Handy utility for turning a collection of strings into a characterwise matrix for comparison within columns and retrieval of substrings from column ranges. Handles simple Range<Int> <---> Range<String.Index> conversions.

StringMatrix.swift

//
//  Created by Joshua Nozzi on 4/29/19.
//  Copyright © 2019 Joshua Nozzi. All rights reserved.
//


import Foundation


struct StringMatrix {
    
    
    var rows: Int { return strings.count }
    var columns: Int { return strings.map{ $0.count }.max() ?? 0 }
    
    init(_ strings: [String]) { self.strings = strings }
    
    
    func substringsAtColumn(_ column: Int) -> [Substring?] {
        
        return (0..<rows).map { self[$0, column] }
        
    }
    
    func substringsWithColumnRange(_ columnRange: Range<Int>) -> [Substring?] {
        
        assert(columnRangeIsValid(columnRange), "Out of column range")
        return (0..<rows).map {
            
            let string = strings[ $0 ]
            guard let stringRange = safeColumnRangeIn(columnRange, inString: string) else { return nil }
            return string[ stringRange ]
            
        }
        
    }
    
    subscript(row: Int, column: Int) -> Substring? {
        
        get {
            
            assert(indexIsValid(row: row, column: column), "Index out of range")
            let string = strings[row]
            guard column < string.count else { return nil }
            guard let range = Range(NSRange(location: column, length: 1), in: string) else { return nil }
            return string[range]
            
        }
        
    }
    
    
    // MARK: Private
    
    
    private let strings: [String]
    
    private func indexIsValid(row: Int, column: Int) -> Bool {
        
        return row >= 0 && row < rows && column >= 0 && column < columns
        
    }
    
    private func columnRangeIsValid(_ columnRange: Range<Int>) -> Bool {
        
        return columnRange.lowerBound >= 0 && columnRange.upperBound <= columns
        
    }
    
    private func safeColumnRangeIn(_ columnRange: Range<Int>, inString string: String) -> Range<String.Index>? {
        
        return Range(NSRange(columnRange.clamped(to: 0..<string.count)), in: string)
        
    }
    
    
}


// MARK: - Helper Extensions


extension Collection where Element == String {
    
    
    var matrix: StringMatrix { return StringMatrix(self as! [String]) }
    
    
}