diff --git a/day4.ml b/day4.ml
new file mode 100644
index 0000000..834f22b
--- /dev/null
+++ b/day4.ml
@@ -0,0 +1,91 @@
+(* Common util. Should probably put this in a module or something.. *)
+let get_one_line file =
+  try Some (input_line file) with
+    End_of_file -> None
+
+let get_lines file =
+  let rec input lines =
+    match get_one_line file with
+      Some line -> input (line :: lines)
+    | None -> List.rev lines
+  in input []
+
+let read_file file =
+  let channel = open_in(file) in
+  get_lines channel
+
+let explode s = (* convert a string to a list of chars *)
+  let rec exp i l =
+   if i < 0 then l else exp (i - 1) (s.[i] :: l) in
+  exp (String.length s - 1) []
+
+let implode l = String.of_seq (List.to_seq l)
+
+(* --- *)
+
+let fix_lines lines = (* puts each passport on 1 line *)
+  let glue s = if String.equal s "" then "\n" else " " in (* which char to concat the lines *)
+  let fix_line acc cur = acc ^ (glue cur) ^ cur in (* our reducer: concat lines with either space or newline *)
+  List.fold_left fix_line "" lines
+
+let read_passport line = (* converts a line to an alist *)
+  let chunks = String.(split_on_char ' ' (trim line)) in (* split each string on space *)
+  let read_chunk chunk =
+    let parts = String.split_on_char ':' chunk in (* split each chunk on colon *)
+    List.(nth parts 0, nth parts 1) in (* return a tuple for each chunk *)
+  List.map read_chunk chunks
+
+let required_fields = ["byr"; "iyr"; "eyr"; "hgt"; "hcl"; "ecl"; "pid";]
+
+let passport_has_required_fields passport =
+  let has_key key = (* does a passport have a particular key *)
+    match List.assoc_opt key passport with
+    | Some _ -> true
+    | _ -> false
+  in
+  List.for_all has_key required_fields (* does it have ALL the keys *)
+
+let is_in_range (min, max) n = int_of_string n >= min && int_of_string n <= max
+
+let is_height_valid h =
+  let len = String.length h in
+  let units = String.sub h (len - 2) 2 in
+  let value = String.sub h 0 (len - 2) in
+  match units with
+  | "cm" -> is_in_range (150, 193) value
+  | "in" -> is_in_range (59, 76) value
+  | _ -> false
+
+let is_char_hex c =
+  let hex_chars = "0123456789abcdef" in
+  String.contains hex_chars c
+
+let is_hair_valid h =
+  let color = (String.sub h 1 (String.length h - 1)) in
+  String.length h = 7 &&
+  String.sub h 0 1 = "#" &&
+  List.for_all is_char_hex (explode color)
+
+let is_eye_color_valid c =
+  List.mem c ["amb"; "blu"; "brn"; "gry"; "grn"; "hzl"; "oth";]
+
+let is_pid_valid p =
+  String.length p = 9 &&
+  List.for_all (fun c -> List.mem c (explode "0123456789")) (explode p)
+
+let is_passport_valid p =
+  is_in_range (1920, 2002) (List.assoc "byr" p) &&
+  is_in_range (2010, 2020) (List.assoc "iyr" p) &&
+  is_in_range (2020, 2030) (List.assoc "eyr" p) &&
+  is_height_valid (List.assoc "hgt" p) &&
+  is_hair_valid (List.assoc "hcl" p) &&
+  is_eye_color_valid (List.assoc "ecl" p) &&
+  is_pid_valid (List.assoc "pid" p)
+
+let lines = read_file "day4.txt" |> fix_lines |> String.split_on_char '\n'
+let passports = List.map read_passport lines
+let passports_with_required_fields = List.filter passport_has_required_fields passports
+let valid_passports = List.filter is_passport_valid passports_with_required_fields;;
+
+Printf.printf "Number of passports with all required fields: %i\n" (List.length passports_with_required_fields);;
+Printf.printf "Number of valid passports: %i\n" (List.length valid_passports);;
diff --git a/day4.txt b/day4.txt
new file mode 100644
index 0000000..8cdfd0a
--- /dev/null
+++ b/day4.txt
@@ -0,0 +1,1000 @@
+hcl:#6b5442 ecl:brn iyr:2019
+pid:637485594 hgt:171cm
+eyr:2021 byr:1986
+
+eyr:2025 iyr:1938 byr:2014 hcl:#341e13
+hgt:66cm
+pid:70195175
+
+hcl:#efcc98
+iyr:2011 ecl:hzl
+eyr:2020 hgt:174cm pid:589700330
+
+hcl:#bba027 eyr:2027 cid:54
+ecl:brn pid:153cm
+iyr:2028 hgt:173cm
+byr:2004
+
+hcl:b45cec
+iyr:2011 ecl:oth hgt:185cm eyr:2029 pid:178cm
+
+hgt:185cm iyr:2016 eyr:2029 hcl:#888785 pid:026540921
+
+eyr:2025
+hcl:6962f7 byr:2015 ecl:oth iyr:1974
+hgt:191cm
+pid:2616015
+
+pid:268398556 iyr:2019 ecl:grn
+eyr:2027 byr:1951 hcl:#18171d hgt:67cm
+
+eyr:2029 hgt:153cm ecl:brn pid:183179186 byr:2013 hcl:#623a2f
+iyr:1957
+
+cid:121 iyr:1922 hcl:752fbc pid:79577560 byr:2025
+hgt:61cm eyr:1971
+
+iyr:2016
+eyr:2024 hcl:#18171d hgt:184cm
+ecl:hzl byr:1992 pid:751161201
+
+eyr:2021 ecl:blu byr:1938 iyr:2016 hcl:#b6652a pid:313406514 hgt:191cm
+
+hcl:#623a2f eyr:2021
+ecl:brn
+pid:145249653 hgt:167cm iyr:2019 byr:1991
+
+iyr:2022 pid:175cm
+byr:2021 eyr:2027 ecl:#f615b1
+hgt:172in hcl:#ceb3a1
+
+hgt:173in
+ecl:#0cba5e pid:1885981567 iyr:1968
+byr:1952
+eyr:1942
+
+ecl:oth eyr:2023 hgt:65cm pid:521737908 byr:1971 hcl:z iyr:2017
+
+byr:1936
+hcl:#cfa07d
+ecl:brn iyr:2011 pid:589047874
+eyr:2025
+
+hcl:#fffffd
+pid:912552538
+cid:159 hgt:160cm iyr:2012
+eyr:2023 ecl:hzl
+byr:1946
+
+iyr:2015
+ecl:amb hgt:72in
+cid:59 pid:782818257 hcl:#18171d eyr:2026
+byr:1952
+
+hgt:173cm iyr:2018 cid:96 ecl:amb byr:1986 pid:783160698 eyr:2026
+hcl:#602927
+
+hcl:#a97842 cid:199 pid:912273414 eyr:2030
+hgt:171cm ecl:hzl iyr:2011 byr:1960
+
+ecl:amb hgt:156cm
+iyr:2013
+hcl:#ceb3a1
+cid:116 pid:567057004 byr:1942
+eyr:2029
+
+ecl:#cddc40
+pid:045090966 cid:254
+hgt:75in hcl:#733820 eyr:2026 byr:1956
+iyr:2015
+
+pid:156cm
+eyr:2040
+hgt:176cm ecl:#02e67d hcl:b7c0e6
+iyr:1959 cid:129 byr:2022
+
+hgt:160cm byr:1933
+ecl:blu eyr:2029 iyr:2012 hcl:#888785 pid:028571975
+
+iyr:2017
+hcl:#390f37 hgt:171cm ecl:brn byr:1931 pid:015365720 eyr:2030
+
+iyr:2014 pid:697057757
+eyr:2026 hgt:188cm
+ecl:gry byr:1926
+
+pid:484310015 hcl:#fffffd hgt:150cm iyr:2018
+cid:53 ecl:gry eyr:2021 byr:1957
+
+hgt:156cm
+eyr:2026 byr:1963
+pid:063272603 ecl:brn iyr:2011
+hcl:#888785
+
+byr:1955 pid:310518398 hgt:191cm iyr:2018
+ecl:oth eyr:2023 cid:132 hcl:#888785
+
+byr:1938 hcl:#623a2f eyr:2023
+iyr:2010
+hgt:165cm
+pid:170304863
+cid:290 ecl:amb
+
+eyr:2026
+pid:021468065 hgt:164cm
+byr:1996 iyr:2016 hcl:#18171d
+ecl:brn
+
+byr:2027 ecl:oth pid:8258823391 hgt:153in hcl:#733820 eyr:1948
+
+byr:2026 ecl:#cd275a iyr:2012 eyr:2036 pid:5917499975
+
+byr:2004
+cid:151
+hcl:99ecb1
+eyr:2033 pid:871137711 iyr:1997
+hgt:184cm ecl:oth
+
+byr:2011
+hcl:z ecl:#eee1d2 hgt:59cm eyr:1925 iyr:2030 pid:#02ee78
+
+pid:742658992
+hcl:#888785
+byr:1995
+eyr:2024 hgt:162cm iyr:2013 cid:169 ecl:gry
+
+hgt:152cm byr:1946
+eyr:2027 iyr:2018
+pid:352799678
+hcl:#238da0
+ecl:amb
+cid:71
+
+hcl:#623a2f pid:723616064 eyr:2021
+hgt:172cm
+byr:1926 iyr:2013
+ecl:grn
+
+iyr:2019 hgt:94 byr:2028 eyr:1986
+pid:#ee7f00
+
+ecl:amb
+eyr:2027 pid:188153423 byr:1957 hcl:#d67ae1
+iyr:2011 hgt:183cm
+
+byr:1950 ecl:#e2495d iyr:2010 hgt:166cm eyr:2034 pid:151457075
+
+eyr:1981
+byr:2016 iyr:2029 pid:153cm ecl:#55c2a4 hcl:z
+hgt:76cm
+
+hgt:184cm ecl:amb eyr:2021
+hcl:#623a2f
+pid:414607669 iyr:1960 byr:2002
+
+eyr:2027 iyr:2020 hgt:179cm byr:1991
+pid:969568248
+ecl:blu
+
+hgt:175cm pid:536803427 hcl:#a97842 iyr:2012
+ecl:grn byr:1950 eyr:2027
+
+eyr:2028 hgt:60in hcl:#733820 iyr:2018 ecl:oth pid:871909483
+byr:1930
+
+hgt:155cm iyr:2020 byr:1960 eyr:2021
+pid:515710074 ecl:amb hcl:#341e13
+
+byr:1922 hcl:z iyr:1977 ecl:brn
+eyr:2023 hgt:119 pid:865700639
+
+ecl:gry hcl:959fcd pid:#633ac1
+byr:2011 hgt:68in
+eyr:2020
+
+iyr:1972 hcl:z cid:149 byr:2020
+hgt:166in pid:4548657 eyr:1960
+ecl:#cc987c
+
+eyr:2023 hcl:#b6652a iyr:2015
+hgt:187in pid:547953710 byr:1979 ecl:grn
+
+iyr:2018
+pid:508691429 ecl:oth eyr:2025 hgt:187cm cid:270
+hcl:#888785 byr:1977
+
+hgt:168cm eyr:2032 byr:2020
+ecl:gry iyr:1982
+hcl:z pid:648015564
+
+hcl:#fffffd pid:911858643 iyr:2016 ecl:gry eyr:2030 byr:1992 hgt:156cm
+
+pid:241562994 eyr:2026 ecl:grn hgt:164cm
+hcl:#c0946f byr:1945 iyr:2015 cid:296
+
+byr:1976 pid:269322775 ecl:hzl
+hgt:162cm hcl:#b6652a
+eyr:2022 cid:335 iyr:2012
+
+eyr:2028
+hgt:106
+pid:268626219 hcl:#a97842
+iyr:2011
+ecl:grn byr:1967
+
+iyr:2016 hcl:#888785 hgt:193cm ecl:oth
+pid:034099334 eyr:2027
+byr:1945
+cid:181
+
+pid:248319556 byr:1987 iyr:2010 cid:122 ecl:utc
+hcl:z hgt:190cm eyr:2030
+
+iyr:2019 hcl:#ceb3a1
+ecl:hzl
+cid:281 hgt:73in byr:1992
+eyr:2023
+
+hcl:#fffffd
+ecl:blu cid:340 hgt:176cm byr:1980 pid:809878309 iyr:2018
+
+hgt:167cm hcl:#866857 byr:1973 cid:143 eyr:2030 iyr:2012
+ecl:hzl pid:168618514
+
+hcl:c97d76 iyr:2016 pid:8439355994 byr:2013 eyr:2036 hgt:71cm
+cid:116 ecl:#055b62
+
+hcl:#341e13 pid:961548527 eyr:2027 hgt:192cm byr:1940 iyr:2011 ecl:oth
+
+byr:1935 hgt:189cm ecl:brn iyr:2012
+eyr:2028 hcl:#602927
+
+byr:2024
+eyr:1939 iyr:2020 hgt:140 pid:889951037
+hcl:#b6652a ecl:blu
+
+ecl:amb byr:1942
+iyr:2012 pid:161703003 hgt:181cm
+eyr:2027 hcl:#602927
+
+hcl:#18171d
+iyr:2015 byr:1935
+cid:204
+ecl:gry
+hgt:180cm eyr:2025 pid:988699528
+
+eyr:2025 byr:1985
+cid:192
+hcl:#866857 hgt:150cm pid:315179208 iyr:2010 ecl:blu
+
+hcl:#341e13 iyr:2013 eyr:2021 cid:62
+byr:1928
+hgt:168cm pid:862861470 ecl:hzl
+
+pid:099158408
+ecl:grn
+eyr:2026 iyr:2018 hcl:#b6652a cid:81
+hgt:185cm byr:1964
+
+byr:1990 hgt:155cm
+ecl:brn
+eyr:2023
+hcl:#ceb3a1 iyr:2012
+
+ecl:brn
+eyr:2026 cid:242 pid:658930205
+hgt:176cm byr:1990 iyr:2016 hcl:#d55f68
+
+hcl:#602927 pid:924899781
+eyr:2024 byr:1964
+iyr:2019
+cid:163
+hgt:181cm ecl:gry
+
+eyr:2026 ecl:blu pid:8812414708 iyr:2017 hcl:#a97842 hgt:190cm
+byr:1970
+
+hgt:152cm
+pid:403682313 iyr:2019
+hcl:#ceb3a1 ecl:oth
+eyr:2021 byr:1957
+
+pid:23799214
+byr:2030 hgt:66cm
+iyr:2022
+hcl:z ecl:#c806fe eyr:2035
+
+eyr:2022 hgt:177cm byr:1967 cid:194
+pid:060293594
+ecl:brn
+iyr:2016
+hcl:#cfa07d
+
+hgt:184cm hcl:#6b5442 eyr:2029
+ecl:oth iyr:2013 pid:26983291 byr:1965
+cid:147
+
+pid:255519852 byr:1975 hgt:192cm
+ecl:lzr
+iyr:2015 eyr:2030
+hcl:#623a2f
+
+iyr:2010
+ecl:blu
+hcl:#881267 hgt:162cm pid:121130250 byr:1935 cid:57 eyr:2025
+
+hgt:189cm
+hcl:#a97842
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+ecl:brn
+pid:972960330
+
+hcl:#623a2f eyr:2026 hgt:193cm cid:87 byr:1982 iyr:2020 pid:158154062 ecl:amb
+
+eyr:2025 hgt:191cm
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+hcl:#341e13
+pid:137048981 iyr:2016 byr:1950
+
+byr:1930 eyr:2029 ecl:hzl hgt:75in
+pid:464272185 cid:341
+iyr:2012 hcl:#c0946f
+
+ecl:brn
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+hgt:170cm
+
+pid:578940518 byr:2025 hgt:190in
+iyr:2030 cid:52 ecl:amb eyr:2027
+
+ecl:amb hgt:185cm cid:237 iyr:2016 pid:490377510 byr:1950 hcl:#18171d
+eyr:2025
+
+iyr:2014 hgt:156in pid:65952131
+ecl:blu byr:1938 hcl:#7d3b0c
+eyr:2023
+
+ecl:gry iyr:2016 pid:818347623 hcl:#888785 eyr:2030 hgt:174cm
+
+ecl:hzl
+hcl:#866857
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+pid:213124752 hgt:179cm
+byr:1989
+
+pid:024846371 byr:1990 iyr:2018
+eyr:2026 hgt:161cm ecl:oth
+
+hcl:z hgt:129 iyr:2016
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+
+hgt:192cm hcl:#602927 ecl:blu iyr:2011
+pid:863613568 byr:1996 eyr:2027
+
+hgt:160cm cid:229 byr:1952
+iyr:2019
+ecl:#0ae2d6 eyr:2027 pid:719697407 hcl:z
+
+pid:040987502 cid:155 iyr:2012 hgt:173cm
+byr:2002
+hcl:#fffffd eyr:2023 ecl:hzl
+
+ecl:oth byr:1993 iyr:2019 pid:319157251 hcl:#733820 hgt:70in eyr:2027
+
+hcl:#9d85d4
+hgt:192cm pid:570514935
+cid:238 eyr:2022 ecl:gry byr:1989
+iyr:2016
+
+hgt:162cm
+cid:201 iyr:2015 eyr:2023 pid:553794028 byr:1922 ecl:amb hcl:#623a2f
+
+cid:56
+eyr:2024 ecl:amb hgt:179cm hcl:#efcc98
+pid:665225721
+iyr:2012 byr:1963
+
+byr:2026
+hcl:#888785
+iyr:1972 eyr:1980 cid:323 pid:153cm
+hgt:170cm ecl:blu
+
+pid:704204892 ecl:gry
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+byr:1920 hgt:168cm iyr:2010 hcl:#3311ec
+
+pid:#7f3caf eyr:2023
+hcl:z hgt:146 byr:1990 ecl:amb
+iyr:2014 cid:270
+
+hgt:171cm ecl:blu pid:383695713
+cid:200 iyr:2010
+hcl:#602927 byr:1950 eyr:2024
+
+hgt:178cm byr:1935 hcl:#2da7db
+pid:597509269
+eyr:2020 iyr:2014
+ecl:blu
+
+eyr:2034
+ecl:#d4719a
+hcl:z hgt:67cm iyr:2023 pid:#268d93 byr:2006
+
+eyr:1939 pid:9942171839
+hgt:104
+iyr:1945
+byr:2011 ecl:#f9bafb hcl:#ceb3a1
+
+byr:1937
+iyr:2010 pid:979528684
+eyr:2028 hcl:#ceb3a1 ecl:gry hgt:164cm
+
+iyr:2019 eyr:2022 pid:044485658 hcl:#18171d byr:1996 hgt:169cm
+ecl:gry
+
+pid:031482456
+eyr:2024
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+hgt:157cm hcl:#7d3b0c byr:1921
+ecl:oth
+
+pid:399398768
+ecl:lzr
+hcl:z
+eyr:1983 hgt:68cm
+byr:2024 iyr:2027 cid:127
+
+hgt:186cm eyr:2026 pid:140032921 ecl:amb cid:278
+byr:1937 iyr:2015
+
+hgt:172cm
+ecl:amb pid:718725983 hcl:#6b5442 eyr:2024
+iyr:2013 byr:1974
+
+ecl:amb iyr:2014 cid:216 hcl:#cfa07d
+eyr:2022 pid:442275714 hgt:68in byr:1999
+
+hgt:152cm cid:193
+iyr:2015 pid:806672342 hcl:#b6652a byr:1927 ecl:oth
+
+hcl:#7d3b0c byr:1925 iyr:2015 hgt:174cm pid:888044223 cid:168 ecl:oth eyr:2029
+
+ecl:gry byr:2009 hgt:156cm
+hcl:#888785 pid:263500722 iyr:2015 eyr:2021
+
+cid:103
+hcl:#ba8b89 ecl:hzl hgt:169cm
+byr:1929 pid:626102979 iyr:2016 eyr:2028
+
+iyr:2016 hgt:188cm cid:133
+byr:1926 ecl:hzl eyr:2023 hcl:#602927 pid:678092780
+
+ecl:utc byr:2025 pid:#584dc1 eyr:2037
+hgt:151cm iyr:1950 hcl:#cfa07d
+
+ecl:oth hgt:140 eyr:1977 hcl:#6b5442 iyr:1955
+byr:1999
+pid:868434068
+
+eyr:2029 hcl:#18171d cid:158 iyr:2016 hgt:166cm ecl:hzl
+pid:100226690 byr:1942
+
+ecl:#806ce8
+cid:153 iyr:2024 byr:1985 hcl:da8a68
+pid:#d9e5b0 eyr:2017
+
+eyr:2020 hgt:164cm cid:222 ecl:hzl byr:1945 hcl:#cfa07d
+iyr:2011
+
+iyr:2018 hgt:165cm
+pid:868536448 eyr:2026 byr:1930
+ecl:hzl hcl:#623a2f cid:128
+
+ecl:grn iyr:2012
+cid:326 byr:1950 hcl:#efcc98 eyr:2029 hgt:177cm pid:685629972
+
+byr:2004 hgt:168cm
+ecl:dne iyr:2020 hcl:z
+
+byr:1964 pid:132604237 ecl:oth hcl:#602927 hgt:188cm
+cid:78
+iyr:2012 eyr:2025
+
+byr:1945
+iyr:2023 ecl:#1a590c hgt:70in
+pid:186cm eyr:2031 hcl:z
+
+cid:178
+ecl:amb eyr:2024 hgt:162cm
+hcl:#18171d iyr:2016
+byr:1945 pid:737813370
+
+hcl:#18171d
+byr:1949
+pid:064917719
+hgt:176cm ecl:amb
+eyr:2034
+iyr:1998
+
+hgt:72in
+pid:711343766 hcl:#623a2f
+iyr:2010 byr:1977 ecl:amb cid:177 eyr:2023
+
+byr:1933 hgt:66 pid:22149379 eyr:2040
+ecl:#92d7a7 hcl:#cfa07d
+
+iyr:2020 byr:1946 eyr:2020 ecl:hzl pid:153cm
+hgt:159cm cid:261 hcl:#888785
+
+iyr:2013 byr:1931
+ecl:#2ced2e hcl:3c49c1 eyr:1950
+hgt:182cm cid:133 pid:#19fc55
+
+hcl:#a9abe6
+iyr:2016
+eyr:2029 ecl:hzl cid:343 pid:691253232 byr:1952 hgt:187cm
+
+hcl:z
+eyr:1964
+ecl:#5995e6
+byr:2021 hgt:72in pid:2103603035 iyr:1951
+
+iyr:2024 hgt:151in byr:1988 ecl:blu
+eyr:1961 cid:117
+hcl:z pid:5371118784
+
+hgt:71cm iyr:2021
+eyr:2033 ecl:amb
+hcl:z cid:202
+pid:207141921 byr:1987
+
+ecl:gry byr:1927 eyr:2024
+hgt:60in iyr:2014
+pid:847799723 cid:285
+hcl:#733820
+
+eyr:2022 hcl:#18171d
+pid:847063261
+byr:1926 ecl:grn
+iyr:2011
+
+pid:647225630 iyr:2016 hcl:#a97842 ecl:oth eyr:2025
+cid:144 hgt:182cm byr:1983
+
+hgt:150 byr:1924
+eyr:2024 hcl:1600da
+ecl:brn
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+
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diff --git a/day5.ml b/day5.ml
new file mode 100644
index 0000000..b82be91
--- /dev/null
+++ b/day5.ml
@@ -0,0 +1,76 @@
+(* Common util. Should probably put this in a module or something.. *)
+let get_one_line file =
+  try Some (input_line file) with
+    End_of_file -> None
+
+let get_lines file =
+  let rec input lines =
+    match get_one_line file with
+      Some line -> input (line :: lines)
+    | None -> List.rev lines
+  in input []
+
+let read_file file =
+  let channel = open_in(file) in
+  get_lines channel
+
+let explode s = (* convert a string to a list of chars *)
+  let rec exp i l =
+   if i < 0 then l else exp (i - 1) (s.[i] :: l) in
+  exp (String.length s - 1) []
+
+let implode l = String.of_seq (List.to_seq l)
+
+let range start len = (* makes a new list from start with length len *)
+  List.init len (fun x -> x + 1) |> List.map (fun x -> x + start)
+
+(* --- *)
+
+type 'a tree = Leaf of 'a | Node of 'a tree * 'a tree
+
+let rec build_tree l = (* build a balanced tree from a list *)
+  match l with (* exception for [] would probably be good here.. *)
+  | x::[] -> Leaf x
+  | x::xs ->
+      let len = List.length l in
+      let mid_val = List.nth l (len/2) in
+      let left = range (x-1) (len/2) in
+      let right = range (mid_val-1) (len/2) in
+      Node (build_tree left, build_tree right)
+
+let rec walk tree steps =
+  match tree with
+  | Leaf a -> a (* if we've hit a leaf, return the value *)
+  | Node (l, r) ->
+      match steps with (* if we get to the end of theh list and we haven't hit a leaf, that's a problem.. if only i knew how to do exceptions *)
+        | 'F' :: xs -> walk l xs
+        | 'L' :: xs -> walk l xs
+        | 'B' :: xs -> walk r xs
+        | 'R' :: xs -> walk r xs
+
+let rows = build_tree (range 0 128)
+let cols = build_tree (range 0 8)
+
+let find_seat map =
+  let row = walk rows (explode (String.sub map 0 7)) - 1 in (* one-off b/c of their non-0-indexed lists? *)
+  let col = walk cols (explode (String.sub map 7 3)) - 1 in
+  (row, col)
+
+let seat_id (row, col) = row * 8 + col
+
+let rec list_max li =
+  match li with (* again non exhaustive, empty list fails. be better *)
+  | x :: [] -> x
+  | x :: xs -> max x (list_max xs)
+
+let rec find_missing seats =
+  match seats with
+  | x :: y :: z -> if y = x + 1 then find_missing (y :: z) else x + 1
+  | x :: [] -> x
+
+let seats = read_file "day5.txt" |> List.map find_seat
+let seat_ids = List.map seat_id seats
+let sorted_seats = List.sort (fun a b -> if a > b then 1 else if a < b then -1 else 0) seat_ids;;
+
+Printf.printf "max seat id %i\n" (list_max seat_ids);;
+Printf.printf "missing seat %i\n" (find_missing sorted_seats);;
diff --git a/day5.txt b/day5.txt
new file mode 100644
index 0000000..77f7b83
--- /dev/null
+++ b/day5.txt
@@ -0,0 +1,897 @@
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