From f013067d264a3002c814e1b758612260fd09fed8 Mon Sep 17 00:00:00 2001
From: Henk Kalkwater <chris@netsoj.nl>
Date: Sun, 13 Dec 2020 21:41:28 +0100
Subject: [PATCH] Brute forcing does not work

---
 source/day13.d | 49 ++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 42 insertions(+), 7 deletions(-)

diff --git a/source/day13.d b/source/day13.d
index 15d1c5e..601eacc 100644
--- a/source/day13.d
+++ b/source/day13.d
@@ -7,8 +7,9 @@ struct ParsedInput {
 
 Variant run(int part, File input, bool bigboy, string[] args) {
 	ParsedInput pInput = input.byLineCopy.array.parseInput();
-	Variant result = parts!int(part, 
-			() => part1(pInput));
+	Variant result = parts!long(part, 
+			() => part1(pInput),
+			() => part2(pInput, 100000000000000));
 	return result;
 }
 
@@ -17,17 +18,15 @@ Variant run(int part, File input, bool bigboy, string[] args) {
 	ParsedInput input;
 	input.timestamp = to!int(range[0]);
 
-	input.busses = range[1].splitter(',').filter!(c => c != "x").map!(to!int).array;
+	input.busses = range[1].splitter(',').map!(c => c == "x" ? -1 : c.to!int).array;
 	writeln(input);
 	return input;
 }
 
 int part1(ParsedInput input) {
-	import std.math;
-
 	int earliestBus = 0;
 	int earliestBusTimestamp = int.max;
-	foreach(int bus; input.busses) {
+	foreach(int bus; input.busses.filter!(x => x != -1)) {
 		int closestTimestamp = (input.timestamp / bus + 1 ) * bus;
 		writeln(closestTimestamp);
 		if (closestTimestamp < earliestBusTimestamp) {
@@ -35,10 +34,45 @@ int part1(ParsedInput input) {
 			earliestBus = bus;
 		}
 	}
-	writefln("Bus %d at %d", earliestBus, earliestBusTimestamp);
+	// debug writefln("Bus %d at %d", earliestBus, earliestBusTimestamp);
 	return earliestBus * (earliestBusTimestamp - input.timestamp);
 }
 
+long part2(ParsedInput input) {
+	long result = 0;
+
+	long startI = startFrom == 0 ? 0 : startFrom / input.busses[0];
+
+	for (long i = startI; result == 0; i++) {
+		long offset = i * input.busses[0];
+		bool fail = false;
+
+		foreach(j, subsequentBus; input.busses[1..$].enumerate(1)) {
+			if (subsequentBus == -1) continue;
+
+			if ((offset + j) % subsequentBus != 0) {
+				fail = true;
+				break;
+			}
+		}
+
+		if (!fail) {
+			result = offset;
+		}
+
+		debug {
+			if (i % 10000 == 0) {
+				writef("\rAt %d", offset);
+				stdout.flush();
+			}
+		}
+
+	}
+	debug writeln();
+
+	return result;
+}
+
 unittest {
 	string[] input = [
 		"939",
@@ -48,4 +82,5 @@ unittest {
 	ParsedInput pInput = parseInput(input);
 
 	assert(part1(pInput) == 295);
+	assert(part2(pInput) == 1068781);
 }