templates: .Truncate can truncate from end of string if length is negative

middleware/context.go

@@ -132,10 +132,16 @@ func (c Context) PathMatches(pattern string) bool {
 	return Path(c.Req.URL.Path).Matches(pattern)
 }
 
-// Truncate truncates the input string to the given length. If
-// input is shorter than length, the entire string is returned.
+// Truncate truncates the input string to the given length.
+// If length is negative, it returns that many characters
+// starting from the end of the string. If the absolute value
+// of length is greater than len(input), the whole input is
+// returned.
 func (c Context) Truncate(input string, length int) string {
-	if len(input) > length {
+	if length < 0 && len(input)+length > 0 {
+		return input[len(input)+length:]
+	}
+	if length >= 0 && len(input) > length {
 		return input[:length]
 	}
 	return input

middleware/context_test.go

@@ -459,12 +459,36 @@ func TestTruncate(t *testing.T) {
 			inputLength: 10,
 			expected:    "string",
 		},
+		// Test 3 - zero length
+		{
+			inputString: "string",
+			inputLength: 0,
+			expected:    "",
+		},
+		// Test 4 - negative, smaller length
+		{
+			inputString: "string",
+			inputLength: -5,
+			expected:    "tring",
+		},
+		// Test 5 - negative, exact length
+		{
+			inputString: "string",
+			inputLength: -6,
+			expected:    "string",
+		},
+		// Test 6 - negative, bigger length
+		{
+			inputString: "string",
+			inputLength: -7,
+			expected:    "string",
+		},
 	}
 
 	for i, test := range tests {
 		actual := context.Truncate(test.inputString, test.inputLength)
 		if actual != test.expected {
-			t.Errorf(getTestPrefix(i)+"Expected %s, found %s. Input was Truncate(%q, %d)", test.expected, actual, test.inputString, test.inputLength)
+			t.Errorf(getTestPrefix(i)+"Expected '%s', found '%s'. Input was Truncate(%q, %d)", test.expected, actual, test.inputString, test.inputLength)
 		}
 	}
 }