2024 day 13 part 2

bin/day2413.ml

@@ -1,23 +1,21 @@
-let parse_machine line_a line_b prize_pos =
+(** [parse_machine line_a line_b line_p] parse the machine description and
+    returns a triple [(a, b, p)] describing the actions of the 'A' and 'B'
+    buttons along with the location of the prize. *)
+let parse_machine line_a line_b line_p =
+  let get_xy re s =
+    let _ = Str.search_forward re s 0 in
+    ( int_of_string (Str.matched_group 1 s),
+      int_of_string (Str.matched_group 2 s) )
+  in
   let re_ab = Str.regexp {|Button [AB]: X\+\([0-9]+\), Y\+\([0-9]+\)|} in
   let re_pos = Str.regexp {|Prize: X=\([0-9]+\), Y=\([0-9]+\)|} in
-  let _ = Str.search_forward re_ab line_a 0 in
-  let a =
-    ( int_of_string (Str.matched_group 1 line_a),
-      int_of_string (Str.matched_group 2 line_a) )
-  in
-  let _ = Str.search_forward re_ab line_b 0 in
-  let b =
-    ( int_of_string (Str.matched_group 1 line_b),
-      int_of_string (Str.matched_group 2 line_b) )
-  in
-  let _ = Str.search_forward re_pos prize_pos 0 in
-  let prize =
-    ( int_of_string (Str.matched_group 1 prize_pos),
-      int_of_string (Str.matched_group 2 prize_pos) )
-  in
+  let a = get_xy re_ab line_a in
+  let b = get_xy re_ab line_b in
+  let prize = get_xy re_pos line_p in
   (a, b, prize)
 
+(** [parse_machines lst] returns a list of the machines parsed from the input
+    list of strings. *)
 let parse_machines =
   let rec impl acc = function
     | "" :: t -> impl acc t
@@ -27,43 +25,67 @@ let parse_machines =
   in
   impl []
 
+(** [machines_of_file fname] returns the list of machines described in the file
+    [fname]. *)
 let machines_of_file fname = Aoc.strings_of_file fname |> parse_machines
 
-(*
-  We want to solve A * a.x + B * b.x = X
-  and              A * a.y + B * b.y = Y
+(** [calc_tokens (a, b p)] calculates how many tokens are needed to get to [p]
+    by pressing button A (moving [a] amount) and button B (moving [b] amount).
+    Returns [None] if no solution possible, or [Some t] if the prize can be got
+    by spending [t] tokens.
 
-  which is
-
-   A * a.x / b.x + B = X / b.x
-   A * a.y / b.y + B = Y / b.y
-   A * (a.x / b.x - a.y / b.y) = X / b.x - Y / b.y
-   A * (a.x * b.y - a.y * b.x) / b.x / b.y = X / b.x - Y / b.y
-   A * (a.x * b.y - a.y * b.x) = X * b.y - Y * b.x
-  
-   A = (X * b.y - Y * b.x) / (a.x * by.y - a.y * b.x)
-   (7870 * 37 - 6450 * 84) / (17 * 37 - 86 * 84)
-   80 - (8400 * 67 - 5400 * 22) / (94 * 67 - 34 * 22)
-   a = (94, 34) b = (22, 67) p = (8400, 5400)
-  [ a.x  b.x ] [ A ]  = [ X ]
-  [ a.y  b.y ] [ B ]    [ Y ]
-*)
+    Note the problem says minimize but if there is a solution there is only one
+    solution. *)
 let calc_tokens ((ax, ay), (bx, by), (x, y)) =
+  (* Solve as a sequence of linear equations: 
+     We want to find A & B in:
+     A * ax + B * bx = X (1)
+     A * ay + B * by = Y (2) 
+
+     dividing (1) by bx and (2) by by gives us:
+
+     A * ax / bx + B = X / bx (3)
+     A * ay / by + B = Y / by (4)
+
+     (3) - (4) gives:
+
+     A * (ax / bx - ay / by) = X / bx - Y / by.
+
+     Multiplying through by (bx * by) gives:
+
+     A * (ax * by - ay * bx) = X * by - Y * bx.
+
+     Dividing by (ax * by - ay * by) gives:
+
+     A = (X * by - Y * bx) / (ax * by - ay * by)
+
+     If ax * by - ay * by is 0 we have an infinite number of answers, and we'll
+     deal with that if that becomes an issue (spoiler alert: it doesn't).
+
+    A needs to be a whole number so (X * by - Y * bx) mod (ax * by - ay * by)
+    must be 0, otherwise there is no solution. *)
   let a_n = (x * by) - (y * bx) in
   let a_d = (ax * by) - (ay * bx) in
-  Printf.printf
-    "a = (%d, %d) b = (%d, %d)  p = (%d, %d), a_n / a_d = %d / %d, mod = %d\n"
-    ax ay bx by x y a_n a_d (a_n mod a_d);
-  if a_n mod a_d <> 0 then None
+  if a_d = 0 then None
+  else if a_n mod a_d <> 0 then None
   else if a_n / a_d <= 0 then None
   else
     let a = a_n / a_d in
     let b = (x - (ax * a)) / bx in
-    Printf.printf "   a = %d b = %d\n" a b;
     Some ((3 * a) + b)
 
-let part machines =
-  List.map calc_tokens machines
-  |> List.filter_map Fun.id |> List.fold_left ( + ) 0
+(** [add_offset offset machine] offsets the prize location for [machine] by
+    [(offset, offset)]. *)
+let add_offset offset (a, b, (x, y)) = (a, b, (x + offset, y + offset))
 
-let _ = Aoc.main machines_of_file [ (string_of_int, part) ]
+(** [part offset machines] calculates the number of tokens needed to win as many
+    prizes as possible from [machines]. All machines prizes are offset by
+    [(offset, offset)]. *)
+let part offset machines =
+  List.map (add_offset offset) machines
+  |> List.filter_map calc_tokens
+  |> List.fold_left ( + ) 0
+
+let _ =
+  Aoc.main machines_of_file
+    [ (string_of_int, part 0); (string_of_int, part 10000000000000) ]