Add Boyer-Moore-Horspool algorithm and tests.

Author: mmazzei

Boyer-Moore/BoyerMoore.swift

@@ -6,7 +6,7 @@
   http://www.drdobbs.com/database/faster-string-searches/184408171
 */
 extension String {
-    func indexOf(pattern: String) -> String.Index? {
+    func indexOf(pattern: String, useHorspoolImprovement: Bool = false) -> String.Index? {
         // Cache the length of the search pattern because we're going to
         // use it a few times and it's expensive to calculate.
         let patternLength = pattern.characters.count
@@ -53,8 +53,16 @@ extension String {
                 // There is a possible match. Do a brute-force search backwards.
                 if let k = backwards() { return k }
 
-                // If no match, we can only safely skip one character ahead.
-                i = index(after: i)
+                if !useHorspoolImprovement {
+                    // If no match, we can only safely skip one character ahead.
+                    i = index(after: i)
+                }
+                else {
+                    // Ensure to jump at least one character (this is needed because the first
+                    // character is in the skipTable, and `skipTable[lastChar] = 0`)
+                    let jumpOffset = max(skipTable[c] ?? patternLength, 1)
+                    i = index(i, offsetBy: jumpOffset, limitedBy: endIndex) ?? endIndex
+                }
             } else {
                 // The characters are not equal, so skip ahead. The amount to skip is
                 // determined by the skip table. If the character is not present in the

Boyer-Moore/Tests/BoyerMooreHorspoolTests.swift

@@ -0,0 +1,14 @@
+//
+//  BoyerMooreHorspoolTests.swift
+//  Tests
+//
+//  Created by Matias Mazzei on 12/24/16.
+//  Copyright © 2016 Swift Algorithm Club. All rights reserved.
+//
+
+class BoyerMooreHorspoolTests: BoyerMooreTest {
+    override func setUp() {
+        super.setUp()
+        useHorspoolImprovement = false
+    }
+}

Boyer-Moore/Tests/BoyerMooreTests.swift

@@ -1,18 +1,28 @@
+//
+//  BoyerMooreHorspoolTests.swift
+//  Tests
+//
+//  Created by Matias Mazzei on 12/24/16.
+//  Copyright © 2016 Swift Algorithm Club. All rights reserved.
+//
+
 import Foundation
 import XCTest
 
 class BoyerMooreTest: XCTestCase {
+    var useHorspoolImprovement = false
+
     override func setUp() {
         super.setUp()
     }
 
     func assert(pattern: String, doesNotExistsIn string: String) {
-        let index = string.indexOf(pattern: pattern)
+        let index = string.indexOf(pattern: pattern, useHorspoolImprovement: useHorspoolImprovement)
         XCTAssertNil(index)
     }
 
     func assert(pattern: String, existsIn string: String) {
-        let index = string.indexOf(pattern: pattern)
+        let index = string.indexOf(pattern: pattern, useHorspoolImprovement: useHorspoolImprovement)
         XCTAssertNotNil(index)
 
         let startIndex = index!

Boyer-Moore/Tests/Tests.xcodeproj/project.pbxproj

@@ -9,13 +9,15 @@
 /* Begin PBXBuildFile section */
 		7B80C3CE1C77A256003CECC7 /* BoyerMooreTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 7B80C3CD1C77A256003CECC7 /* BoyerMooreTests.swift */; };
 		9AED567C1E0ED6DE00958DCC /* BoyerMoore.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9AED567B1E0ED6DE00958DCC /* BoyerMoore.swift */; };
+		9AED56801E0EE60B00958DCC /* BoyerMooreHorspoolTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9AED567F1E0EE60B00958DCC /* BoyerMooreHorspoolTests.swift */; };
 /* End PBXBuildFile section */
 
 /* Begin PBXFileReference section */
 		7B2BBC801C779D720067B71D /* Tests.xctest */ = {isa = PBXFileReference; explicitFileType = wrapper.cfbundle; includeInIndex = 0; path = Tests.xctest; sourceTree = BUILT_PRODUCTS_DIR; };
 		7B2BBC941C779E7B0067B71D /* Info.plist */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.plist.xml; path = Info.plist; sourceTree = SOURCE_ROOT; };
 		7B80C3CD1C77A256003CECC7 /* BoyerMooreTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = BoyerMooreTests.swift; sourceTree = SOURCE_ROOT; };
 		9AED567B1E0ED6DE00958DCC /* BoyerMoore.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; name = BoyerMoore.swift; path = ../../BoyerMoore.swift; sourceTree = "<group>"; };
+		9AED567F1E0EE60B00958DCC /* BoyerMooreHorspoolTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = BoyerMooreHorspoolTests.swift; sourceTree = SOURCE_ROOT; };
 /* End PBXFileReference section */
 
 /* Begin PBXFrameworksBuildPhase section */
@@ -51,6 +53,7 @@
 				9AED567B1E0ED6DE00958DCC /* BoyerMoore.swift */,
 				7B80C3CD1C77A256003CECC7 /* BoyerMooreTests.swift */,
 				7B2BBC941C779E7B0067B71D /* Info.plist */,
+				9AED567F1E0EE60B00958DCC /* BoyerMooreHorspoolTests.swift */,
 			);
 			name = Tests;
 			path = TestsTests;
@@ -125,6 +128,7 @@
 			isa = PBXSourcesBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
+				9AED56801E0EE60B00958DCC /* BoyerMooreHorspoolTests.swift in Sources */,
 				9AED567C1E0ED6DE00958DCC /* BoyerMoore.swift in Sources */,
 				7B80C3CE1C77A256003CECC7 /* BoyerMooreTests.swift in Sources */,
 			);