fix: use absolute value for Ordinal suffixes

ordinals.go

@@ -5,19 +5,28 @@ import "strconv"
 // Ordinal gives you the input number in a rank/ordinal format.
 //
 // Ordinal(3) -> 3rd
+// Ordinal(-1) -> -1st
 func Ordinal(x int) string {
+	// Suffix selection must use the absolute value because Go's % for
+	// negative dividends yields a negative remainder (e.g. -1%10 == -1),
+	// which would never match the positive cases below.
+	n := x
+	if n < 0 {
+		n = -n
+	}
+
 	suffix := "th"
-	switch x % 10 {
+	switch n % 10 {
 	case 1:
-		if x%100 != 11 {
+		if n%100 != 11 {
 			suffix = "st"
 		}
 	case 2:
-		if x%100 != 12 {
+		if n%100 != 12 {
 			suffix = "nd"
 		}
 	case 3:
-		if x%100 != 13 {
+		if n%100 != 13 {
 			suffix = "rd"
 		}
 	}

ordinals_test.go

@@ -24,5 +24,19 @@ func TestOrdinals(t *testing.T) {
 		{"211", Ordinal(211), "211th"},
 		{"212", Ordinal(212), "212th"},
 		{"213", Ordinal(213), "213th"},
+		// Negatives: Go's % keeps the dividend's sign, so suffix selection
+		// must use the absolute value.
+		{"-1", Ordinal(-1), "-1st"},
+		{"-2", Ordinal(-2), "-2nd"},
+		{"-3", Ordinal(-3), "-3rd"},
+		{"-4", Ordinal(-4), "-4th"},
+		{"-11", Ordinal(-11), "-11th"},
+		{"-12", Ordinal(-12), "-12th"},
+		{"-13", Ordinal(-13), "-13th"},
+		{"-21", Ordinal(-21), "-21st"},
+		{"-22", Ordinal(-22), "-22nd"},
+		{"-23", Ordinal(-23), "-23rd"},
+		{"-101", Ordinal(-101), "-101st"},
+		{"-111", Ordinal(-111), "-111th"},
 	}.validate(t)
 }