2025 Day 3

Author: viandoxdev

2025/days/Day03.ml

@@ -1,6 +1,26 @@
 open Utils
 
-let day03 input =
-  ("TODO", "TODO")
+let digit_to_int c = int_of_char c - int_of_char '0'
+
+let max_digit_index s = 
+    let len = String.length s in
+    fold_range_rev (fun (j, m) i -> 
+        let v = digit_to_int s.[i] in
+        if v >= m then (i, v) else (j, m)
+    ) (0, digit_to_int s.[0]) 0 (len - 1)
 
+let rec max_joltage_n n line = let len = String.length line in match n with
+    | 0 -> 0
+    | n when n = len -> int_of_string line
+    | n -> (
+        let (i, d) = max_digit_index (String.sub line 0 (len - n + 1)) in
+        d * (p10 (n - 1)) + max_joltage_n (n - 1) (String.sub line (i + 1) (len - i - 1)))
 
+let solve n lines =
+    List.map (max_joltage_n n) lines |> sum
+
+let day03 input =
+    let lines = String.split_on_char '\n' (trim_end_nl input) in
+    let part1 = solve 2 lines in
+    let part2 = solve 12 lines in
+    (string_of_int part1, string_of_int part2)

2025/days/Utils.ml

@@ -4,7 +4,7 @@ let int_of_bool = function
     | true -> 1
     | false -> 0
 
-let powers_of_10 = [|1;10;100;1000;10000;100000;1000000;10000000;100000000;1000000000;10000000000|]
+let powers_of_10 = [|1;10;100;1000;10000;100000;1000000;10000000;100000000;1000000000;10000000000;100000000000;1000000000000;10000000000000;100000000000000;1000000000000000;10000000000000000;100000000000000000;1000000000000000000;|]
 
 let p10 = function
     | p when p < 0 -> 0 
@@ -19,3 +19,14 @@ let rec sum_range a b f =
     if a <= b then
         f a + sum_range (a + 1) b f
     else 0
+
+let trim_end_nl s = String.sub s 0 (String.length s - 1)
+
+let rec fold_range f acc range_start range_end =
+    if range_start > range_end then acc else
+    fold_range f (f acc range_start) (range_start + 1) range_end
+let rec fold_range_rev f acc range_start range_end =
+    if range_start > range_end then acc else
+    fold_range_rev f (f acc range_end) range_start (range_end - 1)
+
+let sum = List.fold_left (+) 0