err := collector.Wait(); err != nil {
+		return nil, err
+	}
+
+	return m, nil
+}
 ```
 
 ## License