fix up structure

lib/common/hash.go

@@ -1,3 +1,27 @@
 package common
 
+import (
+	"crypto/sha256"
+	"io"
+)
+
+// sha256 hash of some data
 type Hash [32]byte
+
+// calculate sha256 of a byte slice
+func HashData(data []byte) (h Hash) {
+	h = sha256.Sum256(data)
+	return
+}
+
+// calulate sha256 of all data being read from an io.Reader
+// return error if one occurs while reading from reader
+func HashReader(r io.Reader) (h Hash, err error) {
+	sha := sha256.New()
+	_, err = io.Copy(sha, r)
+	if err == nil {
+		d := sha.Sum(nil)
+		copy(h[:], d)
+	}
+	return
+}