From 078230217d035758a6a7ca1c067d5fd0a4bdcf54 Mon Sep 17 00:00:00 2001
From: Kyle Isom <kyle@imap.cc>
Date: Thu, 4 May 2023 13:41:33 -0700
Subject: [PATCH] ahash: add SumLimitedReader

---
 ahash/ahash.go      | 14 +++++++++++++-
 ahash/ahash_test.go | 16 ++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/ahash/ahash.go b/ahash/ahash.go
index 30562c4..c666db3 100644
--- a/ahash/ahash.go
+++ b/ahash/ahash.go
@@ -1,5 +1,6 @@
 // Package ahash provides support for hashing data with a selectable
-//  hash function.
+//
+//	hash function.
 package ahash
 
 import (
@@ -213,6 +214,17 @@ func SumReader(algo string, r io.Reader) ([]byte, error) {
 	return h.Sum(nil), nil
 }
 
+// SumLimitedReader reads n bytes of data from the io.reader and returns the
+// digest (not the hex digest) from the specified algorithm.
+func SumLimitedReader(algo string, r io.Reader, n int64) ([]byte, error) {
+	limit := &io.LimitedReader{
+		R: r,
+		N: n,
+	}
+
+	return SumReader(algo, limit)
+}
+
 var insecureHashList, secureHashList, hashList []string
 
 func init() {
diff --git a/ahash/ahash_test.go b/ahash/ahash_test.go
index 0c9cd26..45ee7e7 100644
--- a/ahash/ahash_test.go
+++ b/ahash/ahash_test.go
@@ -139,3 +139,19 @@ func TestListLengthSanity(t *testing.T) {
 
 	assert.BoolT(t, len(all) == len(secure)+len(insecure))
 }
+
+func TestSumLimitedReader(t *testing.T) {
+	data := bytes.NewBufferString("hello, world")
+	dataLen := data.Len()
+	extendedData := bytes.NewBufferString("hello, world! this is an extended message")
+	expected := "09ca7e4eaa6e8ae9c7d261167129184883644d07dfba7cbfbc4c8a2e08360d5b"
+
+	hash, err := SumReader("sha256", data)
+	assert.NoErrorT(t, err)
+	assert.BoolT(t, fmt.Sprintf("%x", hash) == expected, fmt.Sprintf("have hash %x, want %s", hash, expected))
+
+	extendedHash, err := SumLimitedReader("sha256", extendedData, int64(dataLen))
+	assert.NoErrorT(t, err)
+
+	assert.BoolT(t, bytes.Equal(hash, extendedHash), fmt.Sprintf("have hash %x, want %x", extendedHash, hash))
+}