Add Permutations method to Cards

CHANGELOG

@@ -1,4 +1,5 @@
 0.1.2:
+- Add Permutations method to Cards
 - Rename library from cards to joker
 
 0.1.1:

cards.go

@@ -141,6 +141,32 @@ func (c Cards) Reversed() Cards {
 	return cc
 }
 
+// Permutations returns all permutations of the supplied cards.
+func (c Cards) Permutations() []Cards {
+	var permute func(Cards, int)
+	var res []Cards
+	permute = func(c Cards, n int) {
+		if n == 1 {
+			res = append(res, c.Copy())
+		} else {
+			for i := 0; i < n; i++ {
+				permute(c, n-1)
+				if n%2 == 1 {
+					tmp := c[i]
+					c[i] = c[n-1]
+					c[n-1] = tmp
+				} else {
+					tmp := c[0]
+					c[0] = c[n-1]
+					c[n-1] = tmp
+				}
+			}
+		}
+	}
+	permute(c, len(c))
+	return res
+}
+
 // Low returns the lowest valued card.
 func (c Cards) Low() Card {
 	if len(c) == 0 {

cards_test.go

@@ -73,6 +73,11 @@ func TestCards(t *testing.T) {
 			t.Errorf("failed to sort cards: expected %s, got %s", c.Sorted, sorted)
 		}
 
+		permutations := c.Cards.Permutations()
+		if len(permutations) != factorial(len(c.Cards)) {
+			t.Errorf("failed to generate permutations: expected %d, got %d: %s", factorial(len(c.Cards)), len(permutations), permutations)
+		}
+
 		low := c.Cards.Low()
 		if !low.Equal(c.Low) {
 			t.Errorf("failed to get low card: expected %s, got %s", c.Low, low)
@@ -92,3 +97,10 @@ func BenchmarkCardsSort(b *testing.B) {
 		}
 	}
 }
+
+func factorial(i int) int {
+	if i <= 1 {
+		return 1
+	}
+	return i * factorial(i-1)
+}