diff --git a/Cargo.lock b/Cargo.lock
index 63820ae..a877a59 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -13,4 +13,20 @@ name = "aoc2021"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "itertools",
+]
+
+[[package]]
+name = "either"
+version = "1.6.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e78d4f1cc4ae33bbfc157ed5d5a5ef3bc29227303d595861deb238fcec4e9457"
+
+[[package]]
+name = "itertools"
+version = "0.10.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "69ddb889f9d0d08a67338271fa9b62996bc788c7796a5c18cf057420aaed5eaf"
+dependencies = [
+ "either",
 ]
diff --git a/Cargo.toml b/Cargo.toml
index 8f5f03e..0f2ec28 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -7,3 +7,4 @@ edition = "2021"
 
 [dependencies]
 anyhow = "1.0"
+itertools = "0.10.1"
diff --git a/data/day05-test.txt b/data/day05-test.txt
new file mode 100644
index 0000000..b258f68
--- /dev/null
+++ b/data/day05-test.txt
@@ -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
diff --git a/data/day05.txt b/data/day05.txt
new file mode 100644
index 0000000..302b42d
--- /dev/null
+++ b/data/day05.txt
@@ -0,0 +1,500 @@
+503,977 -> 843,637
+437,518 -> 437,225
+269,250 -> 625,250
+846,751 -> 646,751
+18,731 -> 402,731
+749,923 -> 749,986
+557,758 -> 557,797
+589,54 -> 589,616
+20,136 -> 819,935
+123,983 -> 12,983
+802,624 -> 709,624
+600,458 -> 128,458
+209,703 -> 459,703
+944,415 -> 742,415
+270,718 -> 656,332
+168,339 -> 283,339
+558,172 -> 695,309
+519,524 -> 480,524
+456,400 -> 134,722
+355,961 -> 352,961
+757,158 -> 49,866
+300,254 -> 618,254
+554,275 -> 859,275
+47,612 -> 622,37
+696,885 -> 696,856
+342,803 -> 342,429
+830,649 -> 254,73
+54,921 -> 958,17
+72,691 -> 818,691
+80,72 -> 80,546
+762,649 -> 762,371
+117,39 -> 605,39
+778,605 -> 610,773
+159,25 -> 67,25
+462,825 -> 766,825
+295,167 -> 357,167
+741,379 -> 11,379
+942,230 -> 942,26
+802,672 -> 802,311
+672,759 -> 330,759
+419,97 -> 848,526
+244,262 -> 938,956
+281,988 -> 281,879
+471,451 -> 471,907
+238,387 -> 238,665
+907,129 -> 907,368
+119,734 -> 155,770
+306,119 -> 306,568
+166,175 -> 166,277
+591,32 -> 591,855
+779,324 -> 916,324
+785,245 -> 884,245
+504,884 -> 781,884
+405,967 -> 664,967
+17,16 -> 989,988
+429,944 -> 729,944
+15,31 -> 918,934
+22,963 -> 971,14
+199,280 -> 481,562
+792,550 -> 792,485
+215,141 -> 215,58
+511,560 -> 511,224
+940,134 -> 166,908
+666,212 -> 666,193
+881,514 -> 117,514
+271,416 -> 614,73
+354,784 -> 354,41
+866,152 -> 866,969
+75,924 -> 868,131
+944,507 -> 944,556
+726,315 -> 624,315
+195,122 -> 593,520
+463,311 -> 946,794
+734,698 -> 734,953
+520,98 -> 915,98
+125,139 -> 616,139
+570,841 -> 206,477
+430,442 -> 430,893
+653,59 -> 653,155
+906,883 -> 314,291
+932,157 -> 100,989
+526,244 -> 362,244
+28,837 -> 792,73
+386,426 -> 386,673
+121,66 -> 203,66
+747,121 -> 392,476
+590,989 -> 230,989
+795,83 -> 795,398
+741,522 -> 741,677
+142,166 -> 470,166
+13,645 -> 493,165
+418,567 -> 418,970
+94,877 -> 782,189
+603,426 -> 603,916
+364,304 -> 191,304
+754,146 -> 754,507
+294,678 -> 438,678
+641,633 -> 217,633
+31,133 -> 831,933
+250,976 -> 250,355
+274,558 -> 899,558
+818,507 -> 693,507
+414,465 -> 924,975
+116,729 -> 116,951
+960,843 -> 149,32
+724,127 -> 498,353
+552,43 -> 964,43
+224,853 -> 224,363
+768,214 -> 768,88
+518,414 -> 518,119
+917,824 -> 948,824
+37,81 -> 882,926
+333,390 -> 967,390
+175,453 -> 388,240
+960,395 -> 960,697
+468,37 -> 468,275
+745,318 -> 425,318
+676,425 -> 903,198
+531,663 -> 86,663
+557,834 -> 557,967
+650,931 -> 383,664
+906,197 -> 906,567
+675,637 -> 326,288
+227,977 -> 51,977
+347,524 -> 793,970
+778,850 -> 778,342
+343,155 -> 343,739
+970,167 -> 161,976
+800,127 -> 800,667
+531,533 -> 193,533
+222,776 -> 222,873
+922,29 -> 13,938
+452,808 -> 452,793
+926,142 -> 926,198
+940,93 -> 55,978
+335,51 -> 938,654
+789,352 -> 307,352
+457,419 -> 469,419
+463,12 -> 463,132
+881,95 -> 431,95
+531,57 -> 531,40
+179,308 -> 516,308
+767,907 -> 629,907
+362,457 -> 362,262
+774,896 -> 154,276
+549,243 -> 247,243
+130,933 -> 202,933
+266,639 -> 189,716
+209,717 -> 209,844
+625,296 -> 625,575
+739,785 -> 873,785
+713,857 -> 94,238
+97,28 -> 937,868
+876,734 -> 344,202
+180,608 -> 557,608
+669,566 -> 669,389
+112,779 -> 267,624
+325,669 -> 449,669
+102,915 -> 357,915
+882,839 -> 882,512
+330,699 -> 330,858
+773,851 -> 773,429
+171,187 -> 450,187
+166,726 -> 166,74
+15,134 -> 750,869
+245,126 -> 245,518
+919,484 -> 919,602
+918,900 -> 918,372
+736,515 -> 708,487
+790,777 -> 790,303
+479,541 -> 381,541
+85,243 -> 317,475
+619,441 -> 619,823
+688,658 -> 688,404
+381,475 -> 891,985
+461,529 -> 145,213
+833,885 -> 404,885
+315,502 -> 315,770
+450,934 -> 740,934
+634,334 -> 634,202
+785,866 -> 785,913
+976,627 -> 976,102
+65,491 -> 570,491
+974,257 -> 503,728
+662,938 -> 720,938
+232,472 -> 215,472
+805,504 -> 805,476
+99,909 -> 99,399
+64,947 -> 926,85
+123,645 -> 153,615
+908,10 -> 92,826
+49,174 -> 819,944
+115,136 -> 863,884
+695,91 -> 695,612
+715,527 -> 550,362
+914,125 -> 914,86
+14,980 -> 981,13
+14,308 -> 14,355
+356,895 -> 766,485
+989,10 -> 10,989
+833,292 -> 833,184
+786,785 -> 733,785
+824,53 -> 116,53
+349,547 -> 349,66
+594,189 -> 636,189
+359,399 -> 769,809
+600,751 -> 600,46
+520,236 -> 228,528
+978,610 -> 978,832
+689,575 -> 258,575
+664,734 -> 850,920
+245,672 -> 245,878
+337,509 -> 578,509
+893,613 -> 380,613
+875,608 -> 875,444
+264,701 -> 946,19
+358,267 -> 358,648
+926,61 -> 926,378
+46,885 -> 190,885
+662,131 -> 82,131
+301,53 -> 301,533
+21,839 -> 915,839
+92,174 -> 113,174
+145,680 -> 294,680
+268,773 -> 268,193
+698,893 -> 809,893
+300,512 -> 807,512
+749,408 -> 279,408
+439,214 -> 439,172
+622,740 -> 339,740
+400,253 -> 400,486
+859,686 -> 387,214
+974,485 -> 974,486
+70,987 -> 951,106
+630,449 -> 630,544
+796,212 -> 608,24
+835,959 -> 835,725
+779,755 -> 96,72
+582,778 -> 440,636
+350,479 -> 827,479
+924,40 -> 605,40
+918,832 -> 918,617
+669,590 -> 191,112
+748,214 -> 748,462
+350,703 -> 163,703
+393,791 -> 393,240
+569,857 -> 569,939
+412,375 -> 412,603
+488,975 -> 22,509
+100,372 -> 100,685
+170,669 -> 212,669
+546,734 -> 546,274
+492,172 -> 492,354
+36,134 -> 801,899
+501,773 -> 582,773
+287,694 -> 287,939
+381,988 -> 367,988
+609,360 -> 609,478
+310,158 -> 25,443
+409,716 -> 409,27
+655,959 -> 383,687
+16,697 -> 611,102
+184,290 -> 930,290
+580,79 -> 598,97
+950,65 -> 777,65
+144,288 -> 821,965
+986,685 -> 986,412
+549,702 -> 549,369
+841,148 -> 259,730
+958,31 -> 958,810
+12,908 -> 856,64
+264,793 -> 264,960
+249,115 -> 249,935
+707,714 -> 108,714
+527,192 -> 982,647
+703,883 -> 703,580
+535,346 -> 543,346
+851,185 -> 83,953
+984,586 -> 984,681
+913,574 -> 350,11
+317,221 -> 405,221
+398,673 -> 160,435
+953,264 -> 547,670
+790,115 -> 538,367
+943,236 -> 295,884
+571,746 -> 571,231
+286,318 -> 131,318
+143,251 -> 436,544
+838,435 -> 793,435
+732,782 -> 732,407
+244,287 -> 244,335
+376,29 -> 75,29
+604,732 -> 738,732
+730,30 -> 533,30
+891,474 -> 891,25
+786,140 -> 368,140
+951,583 -> 828,460
+665,897 -> 44,276
+217,905 -> 742,905
+745,583 -> 256,583
+923,22 -> 23,922
+763,336 -> 943,516
+755,678 -> 755,101
+35,790 -> 706,119
+841,658 -> 841,634
+986,66 -> 986,412
+740,69 -> 740,878
+852,733 -> 453,733
+657,273 -> 215,715
+239,824 -> 239,79
+340,482 -> 340,238
+969,834 -> 303,168
+238,718 -> 931,718
+603,63 -> 603,363
+596,135 -> 367,135
+184,474 -> 184,612
+39,60 -> 920,941
+456,103 -> 894,541
+929,35 -> 738,35
+199,528 -> 707,528
+649,251 -> 134,766
+969,209 -> 719,459
+568,45 -> 306,307
+259,703 -> 426,536
+964,737 -> 342,115
+101,890 -> 604,890
+57,223 -> 812,978
+939,99 -> 167,871
+920,438 -> 920,247
+185,384 -> 643,384
+489,783 -> 121,415
+837,938 -> 250,351
+63,920 -> 945,38
+475,45 -> 510,10
+881,872 -> 141,132
+24,238 -> 24,468
+409,523 -> 409,706
+200,309 -> 631,740
+586,385 -> 900,385
+219,250 -> 219,327
+854,526 -> 854,725
+946,343 -> 946,267
+847,746 -> 717,616
+172,203 -> 346,29
+693,652 -> 545,652
+824,115 -> 192,115
+843,908 -> 333,908
+769,784 -> 662,784
+490,535 -> 490,524
+545,699 -> 11,699
+792,544 -> 287,39
+895,712 -> 895,41
+887,350 -> 624,350
+614,475 -> 924,165
+93,961 -> 265,789
+57,71 -> 852,866
+282,738 -> 844,176
+898,251 -> 898,669
+949,872 -> 866,872
+765,408 -> 545,408
+691,503 -> 235,959
+198,491 -> 227,462
+973,40 -> 56,957
+802,402 -> 752,402
+911,60 -> 911,932
+545,244 -> 110,244
+461,26 -> 461,18
+916,308 -> 161,308
+368,476 -> 515,476
+656,916 -> 409,669
+118,950 -> 118,135
+963,294 -> 365,294
+855,713 -> 323,713
+849,930 -> 48,129
+36,337 -> 588,889
+941,394 -> 941,697
+685,170 -> 323,170
+423,683 -> 423,152
+81,522 -> 121,522
+357,598 -> 159,796
+211,192 -> 211,50
+615,607 -> 270,952
+687,384 -> 687,128
+81,896 -> 925,52
+591,988 -> 20,988
+950,740 -> 605,740
+818,772 -> 623,772
+790,405 -> 790,775
+483,34 -> 718,34
+309,190 -> 309,894
+391,83 -> 483,83
+721,201 -> 721,843
+990,464 -> 990,171
+479,707 -> 688,707
+23,775 -> 510,775
+783,863 -> 867,779
+594,151 -> 208,151
+416,936 -> 416,720
+981,972 -> 120,111
+773,476 -> 138,476
+604,900 -> 604,395
+824,437 -> 531,437
+621,948 -> 32,948
+802,26 -> 887,26
+836,335 -> 836,784
+134,585 -> 634,85
+649,87 -> 649,263
+756,804 -> 638,804
+982,26 -> 21,987
+134,976 -> 914,196
+612,539 -> 612,141
+977,11 -> 22,966
+40,80 -> 40,644
+725,562 -> 604,562
+377,649 -> 352,624
+418,146 -> 130,434
+848,927 -> 848,970
+243,350 -> 342,449
+46,10 -> 46,112
+800,654 -> 272,126
+910,633 -> 910,426
+296,619 -> 882,33
+75,922 -> 497,500
+267,616 -> 864,616
+884,694 -> 624,694
+13,656 -> 831,656
+389,390 -> 389,316
+26,24 -> 987,985
+193,557 -> 589,161
+18,13 -> 978,973
+43,951 -> 614,951
+581,398 -> 885,94
+943,525 -> 279,525
+787,83 -> 137,83
+729,271 -> 729,18
+100,383 -> 100,690
+337,266 -> 102,266
+106,640 -> 298,832
+83,65 -> 543,65
+102,872 -> 663,872
+921,765 -> 921,782
+764,392 -> 471,685
+325,987 -> 802,987
+983,43 -> 983,852
+833,475 -> 416,58
+25,270 -> 686,931
+145,433 -> 151,433
+132,329 -> 973,329
+611,494 -> 98,494
+401,633 -> 866,168
+532,126 -> 532,448
+988,894 -> 361,894
+249,177 -> 249,133
+832,71 -> 832,245
+263,70 -> 263,152
+548,333 -> 548,748
+98,570 -> 438,910
+954,41 -> 41,954
+336,199 -> 336,843
+117,974 -> 845,246
+831,456 -> 890,515
+690,114 -> 804,114
+94,108 -> 94,672
+289,104 -> 107,286
+248,580 -> 229,580
+11,284 -> 885,284
+401,802 -> 186,802
+359,245 -> 558,46
+310,85 -> 310,714
+920,577 -> 979,577
+492,236 -> 276,452
+650,961 -> 49,360
+118,705 -> 118,794
+970,24 -> 80,914
+943,454 -> 943,30
+875,935 -> 716,776
+241,717 -> 392,717
+694,345 -> 620,345
+533,435 -> 467,435
+827,166 -> 374,166
+633,849 -> 884,849
+414,640 -> 875,179
+240,790 -> 709,321
+48,222 -> 104,222
+889,897 -> 44,52
+980,438 -> 455,963
+469,875 -> 469,706
+572,869 -> 250,547
+834,11 -> 834,188
+395,966 -> 395,547
+12,681 -> 567,681
+268,957 -> 947,957
+450,478 -> 893,921
+418,707 -> 602,891
+404,303 -> 218,489
+657,232 -> 657,945
+518,392 -> 518,621
+268,959 -> 896,331
+886,616 -> 841,616
+375,503 -> 375,387
diff --git a/src/day05.rs b/src/day05.rs
new file mode 100644
index 0000000..b178f89
--- /dev/null
+++ b/src/day05.rs
@@ -0,0 +1,98 @@
+use anyhow::Result;
+use itertools::Itertools;
+
+#[derive(Debug)]
+struct LineSegment {
+    start: (isize, isize),
+    end: (isize, isize),
+}
+
+impl LineSegment {
+    fn is_horizontal_or_vertical(&self) -> bool {
+        self.start.0 == self.end.0 || self.start.1 == self.end.1
+    }
+
+    fn rasterize(&self) -> Vec<(isize, isize)> {
+        let xstep = (self.end.0 - self.start.0).signum();
+        let ystep = (self.end.1 - self.start.1).signum();
+        let mut ret = vec![self.start];
+        let (mut x, mut y) = self.start;
+        while (x, y) != self.end {
+            x += xstep;
+            y += ystep;
+            ret.push((x, y));
+        }
+        ret
+    }
+}
+
+fn parse_lines(input: &str) -> Vec<LineSegment> {
+    input
+        .lines()
+        .map(|l| {
+            let mut parts = l.split_whitespace();
+            let start_str = parts.next().unwrap();
+            let end_str = parts.nth(1).unwrap();
+            LineSegment {
+                start: start_str
+                    .split(',')
+                    .map(|s| s.parse::<isize>().unwrap())
+                    .collect_tuple()
+                    .unwrap(),
+                end: end_str
+                    .split(',')
+                    .map(|s| s.parse::<isize>().unwrap())
+                    .collect_tuple()
+                    .unwrap(),
+            }
+        })
+        .collect()
+}
+
+fn count_overlapped_cells(segments: &[LineSegment], includ_diag: bool) -> usize {
+    let mut marks: Vec<(isize, isize)> = segments
+        .iter()
+        .filter(|s| s.is_horizontal_or_vertical() || includ_diag)
+        .map(|s| s.rasterize())
+        .flatten()
+        .collect();
+    // sort so we can find duplicates
+    // **NOTE** IterTools has `.duplicates()` but it's significantly slower (more than 2x) than this
+    marks.sort_unstable();
+    marks[..]
+        .iter()
+        .zip(&marks[1..])
+        .filter(|(a, b)| a == b)
+        .unique()
+        .count()
+}
+
+pub(crate) fn run() -> Result<(usize, usize)> {
+    let input = include_str!("../data/day05.txt");
+    let segments = parse_lines(input);
+    Ok((
+        count_overlapped_cells(&segments, false),
+        count_overlapped_cells(&segments, true),
+    ))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_example() {
+        let input = include_str!("../data/day05-test.txt");
+        let segments = parse_lines(input);
+        assert_eq!(count_overlapped_cells(&segments, false), 5);
+        assert_eq!(count_overlapped_cells(&segments, true), 12);
+    }
+
+    #[test]
+    fn test_my_data() {
+        let input = include_str!("../data/day05.txt");
+        let segments = parse_lines(input);
+        assert_eq!(count_overlapped_cells(&segments, false), 5124);
+        assert_eq!(count_overlapped_cells(&segments, true), 19771);
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index daf1dd5..ad33b78 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -4,6 +4,7 @@ mod day01;
 mod day02;
 mod day03;
 mod day04;
+mod day05;
 mod utils;
 
 fn main() -> Result<()> {
@@ -11,5 +12,6 @@ fn main() -> Result<()> {
     println!("day02: {:?}", day02::run()?);
     println!("day03: {:?}", day03::run()?);
     println!("day04: {:?}", day04::run()?);
+    println!("day05: {:?}", day05::run()?);
     Ok(())
 }