1st of December, part two! Using batteries.

dec1_2.ml

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+(*The captcha requires you to review a sequence of digits (your puzzle input) and*)
+(*find the sum of all digits that match the next digit in the list. The list is*)
+(*circular, so the digit after the last digit is the first digit in the list.*)
+
+(*For example:*)
+    (*1122 produces a sum of 3 (1 + 2) because the first digit (1) matches the*)
+    (*second digit and the third digit (2) matches the fourth digit.*)
+    (*1111 produces 4 because each digit (all 1) matches the next.*)
+    (*1234 produces 0 because no digit matches the next.*)
+    (*91212129 produces 9 because the only digit that matches the next one is the*)
+    (*last digit, 9.*)
+
+(*Now, instead of considering the next digit, it wants you to consider the digit halfway around the circular list. That is, if your list contains 10 items, only include a digit in your sum if the digit 10/2 = 5 steps forward matches it. Fortunately, your list has an even number of elements.*)
+
+(*For example:*)
+
+    (*1212 produces 6: the list contains 4 items, and all four digits match the digit 2 items ahead.*)
+    (*1221 produces 0, because every comparison is between a 1 and a 2.*)
+    (*123425 produces 4, because both 2s match each other, but no other digit has a match.*)
+    (*123123 produces 12.*)
+    (*12131415 produces 4.*)
+
+
+open Batteries
+
+let char_to_int x = int_of_string (Char.escaped x)
+
+let compare_char_pos all_chars_length chars_length position =
+    let
+        half_length = all_chars_length / 2
+    in
+        if chars_length <= half_length
+        then half_length - chars_length
+        else position + half_length
+
+
+let get_compare_char all_chars chars_left position =
+    let
+        position = compare_char_pos (List.length all_chars) (List.length chars_left) position
+    in
+        List.at all_chars position
+
+
+let rec algo' (all_chars : char list)
+              (chars_left : char list)
+              (acc : int list)
+              (position : int) =
+
+    match chars_left with
+    | [] -> List.fold_left (+) 0 acc
+    | c1 :: rest ->
+            let
+                compare_char = (get_compare_char all_chars chars_left position)
+            in
+                if (Char.equal c1 compare_char)
+                then algo' all_chars rest (char_to_int c1 :: acc) (position + 1)
+                else algo' all_chars rest acc (position + 1)
+
+
+let algo (chars : char list) =
+    algo' chars chars [] 0
+
+
+let solve_captcha number =
+    number
+    |> String.to_list
+    |> algo
+    |> print_int
+
+
+let () =
+    solve_captcha "57276274387944537823652626177853384411146325384494935924454336611953119173638191671326254832624841593421667683474349154668177743437745965461678636631863541462893547616877914914662358836365421198516263335926544716331814125295712581158399321372683742773423626286669759415959391374744214595682795818615532673877868424196926497731144319736445141728123322962547288572434564178492753681842244888368542423832228211172842456231275738182764232265933625119312598161192193214898949267765417468348935134618964683127194391796165368145548814473129857697989322621368744725685183346825333247866734735894493395218781464346951777873929898961358796274889826894529599645442657423438562423853247543621565468819799931598754753467593832328147439341586125262733737128386961596394728159719292787597426898945198788211417854662948358422729471312456437778978749753927251431677533575752312447488337156956217451965643454445329758327129966657189332824969141448538681979632611199385896965946849725421978137753366252459914913637858783146735469758716752765718189175583956476935185985918536318424248425426398158278111751711911227818826766177996223718837428972784328925743869885232266127727865267881592395643836999244218345184474613129823933659422223685422732186536199153988717455568523781673393698356967355875123554797755491181791593156433735591529495984256519631187849654633243225118132152549712643273819314433877592644693826861523243946998615722951182474773173215527598949553185313259992227879964482121769617218685394776778423378182462422788277997523913176326468957342296368178321958626168785578977414537368686438348124283789748775163821457641135163495649331144436157836647912852483177542224864952271874645274572426458614384917923623627532487625396914111582754953944965462576624728896917137599778828769958626788685374749661741223741834844643725486925886933118382649581481351844943368484853956759877215252766294896496444835264357169642341291412768946589781812493421379575569593678354241223363739129813633236996588711791919421574583924743119867622229659211793468744163297478952475933163259769578345894367855534294493613767564497137369969315192443795512585"
+    (*solve_captcha "123123"*)