day 283: faster version of regular numbers

day283/problem.go

@@ -1,5 +1,45 @@
 package day283
 
+// RegularNumbersFaster returns n regular numbers
+// using brute force.
+func RegularNumbersFaster(n int) []int {
+	if n < 0 {
+		panic("negative values are not possible")
+	} else if n == 0 {
+		return []int{}
+	}
+	result := make([]int, n)
+	result[0] = 1
+	var i2, i3, i5 int
+	next2, next3, next5 := 2, 3, 5
+	for i := 1; i < n; i++ {
+		result[i] = min(next2, next3, next5)
+		if result[i] == next2 {
+			i2++
+			next2 = result[i2] * 2
+		}
+		if result[i] == next3 {
+			i3++
+			next3 = result[i3] * 3
+		}
+		if result[i] == next5 {
+			i5++
+			next5 = result[i5] * 5
+		}
+	}
+	return result
+}
+
+func min(vals ...int) int {
+	smallest := vals[0]
+	for _, v := range vals {
+		if v < smallest {
+			smallest = v
+		}
+	}
+	return smallest
+}
+
 // RegularNumbersBrute returns n regular numbers
 // using brute force.
 func RegularNumbersBrute(n int) []int {

day283/problem_test.go

@@ -17,6 +17,32 @@ var testcases = []struct {
 	{26, []int{1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 16, 18, 20, 24, 25, 27, 30, 32, 36, 40, 45, 48, 50, 54, 60}},
 }
 
+func TestRegularNumbersFaster(t *testing.T) {
+	t.Parallel()
+	for _, tc := range testcases {
+		if result := RegularNumbersFaster(tc.n); !reflect.DeepEqual(result, tc.expected) {
+			t.Errorf("Expected %v, got %v", tc.expected, result)
+		}
+	}
+}
+
+func TestRegularNumbersFasterBadInput(t *testing.T) {
+	defer func() {
+		if err := recover(); err == nil {
+			t.Errorf("Expected a panic for a negative input")
+		}
+	}()
+	RegularNumbersFaster(-2)
+}
+
+func BenchmarkRegularNumbersFaster(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range testcases {
+			RegularNumbersFaster(tc.n)
+		}
+	}
+}
+
 func TestRegularNumbersBrute(t *testing.T) {
 	t.Parallel()
 	for _, tc := range testcases {