using System;

using System.Net.Http;

using System.Collections.Generic;

using System.Linq;

​

​

// Get data

var response = await new HttpClient().GetStringAsync("https://tompafireadventofcode.free.beeceptor.com/day9");

​

var rows = response.Split("\n").Select((line, rowIndex) =>

{

    var values = line.Select((val, columnIndex) => new Cell(int.Parse(val.ToString()), columnIndex, rowIndex)).ToList();

    return new Row(values, rowIndex);

});

​

var heightMap = new HeightMap(rows);

​

var lowestPointsPerRow = heightMap.Rows.SelectMany(row =>

    row.Values.Where(val =>

    {

        int? prev = val.ColumnIndex > 0 ? row.Values[val.ColumnIndex - 1].Value : null;

        int? next = val.ColumnIndex < row.Values.Count - 1 ? row.Values[val.ColumnIndex + 1].Value : null;

​

        return prev.GetValueOrDefault(val.Value + 1) > val.Value && next.GetValueOrDefault(val.Value + 1) > val.Value;

    })

);

​

var lowestPointsPerColumns = heightMap.Columns.SelectMany(col =>

    col.Values.Where(val =>

    {

        int? prev = val.RowIndex > 0 ? col.Values[val.RowIndex - 1].Value : null;

        int? next = val.RowIndex < col.Values.Count - 1 ? col.Values[val.RowIndex + 1].Value : null;

​

        return prev.GetValueOrDefault(val.Value + 1) > val.Value && next.GetValueOrDefault(val.Value + 1) > val.Value;

    })

);

​

var lowestPoints = lowestPointsPerRow.Intersect(lowestPointsPerColumns).ToList();

​

​

// Step 1

​

var riskLevels = lowestPoints.Select(val => 1 + val.Value);

​

var result = riskLevels.Sum();

​

Console.WriteLine(result);

​

​

// Step 2

​

var basins = lowestPoints.Select(val => GetBasin(heightMap, val)).ToList();

​

var top3BasinCounts = basins.Select(b => b.Count()).OrderByDescending(c => c).Take(3);

​

var result2 = top3BasinCounts.Aggregate(1, (x, b) => x * b);

​

Console.WriteLine(result2);

​

​

List<Cell> GetBasin(HeightMap heightMap, Cell cell)

{

    var row = GetRow(heightMap, cell);

    var col = GetColumn(heightMap, cell);

    var basin = new List<Cell>

    {

        cell

    };

​

    var adjacent = new[]

    {

        NavigatePrevious(row.Values, cell, x => x.ColumnIndex),

        NavigateNext(row.Values, cell, x => x.ColumnIndex),

        NavigatePrevious(col.Values, cell, x => x.RowIndex),

        NavigateNext(col.Values, cell, x => x.RowIndex),

    };

​

    var matches = adjacent.Where(x => IsMatch(x, cell.Value)).Select(x => x!).ToList();

​

    if (matches.Any())

    {

        basin.AddRange(matches

                       .SelectMany(x => GetBasin(heightMap, x)));

        basin = basin.Distinct().ToList();

    }

    return basin;

}

​

bool IsMatch(Cell? cell, int compareToValue)

{

    return cell != null && cell.Value != 9 && cell.Value > compareToValue;

}

​

Cell? NavigatePrevious(IEnumerable<Cell> values, Cell cell, Func<Cell, int> indexFunc)

{

    return indexFunc(cell) > 0 ? values.ElementAt(indexFunc(cell) - 1) : null;

}

​

Cell? NavigateNext(IEnumerable<Cell> values, Cell cell, Func<Cell, int> indexFunc)

{

    return indexFunc(cell) < values.Count() - 1 ? values.ElementAt(indexFunc(cell) + 1) : null;

}

​

Row GetRow(HeightMap heightMap, Cell cell)

{

    return heightMap.Rows.Single(row => row.Index == cell.RowIndex);

}

​

Column GetColumn(HeightMap heightMap, Cell cell)

{

    return heightMap.Columns.Single(col => col.Index == cell.ColumnIndex);

}

​

​

public record Row(List<Cell> Values, int Index);

​

public record Column(List<Cell> Values, int Index);

​

public record Cell(int Value, int ColumnIndex, int RowIndex);

​

public record HeightMap(IEnumerable<Row> Rows)

{

    public List<Column> Columns =>

        Rows.SelectMany(row => row.Values)

        .GroupBy(x => x.ColumnIndex)

        .Select(x => new Column(x.ToList(), x.Key)).ToList();

}