Add Horspool string matching algorithm (TheAlgorithms#1397 old accepted PR on Development) (TheAlgorithms#2232)

George B · web-flow · commit 6cb034b07f54 · 2021-06-03T22:53:13.000+08:00
* Add Horspool algorithm
* Add wikipedia link and move to strings package
diff --git a/strings/HorspoolSearch.java b/strings/HorspoolSearch.java
@@ -0,0 +1,162 @@
+package strings;
+
+import java.util.HashMap;
+
+/**
+ * This class is not thread safe<br><br>
+ * (From wikipedia)
+ * In computer science, the Boyer–Moore–Horspool algorithm or Horspool's algorithm is an algorithm for finding
+ * substrings in strings. It was published by Nigel Horspool in 1980. <br>
+ * <a href=https://en.wikipedia.org/wiki/Boyer%E2%80%93Moore%E2%80%93Horspool_algorithm>Wikipedia page</a><br><br>
+ * <p>
+ * An explanation:<br>
+ * <p>
+ * The Horspool algorithm is a simplification of the Boyer-Moore algorithm in that it uses only one of the two heuristic
+ * methods for increasing the number of characters shifted when finding a bad match in the text. This method is usually
+ * called the "bad symbol" or "bad character" shift. The bad symbol shift method is classified as an input enhancement
+ * method in the theory of algorithms. Input enhancement is (from wikipedia) the principle that processing a given input
+ * to a problem and altering it in a specific way will increase runtime efficiency or space efficiency, or both. Both
+ * algorithms try to match the pattern and text comparing the pattern symbols to the text's from right to left.<br><br>
+ * <p>
+ * In the bad symbol shift method, a table is created prior to the search, called the "bad symbol table". The bad symbol
+ * table contains the shift values for any symbol in the text and pattern. For these symbols, the value is the length of
+ * the pattern, if the symbol is not in the first (length - 1) of the pattern. Else it is the distance from its
+ * rightmost occurrence in the pattern to the last symbol of the pattern. In practice, we only calculate the values for
+ * the ones that exist in the first (length - 1) of the pattern.<br><br>
+ * <p>
+ * For more details on the algorithm and the more advanced Boyer-Moore I recommend checking out the wikipedia page and
+ * professor Anany Levitin's book: Introduction To The Design And Analysis Of Algorithms.
+ * </p>
+ */
+public class HorspoolSearch {
+
+    private static HashMap<Character, Integer> shiftValues; // bad symbol table
+    private static Integer patternLength;
+    private static int comparisons = 0; // total comparisons in the current/last search
+
+    /**
+     * Case sensitive version version of the algorithm
+     *
+     * @param pattern the pattern to be searched for (needle)
+     * @param text    the text being searched in (haystack)
+     * @return -1 if not found or first index of the pattern in the text
+     */
+    public static int findFirst(String pattern, String text) {
+        return firstOccurrence(pattern, text, true);
+    }
+
+    /**
+     * Case insensitive version version of the algorithm
+     *
+     * @param pattern the pattern to be searched for (needle)
+     * @param text    the text being searched in (haystack)
+     * @return -1 if not found or first index of the pattern in the text
+     */
+    public static int findFirstInsensitive(String pattern, String text) {
+        return firstOccurrence(pattern, text, false);
+    }
+
+    /**
+     * Utility method that returns comparisons made by last run (mainly for tests)
+     *
+     * @return number of character comparisons of the last search
+     */
+    public static Integer getLastComparisons() {
+        return HorspoolSearch.comparisons;
+    }
+
+    /**
+     * Fairly standard implementation of the Horspool algorithm. Only the index of the last character of the pattern on the
+     * text is saved and shifted by the appropriate amount when a mismatch is found. The algorithm stops at the first
+     * match or when the entire text has been exhausted.
+     *
+     * @param pattern String to be matched in the text
+     * @param text    text String
+     * @return index of first occurrence of the pattern in the text
+     */
+    private static int firstOccurrence(String pattern, String text, boolean caseSensitive) {
+        shiftValues = calcShiftValues(pattern); // build the bad symbol table
+        comparisons = 0; // reset comparisons
+
+        int textIndex = pattern.length() - 1; // align pattern with text start and get index of the last character
+
+        // while pattern is not out of text bounds
+        while (textIndex < text.length()) {
+
+            // try to match pattern with current part of the text starting from last character
+            int i = pattern.length() - 1;
+            while (i >= 0) {
+                comparisons++;
+                char patternChar = pattern.charAt(i);
+                char textChar = text.charAt(
+                        (textIndex + i) - (pattern.length() - 1)
+                );
+                if (!charEquals(patternChar, textChar, caseSensitive)) { // bad character, shift pattern
+                    textIndex += getShiftValue(text.charAt(textIndex));
+                    break;
+                }
+                i--;
+            }
+
+            // check for full match
+            if (i == -1) {
+                return textIndex - pattern.length() + 1;
+            }
+        }
+
+        // text exhausted, return failure
+        return -1;
+    }
+
+    /**
+     * Compares the argument characters
+     *
+     * @param c1            first character
+     * @param c2            second character
+     * @param caseSensitive boolean determining case sensitivity of comparison
+     * @return truth value of the equality comparison
+     */
+    private static boolean charEquals(char c1, char c2, boolean caseSensitive) {
+        if (caseSensitive) {
+            return c1 == c2;
+        }
+        return Character.toLowerCase(c1) == Character.toLowerCase(c2);
+    }
+
+    /**
+     * Builds the bad symbol table required to run the algorithm. The method starts from the second to last character
+     * of the pattern and moves to the left. When it meets a new character, it is by definition its rightmost occurrence
+     * and therefore puts the distance from the current index to the index of the last character into the table. If the
+     * character is already in the table, then it is not a rightmost occurrence, so it continues.
+     *
+     * @param pattern basis for the bad symbol table
+     * @return the bad symbol table
+     */
+    private static HashMap<Character, Integer> calcShiftValues(String pattern) {
+        patternLength = pattern.length();
+        HashMap<Character, Integer> table = new HashMap<>();
+
+        for (int i = pattern.length() - 2; i >= 0; i--) { // length - 2 is the index of the second to last character
+            char c = pattern.charAt(i);
+            int finalI = i;
+            table.computeIfAbsent(c, k -> pattern.length() - 1 - finalI);
+        }
+
+        return table;
+    }
+
+    /**
+     * Helper function that uses the bad symbol shift table to return the appropriate shift value for a given character
+     *
+     * @param c character
+     * @return shift value that corresponds to the character argument
+     */
+    private static Integer getShiftValue(char c) {
+        if (shiftValues.get(c) != null) {
+            return shiftValues.get(c);
+        } else {
+            return patternLength;
+        }
+    }
+
+}
