diff --git a/aoc2021.test/DayTests.cs b/aoc2021.test/DayTests.cs
index 6e2a340..31b1170 100644
--- a/aoc2021.test/DayTests.cs
+++ b/aoc2021.test/DayTests.cs
@@ -42,6 +42,7 @@ public class DayTests
     [DataRow(typeof(Day02), "150", "900")]
     [DataRow(typeof(Day03), "198", "230")]
     [DataRow(typeof(Day04), "4512", "1924")]
+    [DataRow(typeof(Day05), "5", "")]
     public void CheckTestInputs(Type dayType, string part1, string part2)
     {
         Day.UseTestInput = true;
diff --git a/aoc2021/Day05.cs b/aoc2021/Day05.cs
new file mode 100644
index 0000000..aa82eff
--- /dev/null
+++ b/aoc2021/Day05.cs
@@ -0,0 +1,82 @@
+using System.Drawing;
+
+namespace aoc2021;
+
+/// <summary>
+/// Day 5: <see href="https://adventofcode.com/2021/day/5"/>
+/// </summary>
+public sealed class Day05 : Day
+{
+    private readonly List<Line> _lines;
+    
+    public Day05() : base(5, "Hydrothermal Venture")
+    {
+        _lines = Input.Select(l => new Line(l)).ToList();
+    }
+
+    private class Line
+    {
+        public int X1 { get; }
+        public int X2 { get; }
+        public int Y1 { get; }
+        public int Y2 { get; }
+
+        public readonly List<Point> AllPoints = new();
+        
+        public Line(string line)
+        {
+            var s = line
+                .Split(" -> ")
+                .Select(i => i.Split(',').Select(int.Parse))
+                .SelectMany(i => i)
+                .ToList();
+            
+            X1 = s[0];
+            X2 = s[1];
+            Y1 = s[2];
+            Y2 = s[3];
+
+            if (X1 == X2)
+            {
+                var minY = Math.Min(Y1, Y2);
+                var maxY = Math.Max(Y1, Y2);
+                
+                for (var i = 0; i < maxY - minY; i++)
+                {
+                    AllPoints.Add(new(minY + i, X1));
+                }
+            }
+            else if (Y1 == Y2)
+            {
+                var minX = Math.Min(X1, X2);
+                var maxX = Math.Max(X1, X2);
+
+                for (var i = 0; i < maxX - minX; i++)
+                {
+                    AllPoints.Add(new(minX + i, Y1));
+                }
+            }
+            else
+            {
+                var oX = X1 < X2 ? 1 : -1;
+                var oY = Y1 < Y2 ? 1 : -1;
+                for (var i = 0; i < Math.Abs(X1 - X2); i++)
+                {
+                    AllPoints.Add(new(X1 + i * oX, Y1 + i * oY));
+                }
+            }
+        }
+    }
+
+    public override string Part1()
+    {
+        var groups = _lines
+            .Where(l => l.X1 != l.X2 && l.Y1 != l.Y2)
+            .SelectMany(l => l.AllPoints)
+            .GroupBy(x => x);
+            
+        return $"{groups.Count(g => g.Count() > 1)}";
+    }
+
+    public override string Part2() => "";
+}
diff --git a/aoc2021/input/day05.in b/aoc2021/input/day05.in
new file mode 100644
index 0000000..6873750
--- /dev/null
+++ b/aoc2021/input/day05.in
@@ -0,0 +1,500 @@
+959,103 -> 139,923
+899,63 -> 899,53
+591,871 -> 364,644
+739,913 -> 310,484
+460,309 -> 460,705
+351,389 -> 351,837
+420,55 -> 420,541
+916,520 -> 382,520
+136,604 -> 295,604
+343,59 -> 142,59
+951,206 -> 806,206
+984,950 -> 61,27
+739,388 -> 988,388
+374,370 -> 644,370
+403,504 -> 798,899
+943,535 -> 229,535
+149,227 -> 583,661
+62,126 -> 62,352
+131,391 -> 131,717
+254,944 -> 254,220
+572,111 -> 572,47
+336,249 -> 830,743
+898,858 -> 203,163
+203,320 -> 825,942
+19,318 -> 19,120
+740,432 -> 740,39
+591,383 -> 220,754
+445,592 -> 19,592
+202,370 -> 837,370
+473,916 -> 600,789
+118,955 -> 884,189
+377,376 -> 533,532
+160,264 -> 160,62
+627,572 -> 627,679
+217,690 -> 217,629
+685,323 -> 866,504
+391,309 -> 493,207
+872,776 -> 357,776
+116,326 -> 116,426
+82,900 -> 832,900
+594,862 -> 594,593
+802,636 -> 802,223
+862,226 -> 862,787
+313,573 -> 834,573
+145,631 -> 13,499
+545,433 -> 420,308
+427,623 -> 427,808
+898,120 -> 511,120
+859,811 -> 859,28
+715,958 -> 715,893
+115,234 -> 484,234
+125,253 -> 50,253
+737,265 -> 158,265
+523,965 -> 523,983
+118,51 -> 118,766
+455,774 -> 455,357
+680,881 -> 925,881
+191,186 -> 187,186
+821,629 -> 792,658
+397,906 -> 397,962
+988,811 -> 988,427
+91,733 -> 519,733
+46,172 -> 566,172
+740,619 -> 880,759
+609,465 -> 609,702
+289,211 -> 289,620
+622,135 -> 622,929
+113,53 -> 872,53
+559,713 -> 559,132
+894,237 -> 211,920
+237,259 -> 237,39
+738,542 -> 976,542
+163,34 -> 525,34
+681,672 -> 264,255
+37,827 -> 722,827
+60,803 -> 514,349
+433,866 -> 433,257
+379,493 -> 379,643
+697,588 -> 192,83
+875,646 -> 318,89
+634,983 -> 634,111
+636,69 -> 636,41
+911,780 -> 701,570
+792,71 -> 956,71
+682,338 -> 608,412
+257,768 -> 450,575
+112,25 -> 795,708
+730,86 -> 730,65
+966,785 -> 789,608
+390,263 -> 483,356
+90,852 -> 90,471
+507,914 -> 769,914
+803,535 -> 803,245
+710,787 -> 570,787
+138,842 -> 270,710
+862,988 -> 862,656
+56,408 -> 849,408
+16,10 -> 979,973
+982,14 -> 12,984
+647,915 -> 38,306
+797,487 -> 19,487
+539,933 -> 924,933
+509,734 -> 176,734
+813,505 -> 976,505
+474,987 -> 474,896
+21,200 -> 164,200
+986,973 -> 31,18
+919,830 -> 111,22
+32,574 -> 456,150
+743,595 -> 842,595
+623,306 -> 722,306
+878,367 -> 519,367
+924,221 -> 924,231
+86,950 -> 773,263
+950,248 -> 537,248
+149,155 -> 962,968
+449,568 -> 179,568
+186,304 -> 868,986
+921,320 -> 639,602
+602,262 -> 602,500
+602,33 -> 602,248
+380,731 -> 423,774
+535,110 -> 638,110
+552,317 -> 552,75
+173,667 -> 173,847
+707,480 -> 195,480
+833,398 -> 267,964
+276,716 -> 413,716
+342,816 -> 922,816
+24,184 -> 715,875
+762,330 -> 717,285
+718,886 -> 718,551
+707,834 -> 707,704
+479,578 -> 161,896
+145,297 -> 145,435
+760,651 -> 536,875
+954,629 -> 954,816
+305,949 -> 305,919
+55,132 -> 55,233
+469,85 -> 439,85
+653,990 -> 536,990
+876,531 -> 432,87
+698,207 -> 698,672
+11,70 -> 766,825
+591,357 -> 30,918
+697,987 -> 697,823
+610,903 -> 370,663
+319,678 -> 319,504
+337,150 -> 309,150
+876,57 -> 311,57
+673,268 -> 345,596
+895,364 -> 518,741
+327,662 -> 941,48
+77,709 -> 110,742
+194,78 -> 661,78
+587,24 -> 825,24
+503,317 -> 719,317
+459,632 -> 704,387
+717,292 -> 835,292
+912,927 -> 72,87
+510,527 -> 146,527
+336,771 -> 336,266
+566,961 -> 496,961
+969,335 -> 122,335
+925,443 -> 925,397
+316,812 -> 606,812
+815,795 -> 116,795
+169,36 -> 354,36
+358,274 -> 389,274
+302,147 -> 839,684
+762,372 -> 972,372
+172,721 -> 682,211
+265,150 -> 248,167
+753,559 -> 307,559
+823,121 -> 823,126
+498,856 -> 498,135
+75,977 -> 75,381
+541,297 -> 541,320
+735,108 -> 866,108
+434,907 -> 868,907
+915,959 -> 255,959
+967,666 -> 967,209
+361,600 -> 361,222
+314,580 -> 314,497
+175,989 -> 523,641
+957,97 -> 311,743
+956,227 -> 12,227
+95,364 -> 95,742
+857,141 -> 193,805
+388,651 -> 468,731
+582,177 -> 324,177
+68,272 -> 68,720
+543,490 -> 910,490
+508,281 -> 902,281
+823,380 -> 823,296
+23,10 -> 946,933
+813,70 -> 813,450
+881,893 -> 598,893
+535,781 -> 973,781
+80,890 -> 909,61
+604,630 -> 307,927
+836,917 -> 184,917
+76,727 -> 10,727
+727,235 -> 727,578
+629,80 -> 892,80
+110,655 -> 663,102
+985,12 -> 11,986
+830,656 -> 830,761
+660,869 -> 660,543
+381,340 -> 381,562
+392,735 -> 417,735
+855,24 -> 320,24
+801,669 -> 278,146
+730,964 -> 107,964
+523,158 -> 385,20
+27,833 -> 27,987
+569,707 -> 500,707
+527,732 -> 527,424
+74,88 -> 273,287
+143,974 -> 143,735
+247,388 -> 813,954
+577,14 -> 945,382
+49,43 -> 953,947
+332,210 -> 332,143
+69,280 -> 949,280
+25,923 -> 904,44
+306,569 -> 306,470
+158,273 -> 113,228
+771,355 -> 694,278
+515,115 -> 245,385
+427,381 -> 427,729
+16,987 -> 987,16
+319,463 -> 319,234
+854,977 -> 66,189
+794,194 -> 794,183
+576,65 -> 576,843
+37,964 -> 734,964
+740,920 -> 740,877
+245,487 -> 245,957
+404,794 -> 853,794
+660,656 -> 660,756
+921,605 -> 127,605
+650,894 -> 916,894
+968,893 -> 481,406
+986,979 -> 21,14
+154,303 -> 498,647
+720,338 -> 229,338
+62,936 -> 62,897
+55,820 -> 55,923
+812,31 -> 551,31
+338,466 -> 951,466
+663,492 -> 775,604
+449,602 -> 39,602
+44,403 -> 44,144
+58,62 -> 339,62
+713,730 -> 713,502
+704,525 -> 976,797
+372,709 -> 372,680
+709,387 -> 153,387
+922,103 -> 615,103
+629,839 -> 121,839
+206,722 -> 529,722
+232,556 -> 422,746
+300,470 -> 300,726
+376,820 -> 622,574
+834,25 -> 255,604
+271,200 -> 271,875
+804,934 -> 872,934
+900,753 -> 900,632
+604,323 -> 604,70
+890,911 -> 890,41
+464,169 -> 812,169
+850,196 -> 850,903
+34,574 -> 34,54
+718,59 -> 462,315
+431,923 -> 737,923
+433,573 -> 433,420
+297,478 -> 297,775
+756,545 -> 544,545
+247,708 -> 247,702
+736,835 -> 173,272
+319,85 -> 319,827
+931,775 -> 683,775
+292,315 -> 451,315
+397,435 -> 380,435
+987,978 -> 82,73
+227,349 -> 227,724
+349,741 -> 899,191
+965,325 -> 765,125
+849,306 -> 88,306
+516,548 -> 516,902
+919,395 -> 568,395
+736,507 -> 192,507
+960,782 -> 196,18
+431,413 -> 510,492
+911,696 -> 911,830
+888,225 -> 174,225
+57,790 -> 57,953
+858,399 -> 119,399
+59,302 -> 290,302
+456,907 -> 456,599
+374,743 -> 374,565
+183,107 -> 183,171
+58,699 -> 288,699
+886,970 -> 109,193
+940,395 -> 806,261
+781,480 -> 596,665
+456,724 -> 265,724
+414,406 -> 299,521
+115,898 -> 115,863
+34,543 -> 34,496
+900,843 -> 900,457
+165,209 -> 189,209
+976,627 -> 539,190
+252,202 -> 137,202
+584,339 -> 550,373
+580,153 -> 380,353
+232,412 -> 650,830
+910,833 -> 88,11
+418,245 -> 829,245
+298,823 -> 907,214
+91,876 -> 495,876
+315,874 -> 650,539
+907,635 -> 365,635
+339,313 -> 320,313
+362,435 -> 362,938
+152,664 -> 152,391
+253,210 -> 272,210
+216,396 -> 216,726
+852,912 -> 15,75
+882,828 -> 689,828
+674,533 -> 674,523
+469,719 -> 469,79
+733,169 -> 665,101
+734,632 -> 717,632
+615,565 -> 615,114
+979,720 -> 243,720
+827,125 -> 827,919
+605,419 -> 601,419
+749,13 -> 433,329
+990,902 -> 990,843
+186,679 -> 186,457
+374,796 -> 736,796
+133,867 -> 133,801
+757,622 -> 812,567
+351,179 -> 351,509
+214,748 -> 575,748
+177,903 -> 861,219
+747,981 -> 747,64
+588,125 -> 588,557
+464,338 -> 769,338
+645,669 -> 125,149
+579,352 -> 138,352
+77,605 -> 520,605
+698,816 -> 698,917
+112,943 -> 112,834
+731,720 -> 724,720
+887,440 -> 976,351
+676,301 -> 676,741
+870,732 -> 870,648
+250,826 -> 413,826
+399,720 -> 543,864
+834,93 -> 468,459
+415,475 -> 415,641
+793,415 -> 47,415
+365,476 -> 365,31
+195,154 -> 813,154
+503,605 -> 773,605
+553,121 -> 851,121
+25,420 -> 423,818
+943,110 -> 258,110
+775,436 -> 826,436
+16,161 -> 16,889
+702,555 -> 920,555
+589,858 -> 533,802
+932,404 -> 932,539
+647,275 -> 647,962
+87,179 -> 326,179
+931,588 -> 931,287
+868,96 -> 557,96
+879,28 -> 875,28
+375,132 -> 287,44
+484,352 -> 644,512
+448,566 -> 448,214
+734,460 -> 717,460
+550,379 -> 550,674
+964,184 -> 820,328
+167,504 -> 387,504
+594,777 -> 952,777
+328,712 -> 837,712
+600,773 -> 546,773
+955,954 -> 82,81
+863,790 -> 863,86
+831,773 -> 32,773
+987,11 -> 19,979
+901,878 -> 901,177
+427,341 -> 721,635
+690,835 -> 567,835
+557,724 -> 14,181
+591,20 -> 205,406
+846,865 -> 846,859
+644,646 -> 742,548
+187,376 -> 187,563
+367,806 -> 250,923
+332,731 -> 468,731
+378,431 -> 469,431
+844,949 -> 844,452
+172,320 -> 735,320
+597,639 -> 633,639
+353,831 -> 353,307
+355,392 -> 465,392
+624,179 -> 548,255
+441,928 -> 401,888
+442,680 -> 442,569
+567,385 -> 908,44
+10,561 -> 603,561
+851,289 -> 13,289
+832,143 -> 832,64
+366,851 -> 67,851
+890,404 -> 333,961
+83,22 -> 963,902
+10,783 -> 821,783
+369,481 -> 369,611
+943,356 -> 846,356
+675,95 -> 335,435
+442,928 -> 442,764
+500,643 -> 334,643
+90,207 -> 620,207
+520,412 -> 745,187
+586,89 -> 613,89
+411,424 -> 595,424
+938,650 -> 232,650
+216,773 -> 76,773
+895,690 -> 895,294
+250,886 -> 250,605
+296,422 -> 863,989
+534,626 -> 534,707
+577,608 -> 52,83
+61,674 -> 714,21
+844,126 -> 844,694
+565,541 -> 253,229
+62,24 -> 986,948
+588,901 -> 588,212
+541,508 -> 541,141
+516,376 -> 589,449
+390,215 -> 749,215
+324,878 -> 296,850
+592,408 -> 592,158
+433,207 -> 172,207
+139,72 -> 139,121
+471,676 -> 268,676
+374,433 -> 374,95
+672,459 -> 640,427
+348,577 -> 843,82
+903,466 -> 903,348
+437,759 -> 726,470
+152,101 -> 325,274
+933,897 -> 335,897
+516,877 -> 505,866
+890,715 -> 570,715
+78,124 -> 871,917
+360,645 -> 967,645
+645,271 -> 645,57
+693,878 -> 693,159
+49,77 -> 49,744
+935,914 -> 97,76
+941,726 -> 941,464
+756,985 -> 756,480
+887,378 -> 887,529
+405,925 -> 405,533
+533,156 -> 201,156
+565,535 -> 120,90
+51,15 -> 967,931
+660,218 -> 660,339
+522,682 -> 571,682
+958,899 -> 729,899
+521,687 -> 288,687
+643,148 -> 468,323
+989,971 -> 68,50
+729,273 -> 311,691
+245,205 -> 305,205
+634,747 -> 634,605
+280,407 -> 488,199
+109,931 -> 706,334
+849,694 -> 615,928
+794,84 -> 218,84
+669,184 -> 865,184
+936,834 -> 234,132
+691,445 -> 914,668
+423,161 -> 515,69
+81,674 -> 37,674
+292,423 -> 292,741
+188,306 -> 844,962
+204,309 -> 204,705
+961,652 -> 746,652
+985,987 -> 11,13
+139,153 -> 936,950
+436,978 -> 244,978
+921,633 -> 921,340
+872,63 -> 233,63
diff --git a/aoc2021/input/test05.in b/aoc2021/input/test05.in
new file mode 100644
index 0000000..82029a1
--- /dev/null
+++ b/aoc2021/input/test05.in
@@ -0,0 +1,10 @@
+0,9 -> 5,9
+8,0 -> 0,8
+9,4 -> 3,4
+2,2 -> 2,1
+7,0 -> 7,4
+6,4 -> 2,0
+0,9 -> 2,9
+3,4 -> 1,4
+0,0 -> 8,8
+5,5 -> 8,2
\ No newline at end of file