From 4d18082447de888bc4914eab598ac993e5128f5e Mon Sep 17 00:00:00 2001
From: Nihilazo <nico@itwont.work>
Date: Sun, 13 Dec 2020 18:32:33 +0000
Subject: [PATCH] solve part 2, thanks nytpu

---
 day13/main.go | 34 +++++++++++++++++++++++++---------
 1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/day13/main.go b/day13/main.go
index 7fd14dd..4dcd34c 100644
--- a/day13/main.go
+++ b/day13/main.go
@@ -10,29 +10,30 @@ import (
 type Bus struct {
 	Num int
 	Arrival int
+	Minute int
 }
+
 func main() {
 	d, _ := ioutil.ReadFile("input")
 	lines := strings.Split(string(d),"\n")
 	departure, _ := strconv.Atoi(lines[0]) // Earliest departure time
 	busstrings := strings.Split(lines[1],",")
-	var busses []int
-	for _, b := range busstrings {
+	var busses []Bus
+	for i, b := range busstrings {
 		if b != "x" {
 			n, _ := strconv.Atoi(b)
-			busses = append(busses, n)
+			busses = append(busses, Bus{Num: n, Minute: i})
 		} 
 	}
-	var bustimes []Bus
-	for _, b := range busses {
-		i := b
+	for index, b := range busses {
+		i := b.Num
 		for i < departure {
-			i += b
+			i += b.Num
 		}
-		bustimes = append(bustimes, Bus{Num: b, Arrival: i})
+		busses[index].Arrival = i
 	}
 	earliestBus := Bus{}
-	for _, bus := range bustimes {
+	for _, bus := range busses {
 		if earliestBus.Num == 0 { // If we have the first real bus
 			earliestBus = bus
 		} else if earliestBus.Arrival > bus.Arrival {
@@ -40,4 +41,19 @@ func main() {
 		}
 	}
 	fmt.Println("Part 1:", earliestBus.Num * (earliestBus.Arrival - departure))
+
+	fmt.Println("Part 2:", chineseRemainder(busses))
+}
+
+// translation of https://git.sr.ht/~nytpu/advent_of_code_2020/tree/master/day13/problem2.c
+func chineseRemainder(b []Bus) int {
+	jmp := b[0].Num
+	ts := b[0].Num
+	for i := 1; i < len(b); i++ {
+		for (ts + b[i].Minute) % b[i].Num != 0 {
+			ts += jmp
+		}
+		jmp *= b[i].Num
+	}
+	return ts
 }
\ No newline at end of file