add better generate

Signed-off-by: Jess Frazelle <acidburn@microsoft.com>

vendor/github.com/docker/distribution/cmd/digest/main.go

@@ -0,0 +1,100 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"os"
+
+	"github.com/docker/distribution/version"
+	"github.com/opencontainers/go-digest"
+
+	_ "crypto/sha256"
+	_ "crypto/sha512"
+)
+
+var (
+	algorithm   = digest.Canonical
+	showVersion bool
+)
+
+type job struct {
+	name   string
+	reader io.Reader
+}
+
+func init() {
+	flag.Var(&algorithm, "a", "select the digest algorithm (shorthand)")
+	flag.Var(&algorithm, "algorithm", "select the digest algorithm")
+	flag.BoolVar(&showVersion, "version", false, "show the version and exit")
+
+	log.SetFlags(0)
+	log.SetPrefix(os.Args[0] + ": ")
+}
+
+func usage() {
+	fmt.Fprintf(os.Stderr, "usage: %s [files...]\n", os.Args[0])
+	fmt.Fprint(os.Stderr, `
+Calculate the digest of one or more input files, emitting the result
+to standard out. If no files are provided, the digest of stdin will
+be calculated.
+
+`)
+	flag.PrintDefaults()
+}
+
+func unsupported() {
+	log.Fatalf("unsupported digest algorithm: %v", algorithm)
+}
+
+func main() {
+	var jobs []job
+
+	flag.Usage = usage
+	flag.Parse()
+	if showVersion {
+		version.PrintVersion()
+		return
+	}
+
+	var fail bool // if we fail on one item, foul the exit code
+	if flag.NArg() > 0 {
+		for _, path := range flag.Args() {
+			fp, err := os.Open(path)
+
+			if err != nil {
+				log.Printf("%s: %v", path, err)
+				fail = true
+				continue
+			}
+			defer fp.Close()
+
+			jobs = append(jobs, job{name: path, reader: fp})
+		}
+	} else {
+		// just read stdin
+		jobs = append(jobs, job{name: "-", reader: os.Stdin})
+	}
+
+	digestFn := algorithm.FromReader
+
+	if !algorithm.Available() {
+		unsupported()
+	}
+
+	for _, job := range jobs {
+		dgst, err := digestFn(job.reader)
+		if err != nil {
+			log.Printf("%s: %v", job.name, err)
+			fail = true
+			continue
+		}
+
+		fmt.Printf("%v\t%s\n", dgst, job.name)
+	}
+
+	if fail {
+		os.Exit(1)
+	}
+}