Improve Longest Common Substring Implementation (#801)

ref: improve longest common substring implementation

Author: sozelfist

src/dynamic_programming/longest_common_substring.rs

@@ -1,25 +1,36 @@
-// Longest common substring via Dynamic Programming
-// longest_common_substring(a, b) returns the length of longest common substring between the strings a and b.
-pub fn longest_common_substring(text1: &str, text2: &str) -> i32 {
-    let m = text1.len();
-    let n = text2.len();
+//! This module provides a function to find the length of the longest common substring
+//! between two strings using dynamic programming.
 
-    let t1 = text1.as_bytes();
-    let t2 = text2.as_bytes();
+/// Finds the length of the longest common substring between two strings using dynamic programming.
+///
+/// The algorithm uses a 2D dynamic programming table where each cell represents
+/// the length of the longest common substring ending at the corresponding indices in
+/// the two input strings. The maximum value in the DP table is the result, i.e., the
+/// length of the longest common substring.
+///
+/// The time complexity is `O(n * m)`, where `n` and `m` are the lengths of the two strings.
+/// # Arguments
+///
+/// * `s1` - The first input string.
+/// * `s2` - The second input string.
+///
+/// # Returns
+///
+/// Returns the length of the longest common substring between `s1` and `s2`.
+pub fn longest_common_substring(s1: &str, s2: &str) -> usize {
+    let mut substr_len = vec![vec![0; s2.len() + 1]; s1.len() + 1];
+    let mut max_len = 0;
 
-    // BottomUp Tabulation
-    let mut dp = vec![vec![0; n + 1]; m + 1];
-    let mut ans = 0;
-    for i in 1..=m {
-        for j in 1..=n {
-            if t1[i - 1] == t2[j - 1] {
-                dp[i][j] = 1 + dp[i - 1][j - 1];
-                ans = std::cmp::max(ans, dp[i][j]);
+    s1.as_bytes().iter().enumerate().for_each(|(i, &c1)| {
+        s2.as_bytes().iter().enumerate().for_each(|(j, &c2)| {
+            if c1 == c2 {
+                substr_len[i + 1][j + 1] = substr_len[i][j] + 1;
+                max_len = max_len.max(substr_len[i + 1][j + 1]);
             }
-        }
-    }
+        });
+    });
 
-    ans
+    max_len
 }
 
 #[cfg(test)]
@@ -28,28 +39,32 @@ mod tests {
 
     macro_rules! test_longest_common_substring {
         ($($name:ident: $inputs:expr,)*) => {
-        $(
-            #[test]
-            fn $name() {
-                let (text1, text2, expected) = $inputs;
-                assert_eq!(longest_common_substring(text1, text2), expected);
-                assert_eq!(longest_common_substring(text2, text1), expected);
-            }
-        )*
+            $(
+                #[test]
+                fn $name() {
+                    let (s1, s2, expected) = $inputs;
+                    assert_eq!(longest_common_substring(s1, s2), expected);
+                    assert_eq!(longest_common_substring(s2, s1), expected);
+                }
+            )*
         }
     }
 
     test_longest_common_substring! {
-        empty_inputs: ("", "", 0),
-        one_empty_input: ("", "a", 0),
-        single_same_char_input: ("a", "a", 1),
-        single_different_char_input: ("a", "b", 0),
-        regular_input_0: ("abcdef", "bcd", 3),
-        regular_input_1: ("abcdef", "xabded", 2),
-        regular_input_2: ("GeeksforGeeks", "GeeksQuiz", 5),
-        regular_input_3: ("abcdxyz", "xyzabcd", 4),
-        regular_input_4: ("zxabcdezy", "yzabcdezx", 6),
-        regular_input_5: ("OldSite:GeeksforGeeks.org", "NewSite:GeeksQuiz.com", 10),
-        regular_input_6: ("aaaaaaaaaaaaa", "bbb", 0),
+        test_empty_strings: ("", "", 0),
+        test_one_empty_string: ("", "a", 0),
+        test_identical_single_char: ("a", "a", 1),
+        test_different_single_char: ("a", "b", 0),
+        test_common_substring_at_start: ("abcdef", "abc", 3),
+        test_common_substring_at_middle: ("abcdef", "bcd", 3),
+        test_common_substring_at_end: ("abcdef", "def", 3),
+        test_no_common_substring: ("abc", "xyz", 0),
+        test_overlapping_substrings: ("abcdxyz", "xyzabcd", 4),
+        test_special_characters: ("@abc#def$", "#def@", 4),
+        test_case_sensitive: ("abcDEF", "ABCdef", 0),
+        test_full_string_match: ("GeeksforGeeks", "GeeksforGeeks", 13),
+        test_substring_with_repeated_chars: ("aaaaaaaaaaaaa", "aaa", 3),
+        test_longer_strings_with_common_substring: ("OldSite:GeeksforGeeks.org", "NewSite:GeeksQuiz.com", 10),
+        test_no_common_substring_with_special_chars: ("!!!", "???", 0),
     }
 }