day 138: implement a smarter optimal change maker

day138/problem.go

@@ -1,6 +1,8 @@
 package day138
 
-import "errors"
+import (
+	"errors"
+)
 
 var errNotPossible = errors.New("Can not make this value")
 
@@ -31,3 +33,29 @@ func MinCurrencyRequiredBrute(amt int, denomCents []int) (int, error) {
 	}
 	return 0, errNotPossible
 }
+
+// MinCurrencyRequiredSmarter returns the minimum number of currency
+// necessary to make the desired amount.
+func MinCurrencyRequiredSmarter(amt int, denomCents []int) (int, error) {
+	return minCurrencyRequiredSmarter(amt, int(^uint(0)>>1), 0, denomCents)
+}
+
+func minCurrencyRequiredSmarter(amt, min, count int, denomCents []int) (int, error) {
+	if count > min || amt < 0 {
+		return 0, errNotPossible
+	} else if amt == 0 {
+		return count, nil
+	}
+	found := false
+	for i := range denomCents {
+		current := denomCents[len(denomCents)-1-i]
+		if result, err := minCurrencyRequiredSmarter(amt-current, min, count+1, denomCents); err == nil && result < min {
+			found = true
+			min = result
+		}
+	}
+	if found {
+		return min, nil
+	}
+	return 0, errNotPossible
+}

day138/problem_test.go

@@ -10,6 +10,8 @@ var testcases = []struct {
 }{
 	{[]int{1, 5, 10, 25}, 16, 3, nil},
 	{[]int{25}, 16, 0, ErrNotPossible()},
+	{[]int{2, 5}, 16, 5, nil},
+	{[]int{2, 3, 6}, 6, 1, nil},
 }
 
 func TestMinCurrencyRequiredBrute(t *testing.T) {
@@ -28,3 +30,20 @@ func BenchmarkMinCurrencyRequiredBrute(b *testing.B) {
 		}
 	}
 }
+
+func TestMinCurrencyRequiredSmarter(t *testing.T) {
+	t.Parallel()
+	for _, tc := range testcases {
+		if result, err := MinCurrencyRequiredSmarter(tc.n, tc.denom); err != tc.err || result != tc.expected {
+			t.Errorf("Expected (%v,%v) got (%v,%v)", tc.expected, tc.err, result, err)
+		}
+	}
+}
+
+func BenchmarkMinCurrencyRequiredSmarter(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range testcases {
+			MinCurrencyRequiredSmarter(tc.n, tc.denom)
+		}
+	}
+}