First working cut

sha.go

@@ -0,0 +1,43 @@
+package main
+
+import (
+	"crypto/sha256"
+	"fmt"
+	"io"
+	"os"
+)
+
+type fileSha struct {
+	bytes []byte
+	size  int64
+}
+
+func sha256File(path string) fileSha {
+	h := sha256.New()
+	f, err := os.Open(path)
+	if err != nil {
+		panic(err)
+	}
+	defer f.Close()
+	fi, err := f.Stat()
+	if err != nil {
+		panic(err)
+	}
+	io.Copy(h, f)
+	return fileSha{
+		bytes: h.Sum(nil),
+		size:  fi.Size(),
+	}
+}
+
+func (f fileSha) String() string {
+	return fmt.Sprintf("sha256:%x", f.bytes)
+}
+
+func (f fileSha) Name() string {
+	return fmt.Sprintf("%x", f.bytes)
+}
+
+func (f fileSha) Size() int64 {
+	return f.size
+}