utils: oshash: add tests (#1285)

Co-authored-by: WithoutPants <53250216+WithoutPants@users.noreply.github.com>

pkg/utils/oshash_internal_test.go

@@ -0,0 +1,46 @@
+package utils
+
+import (
+	"testing"
+)
+
+// Note that the public API returns "" instead.
+func TestOshashEmpty(t *testing.T) {
+	var size int64 = 0
+	head := make([]byte, chunkSize)
+	tail := make([]byte, chunkSize)
+	want := "0000000000000000"
+	got, err := oshash(size, head, tail)
+	if err != nil {
+		t.Errorf("TestOshashEmpty: Error from oshash: %w", err)
+	}
+	if got != want {
+		t.Errorf("TestOshashEmpty: oshash(0, 0, 0) = %q; want %q", got, want)
+	}
+}
+
+// As oshash sums byte values, causing collisions is trivial.
+func TestOshashCollisions(t *testing.T) {
+	buf1 := []byte("this is dumb")
+	buf2 := []byte("dumb is this")
+	var size int64 = int64(len(buf1))
+	head := make([]byte, chunkSize)
+
+	tail1 := make([]byte, chunkSize)
+	copy(tail1[len(tail1)-len(buf1):], buf1)
+	hash1, err := oshash(size, head, tail1)
+	if err != nil {
+		t.Errorf("TestOshashCollisions: Error from oshash: %v", err)
+	}
+
+	tail2 := make([]byte, chunkSize)
+	copy(tail2[len(tail2)-len(buf2):], buf2)
+	hash2, err := oshash(size, head, tail2)
+	if err != nil {
+		t.Errorf("TestOshashCollisions: Error from oshash: %v", err)
+	}
+
+	if hash1 != hash2 {
+		t.Errorf("TestOshashCollisions: oshash(n, k, ... %v) =! oshash(n, k, ... %v)", buf1, buf2)
+	}
+}