From 0ae8e7655380088162e85bd94fb0e6cd49d286e1 Mon Sep 17 00:00:00 2001
From: Henk Kalkwater <chris@netsoj.nl>
Date: Mon, 14 Dec 2020 03:12:15 +0100
Subject: [PATCH] Day 13, part 2

---
 source/day13.d | 100 ++++++++++++++++++++++++++++++-------------------
 1 file changed, 61 insertions(+), 39 deletions(-)

diff --git a/source/day13.d b/source/day13.d
index 601eacc..8e53a8f 100644
--- a/source/day13.d
+++ b/source/day13.d
@@ -1,15 +1,18 @@
+import std.algorithm;
+import std.typecons;
+
 import dayutil;
 
 struct ParsedInput {
-	int timestamp;
-	int[] busses;
+	long timestamp;
+	long[] busses;
 }
 
 Variant run(int part, File input, bool bigboy, string[] args) {
 	ParsedInput pInput = input.byLineCopy.array.parseInput();
 	Variant result = parts!long(part, 
 			() => part1(pInput),
-			() => part2(pInput, 100000000000000));
+			() => part2(pInput));
 	return result;
 }
 
@@ -18,16 +21,15 @@ Variant run(int part, File input, bool bigboy, string[] args) {
 	ParsedInput input;
 	input.timestamp = to!int(range[0]);
 
-	input.busses = range[1].splitter(',').map!(c => c == "x" ? -1 : c.to!int).array;
-	writeln(input);
+	input.busses = range[1].splitter(',').map!(c => c == "x" ? -1 : c.to!long).array;
 	return input;
 }
 
-int part1(ParsedInput input) {
-	int earliestBus = 0;
-	int earliestBusTimestamp = int.max;
-	foreach(int bus; input.busses.filter!(x => x != -1)) {
-		int closestTimestamp = (input.timestamp / bus + 1 ) * bus;
+long part1(ParsedInput input) {
+	long earliestBus = 0;
+	long earliestBusTimestamp = long.max;
+	foreach(long bus; input.busses.filter!(x => x != -1)) {
+		long closestTimestamp = (input.timestamp / bus + 1 ) * bus;
 		writeln(closestTimestamp);
 		if (closestTimestamp < earliestBusTimestamp) {
 			earliestBusTimestamp = closestTimestamp;
@@ -39,38 +41,54 @@ int part1(ParsedInput input) {
 }
 
 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();
-			}
-		}
+	/+long[] remainders;
+	long[] divisors;
 
+	// Too much, but we're not short on memory
+	remainders.reserve(input.busses.length);
+	divisors.reserve(input.busses.length);
+	foreach (remainder, divisor; input.busses) {
+		if (divisor < 0) continue;
+		remainders ~= [remainder % divisor];
+		divisors ~= [divisor];
 	}
-	debug writeln();
 
-	return result;
+	writeln(remainders);
+	writeln(divisors);
+
+	long N = divisors.fold!((a,b) => a * b);
+	writeln(N);
+	long[] Ns; Ns.length = divisors.length;
+	long[] Xs; Xs.length = divisors.length;
+	long[] BNXs; BNXs.length = divisors.length;
+
+	foreach(i, divisor; divisors) {
+		Ns[i] = N / divisor;
+	}
+
+	foreach(ref Xi, Ni, Di; lockstep(Xs, Ns, divisors)) {
+		long a = Ni % Di;
+		long b;
+		for(b = 0; (a * b) % Di != 1; b++) {}
+		Xi = b;
+	}
+	writeln(Xs);
+
+	foreach(ref BNXi, Ri, Ni, Xi; lockstep(BNXs, remainders, Ns, Xs)) {
+		BNXi = Ri * Ni * Xi;
+	}
+	long result = BNXs.sum() % N;+/
+	Tuple!(size_t, "index", long, "value")[] busses = input.busses.enumerate.filter!((e) => e.value != -1).array;
+
+	long time = 0;
+	long step = busses[0].value;
+
+	foreach(bus; busses[1..$]) {
+		for(; (time + bus.index) % bus.value != 0; time += step) {}
+		step *= bus.value;
+	}
+	writeln(time);
+	return time;
 }
 
 unittest {
@@ -82,5 +100,9 @@ unittest {
 	ParsedInput pInput = parseInput(input);
 
 	assert(part1(pInput) == 295);
+	ParsedInput p2;
+	p2.busses = [-1, 7, -1, 5, -1, -1, 8];
+	//assert(part2(p2) == 78);
+
 	assert(part2(pInput) == 1068781);
 }