Merge pull request #138 from gammazero/read-from-stream

New SumStream function reads from io.Reader

opts/opts.go

@@ -8,7 +8,6 @@ import (
 	"flag"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"sort"
 	"strings"
 
@@ -149,10 +148,5 @@ func (o *Options) Check(r io.Reader, h1 mh.Multihash) error {
 
 // Multihash reads all the data in r and calculates its multihash.
 func (o *Options) Multihash(r io.Reader) (mh.Multihash, error) {
-	b, err := ioutil.ReadAll(r)
-	if err != nil {
-		return nil, err
-	}
-
-	return mh.Sum(b, o.AlgorithmCode, o.Length)
+	return mh.SumStream(r, o.AlgorithmCode, o.Length)
 }

sum.go

@@ -3,6 +3,8 @@ package multihash
 import (
 	"errors"
 	"fmt"
+	"hash"
+	"io"
 
 	mhreg "github.com/multiformats/go-multihash/core"
 )
@@ -13,8 +15,8 @@ var ErrSumNotSupported = mhreg.ErrSumNotSupported
 var ErrLenTooLarge = errors.New("requested length was too large for digest")
 
 // Sum obtains the cryptographic sum of a given buffer. The length parameter
-// indicates the length of the resulting digest and passing a negative value
-// use default length values for the selected hash function.
+// indicates the length of the resulting digest. Passing a negative value uses
+// default length values for the selected hash function.
 func Sum(data []byte, code uint64, length int) (Multihash, error) {
 	// Get the algorithm.
 	hasher, err := GetHasher(code)
@@ -25,6 +27,28 @@ func Sum(data []byte, code uint64, length int) (Multihash, error) {
 	// Feed data in.
 	hasher.Write(data)
 
+	return encodeHash(hasher, code, length)
+}
+
+// SumStream obtains the cryptographic sum of a given stream. The length
+// parameter indicates the length of the resulting digest. Passing a negative
+// value uses default length values for the selected hash function.
+func SumStream(r io.Reader, code uint64, length int) (Multihash, error) {
+	// Get the algorithm.
+	hasher, err := GetHasher(code)
+	if err != nil {
+		return nil, err
+	}
+
+	// Feed data in.
+	if _, err = io.Copy(hasher, r); err != nil {
+		return nil, err
+	}
+
+	return encodeHash(hasher, code, length)
+}
+
+func encodeHash(hasher hash.Hash, code uint64, length int) (Multihash, error) {
 	// Compute final hash.
 	//  A new slice is allocated.  FUTURE: see other comment below about allocation, and review together with this line to try to improve.
 	sum := hasher.Sum(nil)