Use a regular expressions to get indices from one string. Then use those indices to get the content from another. #AdventOfCode2022 #Day5 https://adventofcode.com/2022/day/5

regex_indexParser.swift

import UIKit



let stringA = " 1   2   3   4   5   6   7   8   9"
let stringB = "[F] [R] [C] [F] [L] [Q] [F] [D] [P]"

let matches = stringA.matches(of: /\b[0-9]+\b/)

for match in matches {
    print(match.0, terminator: " ")
    print(match.range.lowerBound) // to prove a point about String.Index
}
print()

for match in matches {
    print(stringB[match.range], terminator: "")
}
print()

func getMatchRanges(from templateString:String, using regEx:String) -> [Range<String.Index>] {
    guard let regex = try? Regex(regEx) else {
        print("Regex init from \(regEx) failed")
        return []
    }
    return templateString.matches(of: regex).map( {$0.range} )
}

let ranges = getMatchRanges(from: stringA, using: #"\b[0-9]+\b"#)
let results = ranges.compactMap({ stringB[$0] })

print(results)

//There is a problem if the lines is shorter
var stringC = "[M] [V] [C]"

//It's actually pretty hard to check the bounds in a normal way.
//String.Index is weird ...
//https://stackoverflow.com/questions/39676939/how-does-string-index-work-in-swift
stringA[(String.Index(utf16Offset: 3, in: stringA)...String.Index(utf16Offset: 8, in: stringA))]


//One solution is just to make the string longer
let dif = stringA.count - stringC.count
let whiteSpace = String(repeating: " ", count: dif)
let longEnough:String = stringC.appending(whiteSpace)

let resultsFromShortString = ranges.compactMap({ longEnough[$0] })
print(resultsFromShortString)

//Or instead make it fail quietly
//https://www.vadimbulavin.com/handling-out-of-bounds-exception/
extension String {
    subscript(quiet range: (Range<String.Index>)) -> Substring? {
        return indices.contains(range.lowerBound) && indices.contains(range.upperBound) ? self[range] : nil
    }
}

let resultsWithQuietFail = ranges.compactMap({ stringC[quiet: $0] })
print(resultsWithQuietFail)