TSCBasic: Use collection diff api for EditDistance if possible (#302)

* Use collection diff api if possible
* Separate func for unit test

Author: PetrGuan

Sources/TSCBasic/EditDistance.swift

@@ -8,13 +8,28 @@
  See http://swift.org/CONTRIBUTORS.txt for Swift project authors
 */
 
-/// Computes the number of edits needed to transform first string to second.
+/// Computes the number of edits needed to transform source string to target.
 ///
-/// - Complexity: O(_n*m_), where *n* is the length of the first String and
-///   *m* is the length of the second one.
-public func editDistance(_ first: String, _ second: String) -> Int {
+/// - Complexity: O(_n*m_), where *n* is the length of the source String and
+///   *m* is the length of the target one.
+public func editDistance(_ source: String, _ target: String) -> Int {
     // FIXME: We should use the new `CollectionDifference` API once the
     // deployment target is bumped.
+    if #available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *) {
+        return collectionDiffEditDistance(source, target)
+    }
+    else {
+        return internalEditDistance(source, target)
+    }
+}
+
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
+func collectionDiffEditDistance(_ source: String, _ target: String) -> Int {
+    let difference = target.difference(from: source)
+    return max(difference.insertions.count, difference.removals.count)
+}
+
+func internalEditDistance(_ first: String, _ second: String) -> Int {
     let a = Array(first.utf16)
     let b = Array(second.utf16)
     var distance = [[Int]](repeating: [Int](repeating: 0, count: b.count + 1), count: a.count + 1)

Tests/TSCBasicTests/EditDistanceTests.swift

@@ -8,18 +8,29 @@
  See http://swift.org/CONTRIBUTORS.txt for Swift project authors
 */
 
+@testable import TSCBasic
 import XCTest
 
 import TSCBasic
 
 class EditDistanceTests: XCTestCase {
 
-    func testBasics() {
-        XCTAssertEqual(editDistance("Foo", "Fo"), 1)
-        XCTAssertEqual(editDistance("Foo", "Foo"), 0)
-        XCTAssertEqual(editDistance("Bar", "Foo"), 3)
-        XCTAssertEqual(editDistance("ABCDE", "ABDE"), 1)
-        XCTAssertEqual(editDistance("sunday", "saturday"), 3)
-        XCTAssertEqual(editDistance("FOO", "foo"), 3)
+    @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
+    func testEditDistanceWithCollectionDiff() {
+        XCTAssertEqual(collectionDiffEditDistance("Foo", "Fo"), 1)
+        XCTAssertEqual(collectionDiffEditDistance("Foo", "Foo"), 0)
+        XCTAssertEqual(collectionDiffEditDistance("Bar", "Foo"), 3)
+        XCTAssertEqual(collectionDiffEditDistance("ABCDE", "ABDE"), 1)
+        XCTAssertEqual(collectionDiffEditDistance("sunday", "saturday"), 3)
+        XCTAssertEqual(collectionDiffEditDistance("FOO", "foo"), 3)
+    }
+
+    func testInternalEditDistance() {
+        XCTAssertEqual(internalEditDistance("Foo", "Fo"), 1)
+        XCTAssertEqual(internalEditDistance("Foo", "Foo"), 0)
+        XCTAssertEqual(internalEditDistance("Bar", "Foo"), 3)
+        XCTAssertEqual(internalEditDistance("ABCDE", "ABDE"), 1)
+        XCTAssertEqual(internalEditDistance("sunday", "saturday"), 3)
+        XCTAssertEqual(internalEditDistance("FOO", "foo"), 3)
     }
 }