Add Boyer-Moore string search algorithm (TheAlgorithms#990)

ayDavidGitHere · github-actions · web-flow · commit 7881cb5f16b0 · 2022-04-23T13:06:41.000+02:00
Co-authored-by: github-actions &lt;${GITHUB_ACTOR}@users.noreply.github.com&gt;
diff --git a/DIRECTORY.md b/DIRECTORY.md
@@ -295,6 +295,7 @@
 ## String
   * [AlphaNumericPalindrome](https://github.com/TheAlgorithms/Javascript/blob/master/String/AlphaNumericPalindrome.js)
   * [AlternativeStringArrange](https://github.com/TheAlgorithms/Javascript/blob/master/String/AlternativeStringArrange.js)
+  * [BoyerMoore](https://github.com/TheAlgorithms/Javascript/blob/master/String/BoyerMoore.js)
   * [CheckAnagram](https://github.com/TheAlgorithms/Javascript/blob/master/String/CheckAnagram.js)
   * [CheckCamelCase](https://github.com/TheAlgorithms/Javascript/blob/master/String/CheckCamelCase.js)
   * [CheckExceeding](https://github.com/TheAlgorithms/Javascript/blob/master/String/CheckExceeding.js)
diff --git a/String/BoyerMoore.js b/String/BoyerMoore.js
@@ -0,0 +1,49 @@
+/*
+ *
+ *
+ *Implementation of the Boyer-Moore String Search Algorithm.
+ *The Boyer–Moore string search algorithm allows linear time in
+ *search by skipping indices when searching inside a string for a pattern.
+ *
+ *
+ *
+ *
+ **/
+const buildBadMatchTable = (str) => {
+  const tableObj = {}
+  const strLength = str.length
+  for (let i = 0; i < strLength - 1; i++) {
+    tableObj[str[i]] = strLength - 1 - i
+  }
+  if (tableObj[str[strLength - 1]] === undefined) {
+    tableObj[str[strLength - 1]] = strLength
+  }
+  return tableObj
+}
+
+const boyerMoore = (str, pattern) => {
+  const badMatchTable = buildBadMatchTable(pattern)
+  let offset = 0
+  const patternLastIndex = pattern.length - 1
+  const maxOffset = str.length - pattern.length
+  // if the offset is bigger than maxOffset, cannot be found
+  while (offset <= maxOffset) {
+    let scanIndex = 0
+    while (pattern[scanIndex] === str[scanIndex + offset]) {
+      if (scanIndex === patternLastIndex) {
+        // found at this index
+        return offset
+      }
+      scanIndex++
+    }
+    const badMatchString = str[offset + patternLastIndex]
+    if (badMatchTable[badMatchString]) {
+      // increase the offset if it exists
+      offset += badMatchTable[badMatchString]
+    } else {
+      offset++
+    }
+  }
+  return -1
+}
+export { boyerMoore }
