Add test for dropLastLine

internal/magic/magic_test.go

@@ -18,3 +18,31 @@ func TestMagic(t *testing.T) {
 		}
 	}
 }
+
+var dropTests = []struct {
+	raw   string
+	cutAt uint32
+	res   string
+}{
+	{"", 0, ""},
+	{"", 1, ""},
+	{"å", 2, "å"},
+	{"\n", 0, "\n"},
+	{"\n", 1, "\n"},
+	{"\n\n", 1, "\n"},
+	{"\n\n", 3, "\n\n"},
+	{"a\n\n", 3, "a\n"},
+	{"\na\n", 3, "\na"},
+	{"å\n\n", 5, "å\n\n"},
+	{"\nå\n", 5, "\nå\n"},
+}
+
+func TestDropLastLine(t *testing.T) {
+	for i, tt := range dropTests {
+		gotR := dropLastLine([]byte(tt.raw), tt.cutAt)
+		got, _ := io.ReadAll(gotR)
+		if got := string(got); got != tt.res {
+			t.Errorf("dropLastLine %d error: expected %q; got %q", i, tt.res, got)
+		}
+	}
+}