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Java/strings/HorspoolSearch.java at master · rcholic/Java · GitHub
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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.
*/
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
;
}
}
}
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