実用的なString Extension

SwiftのStringは色々な考慮上、便利なSubstring方法を提供されていない。LeetCodeをやる時Stringの問題が合うとかなり面倒くさい。下記のextensionをコピペーすれば大変助かる。

extension StringProtocol {
    subscript(_ offset: Int)                     -> Element     { self[index(startIndex, offsetBy: offset)] }
    subscript(_ range: Range<Int>)               -> SubSequence { prefix(range.lowerBound+range.count).suffix(range.count) }
    subscript(_ range: ClosedRange<Int>)         -> SubSequence { prefix(range.lowerBound+range.count).suffix(range.count) }
    subscript(_ range: PartialRangeThrough<Int>) -> SubSequence { prefix(range.upperBound.advanced(by: 1)) }
    subscript(_ range: PartialRangeUpTo<Int>)    -> SubSequence { prefix(range.upperBound) }
    subscript(_ range: PartialRangeFrom<Int>)    -> SubSequence { suffix(Swift.max(0, count-range.lowerBound)) }
}

参考：https://stackoverflow.com/questions/24092884/