Solve 'Gap in Primes' kata

Sources/SwiftKatas/GapInPrimes/gap.swift

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+/**
+ https://www.codewars.com/kata/561e9c843a2ef5a40c0000a4/train/swift
+ */
+func gap(_ g: Int, _ m: Int, _ n: Int) -> (Int, Int)? {
+    return (m ... n)
+        .filter { $0.isPrime() }
+        .zipWithNext()
+        .map { ($0, $1) }
+        .first { $0.1 - $0.0 == g }
+}
+
+public extension Int {
+    func isPrime() -> Bool {
+        if self <= 1 { return false }
+        if self <= 3 { return true }
+        if self % 2 == 0 || self % 3 == 0 { return false }
+
+        var i = 5
+        while i * i <= self {
+            if self % i == 0 || self % (i + 2) == 0 {
+                return false
+            }
+            i += 6
+        }
+
+        return true
+    }
+}
+
+extension Sequence {
+    func zipWithNext() -> Zip2Sequence<Self, DropFirstSequence<Self>> {
+        return zip(self, dropFirst())
+    }
+}

Tests/SwiftKatasTests/GapInPrimes/gapTests.swift

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+@testable import SwiftKatas
+import XCTest
+
+class GapInPrimesTest: XCTestCase {
+    func testing(_ g: Int, _ m: Int, _ n: Int, _ expected: (Int, Int)?) {
+        XCTAssert(gap(g, m, n)! == expected!, "should return \(expected!), but returned \(gap(g, m, n)!)")
+    }
+
+    func testingNil(_ g: Int, _ m: Int, _ n: Int) {
+        XCTAssertTrue(gap(g, m, n) == nil, "Should return nil")
+    }
+
+    func testExample() {
+        testing(2, 100, 110, (101, 103))
+        testing(4, 100, 110, (103, 107))
+        testingNil(6, 100, 110)
+        testingNil(11, 30000, 100_000)
+        testing(2, 10_000_000, 11_000_000, (10_000_139, 10_000_141))
+    }
+}