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HegelPro's solution

to Matching Brackets in the Elm Track

Published at Jul 05 2019 · 0 comments
Instructions
Test suite
Solution

Note:

This exercise has changed since this solution was written.

Bracket Push

Given a string containing brackets [], braces {}, parentheses (), or any combination thereof, verify that any and all pairs are matched and nested correctly.

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

Ginna Baker

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 BracketPush exposing (isPaired)
import Expect
import String
import Test exposing (..)


tests : Test
tests =
    describe "BracketPush"
        [ test "paired square brackets" <|
            \_ ->
                Expect.equal True (isPaired "[]")
        , skip <|
            test "empty string" <|
                \_ ->
                    Expect.equal True (isPaired "")
        , skip <|
            test "unpaired brackets" <|
                \_ ->
                    Expect.equal False (isPaired "[[")
        , skip <|
            test "wrong ordered brackets" <|
                \_ ->
                    Expect.equal False (isPaired "}{")
        , skip <|
            test "wrong closing bracket" <|
                \_ ->
                    Expect.equal False (isPaired "{]")
        , skip <|
            test "paired with whitespace" <|
                \_ ->
                    Expect.equal True (isPaired "{ }")
        , skip <|
            test "partially paired brackets" <|
                \_ ->
                    Expect.equal False (isPaired "{[])")
        , skip <|
            test "simple nested brackets" <|
                \_ ->
                    Expect.equal True (isPaired "{[]}")
        , skip <|
            test "seceral paired brackets" <|
                \_ ->
                    Expect.equal True (isPaired "{}[]")
        , skip <|
            test "paired and nested brackets" <|
                \_ ->
                    Expect.equal True (isPaired "([{}({}[])])")
        , skip <|
            test "unopened closing brackets" <|
                \_ ->
                    Expect.equal False (isPaired "{[)][]}")
        , skip <|
            test "unpaired and nested brackets" <|
                \_ ->
                    Expect.equal False (isPaired "([{])")
        , skip <|
            test "paired and wrong nested brackets" <|
                \_ ->
                    Expect.equal False (isPaired "[({]})")
        , skip <|
            test "math expression" <|
                \_ ->
                    Expect.equal True (isPaired "(((185 + 223.85) * 15) - 543)/2")
        , skip <|
            test "complex latex expression" <|
                \_ ->
                    Expect.equal True (isPaired "\\left(\\begin{array}{cc} \\frac{1}{3} & x\\\\ \\mathrm{e}^{x} &... x^2 \\end{array}\\right)")
        ]
module BracketPush exposing (isPaired)

import Dict exposing (..)

openBracketToCloseBracket : String -> String
openBracketToCloseBracket openChar =
    case openChar of
        "[" -> "]"
        "{" -> "}"
        "(" -> ")"
        _ -> ""

isBracket : String -> Bool
isBracket char
    = isOpenBracket char
    || isCloseBracket char

isOpenBracket : String -> Bool
isOpenBracket char
    = char == "["
    || char == "{"
    || char == "("

isCloseBracket : String -> Bool
isCloseBracket char
    = char == "]"
    || char == "}"
    || char == ")"


isPaired : String -> Bool
isPaired input =
    let
        char = String.left 1 input
    in
        if String.length input == 0 then
            True
        else
            if isBracket char then
                if isOpenBracket char then
                    case findLastCloseBracketIndex char input of
                        Just closeBracketIndex ->
                            isPaired (String.dropLeft 1 (String.left (closeBracketIndex + 1) input))
                            && isPaired (String.dropLeft (closeBracketIndex + 2) input)
                        Nothing -> False
                else
                    False
            else
                isPaired (String.dropLeft 1 input)

findFirstIndex : String -> String -> Maybe Int
findFirstIndex char str =
    str
        |> String.indexes char
        |> List.head

findSecondIndex : String -> String -> Maybe Int
findSecondIndex char str =
    str
        |> String.indexes char
        |> List.drop 1
        |> List.head

findLastIndex : String -> String -> Maybe Int
findLastIndex char str =
    str
        |> String.indexes char
        |> List.reverse
        |> List.head
            
findLastCloseBracketIndex : String -> String -> Maybe Int
findLastCloseBracketIndex char str =
    String.foldl (\n res -> funcFo char n res) { index = 0, z = 0, result = Nothing, isDone = False } (str)
        |> \n -> n.result

funcFo : String -> Char -> { index : Int, z : Int, result : Maybe Int, isDone : Bool } -> { index : Int, z : Int, result : Maybe Int, isDone : Bool }
funcFo findChar char res =
    if res.isDone then
        res
    else
        if String.fromChar (char) == findChar then
            { res | index = res.index + 1, z = res.z + 1 }
        else if openBracketToCloseBracket findChar == String.fromChar char then
            if res.index == 1 then
                {res | isDone = True, result = Just (res.z - 1) }
            else
                { res | index = res.index - 1, z = res.z + 1 }
        else
            { res | z = res.z + 1 }

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