Day 15 (Golang)

day15/main.go

@@ -0,0 +1,49 @@
+package main
+
+import "fmt"
+
+type recitation map[uint]num
+
+func (r *recitation) remember(k uint, v uint) {
+	lookup, ok := (*r)[k]
+	if ok {
+		(*r)[k] = append(lookup, v)
+	} else {
+		(*r)[k] = num{v}
+	}
+}
+
+type num []uint
+
+func (n num) turnsApart() uint {
+	if len(n) < 2 {
+		return 0
+	}
+	return n[len(n)-1] - n[len(n)-2]
+}
+
+func main() {
+	pt1 := solve([]uint{1, 0, 16, 5, 17, 4}, 2020)
+	pt2 := solve([]uint{1, 0, 16, 5, 17, 4}, 30000000)
+
+	fmt.Println("Part 1:", pt1)
+	fmt.Println("Part 2:", pt2)
+}
+
+func solve(s []uint, t uint) int {
+	var lastNum uint
+
+	rec := make(recitation)
+	for i, n := range s {
+		lastNum = n
+		rec.remember(n, uint(i+1))
+	}
+
+	for i := uint(len(rec) + 1); i <= t; i++ {
+		lastHist := rec[lastNum]
+		lastNum = lastHist.turnsApart()
+		rec.remember(lastNum, i)
+	}
+
+	return int(lastNum)
+}

day15/main_test.go

@@ -0,0 +1,25 @@
+package main
+
+import "testing"
+
+func Test_part1(t *testing.T) {
+	exp := 438
+	got := solve([]uint{3, 2, 1}, uint(2020))
+	if exp != got {
+		t.Errorf("Part 1: expected %d, got %d", exp, got)
+	}
+}
+
+func Test_part2(t *testing.T) {
+	exp := 18
+	got := solve([]uint{3, 2, 1}, uint(30000000))
+	if exp != got {
+		t.Errorf("Part 1: expected %d, got %d", exp, got)
+	}
+}
+
+func Benchmark_part1(b *testing.B) {
+	for n := 0; n < b.N; n++ {
+		solve([]uint{3, 2, 1}, uint(2020))
+	}
+}