public static long Part2(IEnumerable <string> busIds)
{
if (busIds == null)
{
throw new ArgumentNullException(nameof(busIds));
}
IEnumerable <(long, int)> validBusIdsWithIndexes = busIds
.Select((busId, i) => (busId, i))
.Where(t => t.Item1 != "x")
.Select(t => (long.Parse(t.Item1, CultureInfo.InvariantCulture), t.Item2));
return(ChineseRemainderTheorem.Solve(
validBusIdsWithIndexes.Select(t => t.Item1).ToArray(),
validBusIdsWithIndexes.Select(t => t.Item1 - t.Item2).ToArray()
));
}
static long Part2(string[] lines)
{
int i = -1;
List <Congruence> congruences = new List <Congruence>();
foreach (var l in lines[1].Split(','))
{
i++;
if (l == "x")
{
continue;
}
int m = int.Parse(l);
congruences.Add(new Congruence(m - i, int.Parse(l)));
}
return(ChineseRemainderTheorem.Solve(congruences));
}
static void Main(string[] args)
{
var input = File.ReadAllLines("Input.txt");
int time = Convert.ToInt32(input[0]);
var busses = new List <int>();
foreach (var bus in input[1].Split(','))
{
if (Int32.TryParse(bus, out int busnum))
{
busses.Add(busnum);
}
}
var nextBus = (from busNumber in busses
select(Number: busNumber, Time: GetNextBusTime(time, busNumber)))
.OrderBy(x => x.Time).First();
var part1Answer = (nextBus.Time - time) * nextBus.Number;
Console.WriteLine($"Part1: {part1Answer}");
// Part2: Notes
// Start at first bus arrival from index 1. This is `t`
// Take the next bus, look up the index. Figure out if t + index is divisible by busNumber
// If true, keep going untill all numbers are exhausted
// Recursion? -- No.
// If false increment t by first bus number and try again
// Forget all above, use chinese remained theroem copypasta
var valueOffset = input[1].Split(',')
.Select((value, offset) => (value, offset))
.Where(x => x.value != "x")
.Select(x => (Value: Convert.ToInt64(x.value), Offset: x.offset));
var n = valueOffset.Select(x => x.Value).ToArray();
var a = valueOffset.Select(x => x.Value - x.Offset).ToArray();
a[0] = 0;
var part2Answer = ChineseRemainderTheorem.Solve(n, a);
Console.WriteLine($"Part2: {part2Answer}");
}