HegelPro's solution to Largest Series Product on the Elm track

Instructions

Test suite

Solution

Given a string of digits, calculate the largest product for a contiguous substring of digits of length n.

For example, for the input '1027839564', the largest product for a series of 3 digits is 270 (9 * 5 * 6), and the largest product for a series of 5 digits is 7560 (7 * 8 * 3 * 9 * 5).

Note that these series are only required to occupy adjacent positions in the input; the digits need not be numerically consecutive.

For the input '73167176531330624919225119674426574742355349194934', the largest product for a series of 6 digits is 23520.

Elm Installation

Refer to the Installing Elm page for information about installing elm.

Writing the Code

The first time you start an exercise, you'll need to ensure you have the appropriate dependencies installed. Thankfully, Elm makes that easy for you and will install dependencies when you try to run tests or build the code.

Execute the tests with:

$ elm-test

Automatically run tests again when you save changes:

$ elm-test --watch

As you work your way through the test suite, be sure to remove the skip <| calls from each test until you get them all passing!

Source

A variation on Problem 8 at Project Euler http://projecteuler.net/problem=8

Submitting Incomplete Solutions

It is possible to submit an incomplete solution so you can see how others have completed the exercise.

Tests.elm

module Tests exposing (tests)

import Expect
import LargestSeriesProduct exposing (largestProduct)
import Test exposing (..)


tests : Test
tests =
    describe "largestProduct"
        [ test "can find the largest product of 2 with numbers in order" <|
            \() -> Expect.equal (Just 72) (largestProduct 2 "0123456789")
        , skip <|
            test "can find the largest product of 2" <|
                \() -> Expect.equal (Just 48) (largestProduct 2 "576802143")
        , skip <|
            test "finds the largest product if span equals length" <|
                \() -> Expect.equal (Just 18) (largestProduct 2 "29")
        , skip <|
            test "can find the largest product of 3 with numbers in order" <|
                \() -> Expect.equal (Just 504) (largestProduct 3 "0123456789")
        , skip <|
            test "can find the largest product of 3" <|
                \() -> Expect.equal (Just 270) (largestProduct 3 "1027839564")
        , skip <|
            test "can find the largest product of 5 with numbers in order" <|
                \() -> Expect.equal (Just 15120) (largestProduct 5 "0123456789")
        , skip <|
            test "can get the largest product of a big number" <|
                \() -> Expect.equal (Just 23520) (largestProduct 6 "73167176531330624919225119674426574742355349194934")
        , skip <|
            test "can get the largest product of a big number II" <|
                \() -> Expect.equal (Just 28350) (largestProduct 6 "52677741234314237566414902593461595376319419139427")
        , skip <|
            test "can get the largest product of a big number (Project Euler)" <|
                \() -> Expect.equal (Just 23514624000) (largestProduct 13 "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450")
        , skip <|
            test "reports zero if the only digits are zero" <|
                \() -> Expect.equal (Just 0) (largestProduct 2 "0000")
        , skip <|
            test "reports zero if all spans include zero" <|
                \() -> Expect.equal (Just 0) (largestProduct 3 "99099")
        , skip <|
            test "rejects span longer than string length" <|
                \() -> Expect.equal Nothing (largestProduct 4 "123")
        , skip <|
            test "reports 1 for empty string and empty product (0 span)" <|
                \() -> Expect.equal (Just 1) (largestProduct 0 "")
        , skip <|
            test "reports 1 for nonempty string and empty product (0 span)" <|
                \() -> Expect.equal (Just 1) (largestProduct 0 "123")
        , skip <|
            test "rejects empty string and nonzero span" <|
                \() -> Expect.equal Nothing (largestProduct 1 "")
        , skip <|
            test "rejects invalid character in digits" <|
                \() -> Expect.equal Nothing (largestProduct 2 "1234a5")
        , skip <|
            test "rejects negative span" <|
                \() -> Expect.equal Nothing (largestProduct -1 "12345")
        ]

module LargestSeriesProduct exposing (largestProduct)


largestProduct : Int -> String -> Maybe Int
largestProduct length series =
    if String.length series >= length
        && length >= 0
        && String.all Char.isDigit series then
            series
                |> String.left length
                |> String.toList
                |> List.map (String.fromChar >> String.toInt >> Maybe.withDefault 0)
                |> List.foldr (*) 1
                |> \prevNumber -> (
                    if String.length series > 0 then
                        case largestProduct length (String.dropLeft 1 series) of
                            Just nextNumber -> max nextNumber prevNumber
                            Nothing -> prevNumber
                    else
                        prevNumber
                )
                |> \result -> Just result
    else
        Nothing

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HegelPro's solution

to Largest Series Product in the Elm Track