Code solution for Advent of Code 2017 Day 1 (http://adventofcode.com/2017/day/1)

advent-of-code-2017-elm-day1-part2.elm

module Main exposing (main)

import Html exposing (Html, li, text, ul)
import Html.Attributes exposing (style)

-- Advent of Code 2017 - Day 1 Part 2 (http://adventofcode.com/2017/day/1)

main : Html msg
main =
    ul []
        (List.map
            (\c ->
                let
                    result =
                        dayOneFacade (Tuple.first c)
                in
                result
                    |> toString
                    |> (\s ->
                            "Expected "
                                ++ toString (Tuple.second c)
                                ++ " got "
                                ++ s
                                |> text
                                |> (\t ->
                                        li
                                            [ style
                                                [ ( "background"
                                                  , if result == Tuple.second c then
                                                        "green"
                                                    else
                                                        "red"
                                                  )
                                                ]
                                            ]
                                            [ t ]
                                   )
                       )
            )
            cases
        )

cases =
    [ ( "1212", 6 ), 
      ( "1221", 0 ), 
      ( "123425", 4 ), 
      ( "123123", 12 ),
      ( "12131415", 4 ),
      ( "181445682966897848665963472661939865313976877194312684993521259486517527961396717561854825453963181134379574918373213732184697746668399631642622373684425326112585283946462323363991753895647177797691214784149215198715986947573668987188746878678399624533792551651335979847131975965677957755571358934665327487287312467771187981424785514785421781781976477326712674311994735947987383516699897916595433228294198759715959469578766739518475118771755787196238772345762941477359483456641194685333528329581113788599843621326313592354167846466415943566183192946217689936174884493199368681514958669615226362538622898367728662941275658917124167353496334664239539753835439929664552886538885727235662548783529353611441231681613535447417941911479391558481443933134283852879511395429489152435996669232681215627723723565872291296878528334773391626672491878762288953597499218397146685679387438634857358552943964839321464529237533868734473777756775687759355878519113426969197211824325893376812556798483325994128743242544899625215765851923959798197562831313891371735973761384464685316273343541852758525318144681364492173465174512856618292785483181956548813344752352933634979165667651165776587656468598791994573513652324764687515345959621493346623821965554755615219855842969932269414839446887613738174567989512857785566352285988991946436148652839391593178736624957214917527759574235133666461988355855613377789115472297915429318142824465141688559333787512328799783539285826471818279818457674417354335454395644435889386297695625378256613558911695145397779576526397241795181294322797687168326696497256684943829666672341162656479563522892141714998477865114944671225898297338685958644728534192317628618817551492975251364233974374724968483637518876583946828819994321129556511537619253381981544394112184655586964655164192552352534626295996968762388827294873362719636616182786976922445125551927969267591395292198155775434997827738862786341543524544822321112131815475829945625787561369956264826651461575948462782869972654343749617939132353399334744265286151177931594514857563664329299713436914721119746932159456287267887878779218815883191236858656959258484139254446341", 2)
      ]


dayOneFacade : String -> Int
dayOneFacade digits =
    let
        dayOne : List (Int, Int) -> Int -> Int
        dayOne list sum =
            case list of
                (a, b) :: tail ->
                    dayOne (tail)
                        (if a == b then
                            sum + a
                         else
                            sum
                        )

                _ ->
                    sum

        half = (String.length digits) // 2

        list =
            String.split "" digits
                |> List.map (String.toInt >> Result.toMaybe >> Maybe.withDefault 0)
                |> \numbers -> List.map2 (\x y -> (x, y)) numbers (List.drop half numbers ++ List.take half numbers)
    in
    dayOne list 0