string truncate extension

String+truncate.swift

import Foundation

// https://qiita.com/risuke/items/cdd6a75f236faae3ce6b
// https://gist.github.com/budidino/8585eecd55fd4284afaaef762450f98e
extension String {
    enum TruncationPosition {
        case head
        case middle
        case tail
    }

    func truncated(with position: TruncationPosition, limit: Int, leader: String = "...") -> String {
        guard self.count > limit else { return self }

        switch position {
        case .head:
            return leader + self.suffix(limit)
        case .middle:
            let headCharactersCount = Int(ceil(Float(limit - leader.count) / 2.0))
            let tailCharactersCount = Int(floor(Float(limit - leader.count) / 2.0))
            return "\(self.prefix(headCharactersCount))\(leader)\(self.suffix(tailCharactersCount))"
        case .tail:
            return self.prefix(limit) + leader
        }
    }

    /// 文字列を切り取って任意個数分を置換する
    /// - Parameters:
    ///   - position: 切り取り位置
    ///   - limit: 処理対象とする文字数
    ///   - count: 置き換える文字数
    ///   - leader: 置き換え文字
    func truncatedToReplace(with position: TruncationPosition,
                            limit: Int,
                            count: Int = 3,
                            leader: Character = ".") -> String {
        guard self.count > limit else { return self }
        let replacing = String(repeating: leader, count: count)
        switch position {
        case .head:
            return replacing + self.suffix(limit - count)
        case .middle:
            let headCharactersCount = Int(ceil(Float(limit - count) / 2.0))
            let tailCharactersCount = Int(floor(Float(limit - count) / 2.0))
            return "\(self.prefix(headCharactersCount))\(replacing)\(self.suffix(tailCharactersCount))"
        case .tail:
            return self.prefix(limit - count) + replacing
        }
    }
}

// MARK: - exec

let str = "abcdefghij01234567890" // count 20

str.truncated(with: .head, limit: 10)   // "...1234567890"
str.truncated(with: .middle, limit: 10) // "abcd...890"
str.truncated(with: .tail, limit: 10)   // "abcdefghij..."

str.truncatedToReplace(with: .head, limit: 10)   // "...4567890"
str.truncatedToReplace(with: .middle, limit: 10) // "abcd...890"
str.truncatedToReplace(with: .tail, limit: 10)   // "abcdefg..."