Day 25 solved.

2020/day25.go

@@ -0,0 +1,51 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"git.annabunch.es/annabunches/adventofcode/2020/lib/util"
+)
+
+func findLoopSize(pubKey, subject int) int {
+	value := 1
+	for i := 0; ; i++ {
+		value *= subject
+		value %= 20201227
+
+		if value == pubKey {
+			return i + 1
+		}
+	}
+}
+
+func transformLoop(pubKey, loopSize int) int {
+	value := 1
+	for i := 0; i < loopSize; i++ {
+		value *= pubKey
+		value %= 20201227
+	}
+
+	return value
+}
+
+func main() {
+	step := os.Args[1]
+	values := util.InputParserInts(os.Args[2])
+
+	keyPub := values[0]
+	doorPub := values[1]
+
+	keyLoop := findLoopSize(keyPub, 7)
+	doorLoop := findLoopSize(doorPub, 7)
+
+	encryptionKey1 := transformLoop(keyPub, doorLoop)
+	encryptionKey2 := transformLoop(doorPub, keyLoop)
+
+	switch step {
+	case "1":
+		fmt.Println(encryptionKey1)
+		fmt.Println(encryptionKey2)
+	case "2":
+	}
+}