day 1

aoc2020.ml

@@ -3,4 +3,4 @@ let days = [|Day1.main|]
 let _ =
   match Array.to_list Sys.argv with
   | _exename :: day :: args -> days.(int_of_string day - 1) args
-  | _ -> Usage.exit "[day_num] args..."
+  | _ -> Usage.exit "<day> args..."

bracket.ml

@@ -0,0 +1,17 @@
+let bracket ~make ~act ~free =
+  let x = make () in
+  match act x with
+  | result      -> free x; result
+  | exception e -> free x; raise e
+
+let infile ~act name =
+  bracket ~make:(fun () -> open_in name) ~act ~free:close_in_noerr
+
+let infile_lines ~line:f ~of_seq name =
+  let get_line file =
+    match input_line file with
+    | exception End_of_file -> None
+    | line                  -> Some (f line, file)
+  in
+  let act file = of_seq (Seq.unfold get_line file) in
+  infile name ~act

bracket.mli

@@ -0,0 +1,5 @@
+val bracket: make:(unit -> 'a) -> act:('a -> 'b) -> free:('a -> unit) -> 'b
+
+val infile: act:(in_channel -> 'b) -> string -> 'b
+
+val infile_lines: line:(string -> 'a) -> of_seq:('a Seq.t -> 'b) -> string -> 'b

data/day1

@@ -0,0 +1,200 @@
+1293
+1207
+1623
+1675
+1842
+1410
+85
+1108
+557
+1217
+1506
+1956
+1579
+1614
+1360
+1544
+1946
+1666
+1972
+1814
+1699
+1778
+1529
+2002
+1768
+1173
+1407
+1201
+1264
+1739
+1774
+1951
+1980
+1428
+1381
+1714
+884
+1939
+1295
+1694
+1168
+1971
+1352
+1462
+1828
+1402
+1433
+1542
+1144
+1331
+1427
+1261
+1663
+1820
+1570
+1874
+1486
+1613
+1769
+1721
+1753
+1142
+1677
+2010
+1640
+1465
+1171
+534
+1790
+2005
+1604
+1891
+1247
+1281
+1867
+1403
+2004
+1668
+1416
+2001
+1359
+686
+1965
+1728
+1551
+1565
+1128
+1832
+1757
+1350
+1808
+1711
+1799
+1590
+1989
+1547
+1140
+1905
+1368
+1179
+1902
+1473
+1908
+1859
+1257
+1394
+1244
+1800
+1695
+1731
+1474
+1781
+1885
+1154
+1990
+1929
+1193
+1302
+1831
+1226
+1418
+1400
+1435
+1645
+1655
+1843
+1227
+1481
+1754
+1290
+1685
+1498
+71
+1286
+1137
+1288
+1758
+1987
+1471
+1839
+1545
+1682
+1615
+1475
+1849
+1985
+1568
+1795
+1184
+1863
+1362
+1271
+1802
+1944
+1821
+1880
+1788
+1733
+1150
+1314
+1727
+1434
+1833
+1312
+1457
+160
+1629
+1967
+1505
+1239
+1266
+1838
+1687
+1630
+1591
+1893
+1450
+1234
+1755
+1523
+1533
+1499
+1865
+1725
+1444
+1517
+1167
+1738
+1519
+1263
+1901
+1627
+1644
+1771
+1812
+1270
+1497
+1707
+1708
+1396

day1.ml

@@ -1,6 +1,36 @@
-let main2 _input =
-  print_endline "boop"
+module IntSet = Set.Make(Int)
+
+let read_ints =
+  Bracket.infile_lines ~line:int_of_string ~of_seq:IntSet.of_seq
+
+let find_with target ints =
+  let open IntSet in
+  let p x = mem (target - x) ints in
+  let res = min_elt (filter p ints) in
+  res, target - res
+
+let find = find_with 2020
+
+let main_part_1 input =
+  let ints = read_ints input in
+  let x, y = find ints in
+  Printf.printf "%d * %d = %d\n" x y (x * y)
+
+let main_part_2 input =
+  let ints = read_ints input in
+  let exception Found of int * int * int in
+  let go x =
+    match find_with (2020 - x) ints with
+    | exception Not_found -> ()
+    | y, z -> raise (Found (x, y, z))
+  in
+  match IntSet.iter go ints with
+  | exception Found (x, y, z) ->
+      Printf.printf "%d * %d * %d = %d\n" x y z (x * y * z)
+  | _ -> raise Not_found
+
+let mains = [|main_part_1; main_part_2|]
 
 let main = function
-  | [input] -> main2 input
-  | _ -> failwith "usage: $0 1 infile"
+  | [part; input] -> mains.(int_of_string part - 1) input
+  | _ -> Usage.exit "1 <part> <infile>"