String prefix and suffix for Swift 3.0

StringCutter.swift

import Foundation

extension String {
    /**
        Drops characters from the front and the end of this string, returning
        what remains.

        - Parameter first: The number of characters to drop from the beginning.
            Values smaller than 0 are treated as 0.
        - Parameter last: The number of characters to drop from the end.
            Values smaller than 0 are treated as 0.
    */
    func dropping(first: Int = 0, last: Int = 0) -> String {
        let from = self.index(self.startIndex, offsetBy: max(first, 0),
                limitedBy: self.endIndex)!
        let to   = self.index(self.endIndex,   offsetBy: -max(last, 0),
                limitedBy: self.startIndex)!

        if to < from {
            return ""
        } else {
            return self.substring(with: from..<to)
        }
    }

    /**
        - Parameter first: The number of characters to take.
            Values smaller than 0 are treated as 0.
        - Returns: the string consisting of the first `first` characters of this string.
            If `first` is larger than the total number of characters, the whole
            string is returned.
    */
    func taking(first: Int) -> String {
        if first <= 0 {
            return ""
        } else if let to = self.index(self.startIndex, offsetBy: first, limitedBy: self.endIndex) {
            return self.substring(to: to)
        } else {
            return self
        }
    }

    /**
        - Parameter last: The number of characters to take.
            Values smaller than 0 are treated as 0.
        - Returns: the string consisting of the last `take` characters of this string.
            If `last` is larger than the total number of characters, the whole
            string is returned.
    */
    func taking(last: Int) -> String {
        if last <= 0 {
            return ""
        } else if let from = self.index(self.endIndex, offsetBy: -last, limitedBy: self.startIndex) {
            return self.substring(from: from)
        } else {
            return self
        }
    }
}

StringCutterTest.swift

import Foundation
import XCTest

class StringCutterTests: XCTestCase {
    func testDrop() {
        XCTAssertEqual("monkey", "monkey".dropping(first: 0, last: 0))
        XCTAssertEqual("monk",   "monkey".dropping(first: 0, last: 2))
        XCTAssertEqual("nkey",   "monkey".dropping(first: 2, last: 0))
        XCTAssertEqual("nk",     "monkey".dropping(first: 2, last: 2))
        XCTAssertEqual("",       "monkey".dropping(first: 3, last: 3))

        XCTAssertEqual("",       "monkey".dropping(first: 4,  last: 4))
        XCTAssertEqual("monke",  "monkey".dropping(first: -1, last: 1))
        XCTAssertEqual("onkey",  "monkey".dropping(first: 1,  last: -1))
        XCTAssertEqual("monkey", "monkey".dropping(first: -1, last: -1))

    }

    func testPrefix() {
        XCTAssertEqual("", "monkey".taking(first: -1))
        XCTAssertEqual("", "monkey".taking(first: 0))
        XCTAssertEqual("m", "monkey".taking(first: 1))
        XCTAssertEqual("mon", "monkey".taking(first: 3))
        XCTAssertEqual("monkey", "monkey".taking(first: 7))
    }

    func testSuffix() {
        XCTAssertEqual("", "monkey".taking(last: -1))
        XCTAssertEqual("", "monkey".taking(last: 0))
        XCTAssertEqual("y", "monkey".taking(last: 1))
        XCTAssertEqual("key", "monkey".taking(last: 3))
        XCTAssertEqual("monkey", "monkey".taking(last: 7))
    }
}