add hash output length get function

hash.go

@@ -10,25 +10,27 @@ import (
 )
 
 const (
-	GNUTLS_MD5    = 2
-	GNUTLS_SHA1   = 3
-	GNUTLS_MD2    = 5
-	GNUTLS_SHA256 = 6
-	GNUTLS_SHA384 = 7
-	GNUTLS_SHA512 = 8
-	GNUTLS_SHA224 = 9
+	GNUTLS_HASH_MD5    = 2
+	GNUTLS_HASH_SHA1   = 3
+	GNUTLS_HASH_MD2    = 5
+	GNUTLS_HASH_SHA256 = 6
+	GNUTLS_HASH_SHA384 = 7
+	GNUTLS_HASH_SHA512 = 8
+	GNUTLS_HASH_SHA224 = 9
 )
 
 // Hash hash struct
 type Hash struct {
 	hash    C.gnutls_hash_hd_t
 	t       int
+	hashLen C.int
 }
 
 // NewHash new hash struct
 func NewHash(t int) *Hash {
 	h := C.new_hash(C.int(t))
-	return &Hash{h, t}
+	hashOutLen := GetHashOutputLen(t)
+	return &Hash{h, t, C.int(hashOutLen)}
 }
 
 // Write write data to hash context
@@ -51,14 +53,12 @@ func (h *Hash) Sum(buf []byte) []byte {
 		h.Write(buf)
 	}
 
-	hashOutLen := C.get_hash_len(C.int(h.t))
-
-	dstBuf := C.malloc(C.size_t(hashOutLen))
+	dstBuf := C.malloc(C.size_t(h.hashLen))
 	defer C.free(dstBuf)
 
 	C.gnutls_hash_output(h.hash, dstBuf)
 
-	gobuf := C.GoBytes(dstBuf, hashOutLen)
+	gobuf := C.GoBytes(dstBuf, h.hashLen)
 
 	return gobuf
 }
@@ -68,3 +68,9 @@ func (h *Hash) Close() error {
 	C.gnutls_hash_deinit(h.hash, nil)
 	return nil
 }
+
+// GetHashOutputLen get the hash algorithm output length
+// example GNUTLS_MD5 is 16
+func GetHashOutputLen(t int) int {
+	return int(C.gnutls_hash_get_len(C.gnutls_digest_algorithm_t(t)))
+}