add boyer moore string search (TheAlgorithms#388)

Author: ongspxm

strings/search/boyermoore.go

@@ -0,0 +1,57 @@
+package search
+
+// Implementation of boyer moore string search
+// O(l) where l=len(text)
+func BoyerMoore(text string, pattern string) []int {
+	var positions []int
+
+	l := len(text)
+	n := len(pattern)
+
+	// using booyer moore horspool modification
+	// O(n) space instead of O(n**2)
+	bcr := make(map[byte]int)
+	for i := 0; i < n-1; i++ {
+		bcr[pattern[i]] = n - i - 1
+	}
+
+	// Apostolico–Giancarlo modification
+	// allow to skip patterns that we know matches
+	// let us do O(l) instead of O(ln)
+	skips := make(map[int]int)
+	for _, s := range bcr {
+		i := 0
+		for ; i < n-s; i++ {
+			if pattern[n-1-i] != pattern[n-1-s-i] {
+				break
+			}
+		}
+		skips[s] = i
+	}
+
+	skip := 0
+	jump := n
+	for i := 0; i < l-n+1; {
+		skip = skips[jump]
+		for k := n - 1; k > -1; k-- {
+			if text[i+k] != pattern[k] {
+				jump, ok := bcr[text[i+k]]
+				if !ok {
+					jump = n
+				}
+				i += jump
+				break
+			}
+			if k == n-jump {
+				k -= skip
+			}
+			if k == 0 {
+				positions = append(positions, i)
+				jump = 1
+				i += jump
+			}
+		}
+	}
+
+	return positions
+}

strings/search/patternsearch.go renamed to strings/search/naive.go

@@ -1,15 +1,7 @@
-/*
-This algorithm tries to search the given pattern in the given text.
-If pattern is found from index position i in the text,
-it is added to positions.
-
-Time Complexity : O(n*m)
-    n = length of text
-    m = length of pattern
-*/
-
 package search
 
+// Implementation of naive string search
+// O(n*m) where n=len(txt) and m=len(pattern)
 func Naive(text string, pattern string) []int {
 	var positions []int
 	for i := 0; i <= len(text)-len(pattern); i++ {

strings/search/patternsearch_test.go

@@ -35,6 +35,18 @@ var testCases = []struct {
 		"XYZ",
 		[]int(nil),
 	},
+	{
+		"worse case 1",
+		"AAAAAAAAAA",
+		"AAA",
+		[]int{0, 1, 2, 3, 4, 5, 6, 7},
+	},
+	{
+		"worse case 2",
+		"NANANANANANANANANA",
+		"NANANA",
+		[]int{0, 2, 4, 6, 8, 10, 12},
+	},
 }
 
 func TestNaive(t *testing.T) {
@@ -47,3 +59,14 @@ func TestNaive(t *testing.T) {
 		})
 	}
 }
+
+func TestBooyerMoore(t *testing.T) {
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			actual := BoyerMoore(tc.input, tc.pattern)
+			if !reflect.DeepEqual(actual, tc.expected) {
+				t.Errorf("Expected matches for pattern '%s' for string '%s' are: %v, but actual matches are: %v", tc.pattern, tc.input, tc.expected, actual)
+			}
+		})
+	}
+}