ali-ghanbari commented on a change in pull request #233:
URL: https://github.com/apache/commons-text/pull/233#discussion_r674460266
##########
File path:
src/main/java/org/apache/commons/text/similarity/LongestCommonSubsequence.java
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@@ -58,7 +58,57 @@ public Integer apply(final CharSequence left, final
CharSequence right) {
if (left == null || right == null) {
throw new IllegalArgumentException("Inputs must not be null");
}
- return longestCommonSubsequence(left, right).length();
+ // Find lengths of two strings
+ final int leftSz = left.length();
+ final int rightSz = right.length();
+
+ // Check if we can save even more space
+ if (leftSz < rightSz) {
+ return algorithmB(right, rightSz, left, leftSz)[leftSz];
+ }
+ return algorithmB(left, leftSz, right, rightSz)[rightSz];
+ }
+
+ /**
+ * An implementation of "ALG B" from Hirschberg's paper <a
href="https://dl.acm.org/doi/10.1145/360825.360861";>A linear space algorithm
for computing maximal common subsequences</a>.
+ * Assuming the sequence <code>left</code> is of size <code>m</code> and
the sequence <code>right</code> is of size <code>n</code>,
+ * this method returns the last row of the dynamic programming table when
calculating LCS the two sequences.
+ * Therefore, the last element of the returned array, is the size of LCS
of <code>left</code> and <code>right</code>.
+ * This method runs in O(m * n) time and O(n) space.
+ * To save more space, it is preferable to pass the shorter sequence as
<code>right</code>.
Review comment:
Yes, that would be more _user-friendly_, but as algorithmB is being
called from algorithmC as an auxiliary function and swapping the parameters
causes the method to return an array of some length that is not expected by
algorithmC. That's why I didn't adopt that approach.
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