Day 15 solution.

2020/day15.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	step := os.Args[1]
+	indexMap := make(map[int]int)
+	indexMap[0] = 0
+	indexMap[14] = 1
+	indexMap[1] = 2
+	indexMap[3] = 3
+	indexMap[7] = 4
+
+	last := 9
+	loops := 0
+	switch step {
+	case "1":
+		loops = 2020
+	case "2":
+		loops = 30000000
+	}
+
+	for i := 5; i < loops-1; i++ {
+		if x, ok := indexMap[last]; ok {
+			indexMap[last] = i
+			// fmt.Printf("%d was last seen %d ago\n", last, i-x)
+			last = i - x
+		} else {
+			indexMap[last] = i
+			// fmt.Printf("%d has not been seen before\n", last)
+			last = 0
+		}
+	}
+	fmt.Println(last)
+}