Parses outermost paired delimiters, gracefully handling mismatches

GroupedString.swift

// #import "https://gist.githubusercontent.com/JadenGeller/7a4ae11be08a6798002775bbd07887e3/raw/7187bf2c72cd8ab29d64a0eea54a90e7c979dc41/Abort.swift"

extension String {
    enum GroupedString {
        enum Element {
            case character(Character)
            case group(Substring)
        }
        
        case balanced([Element])
        case hanging(Int, [Element], Int)
        
        init(leadingDepth: Int, elements: [Element], trailingDepth: Int) {
            if leadingDepth == 0 && trailingDepth == 0 {
                self = .balanced(elements)
            } else {
                self = .hanging(leadingDepth, elements, trailingDepth)
            }
        }
    }
    
    func grouping(between openingDelimiter: Character, _ closingDelimiter: Character) -> GroupedString {
        var leadingDepth = 0
        var currentDepth = 0
        let components = try! Abort.retry { () -> [Substring] in
            currentDepth = leadingDepth
            return try split(omittingEmptySubsequences: false) {
                switch $0 {
                case openingDelimiter:
                    defer { currentDepth += 1 }
                    return currentDepth == 0
                case closingDelimiter:
                    guard currentDepth > 0 else {
                        leadingDepth += 1
                        throw Abort()
                    }
                    currentDepth -= 1
                    return currentDepth == 0
                default:
                    return false
                }
            }
        }
        let elements = components.enumerated().flatMap { index, component -> [GroupedString.Element] in
            if index % 2 == (leadingDepth > 0 ? 1 : 0) {
                return component.map(GroupedString.Element.character)
            } else {
                return [GroupedString.Element.group(component)]
            }
        }
        return .init(leadingDepth: leadingDepth, elements: elements, trailingDepth: currentDepth)
    }
}