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strings/HorspoolSearch.java
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@ -3,30 +3,38 @@ 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>
* 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.
*/
public class HorspoolSearch {
@ -66,9 +74,9 @@ public class HorspoolSearch {
}
/**
* 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.
* 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
@ -78,7 +86,8 @@ public class HorspoolSearch {
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
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()) {
@ -88,9 +97,7 @@ public class HorspoolSearch {
while (i >= 0) {
comparisons++;
char patternChar = pattern.charAt(i);
char textChar = text.charAt(
(textIndex + i) - (pattern.length() - 1)
);
char textChar = text.charAt((textIndex + i) - (pattern.length() - 1));
if (!charEquals(patternChar, textChar, caseSensitive)) { // bad character, shift pattern
textIndex += getShiftValue(text.charAt(textIndex));
break;
@ -124,10 +131,11 @@ public class HorspoolSearch {
}
/**
* 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.
* 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
@ -136,7 +144,9 @@ public class HorspoolSearch {
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
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);
@ -146,7 +156,8 @@ public class HorspoolSearch {
}
/**
* Helper function that uses the bad symbol shift table to return the appropriate shift value for a given character
* 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
@ -158,5 +169,4 @@ public class HorspoolSearch {
return patternLength;
}
}
}