Refactor Rabin Karp (#810)

* ref: refactor rabin karp

* chore(tests): add some edge tests

Author: sozelfist

src/string/rabin_karp.rs

@@ -1,137 +1,123 @@
-const MODULUS: u16 = 101;
-const BASE: u16 = 256;
-
-pub fn rabin_karp(target: &str, pattern: &str) -> Vec<usize> {
-    // Quick exit
-    if target.is_empty() || pattern.is_empty() || pattern.len() > target.len() {
+//! This module implements the Rabin-Karp string searching algorithm.
+//! It uses a rolling hash technique to find all occurrences of a pattern
+//! within a target string efficiently.
+
+const MOD: usize = 101;
+const RADIX: usize = 256;
+
+/// Finds all starting indices where the `pattern` appears in the `text`.
+///
+/// # Arguments
+/// * `text` - The string where the search is performed.
+/// * `pattern` - The substring pattern to search for.
+///
+/// # Returns
+/// A vector of starting indices where the pattern is found.
+pub fn rabin_karp(text: &str, pattern: &str) -> Vec<usize> {
+    if text.is_empty() || pattern.is_empty() || pattern.len() > text.len() {
         return vec![];
     }
 
-    let pattern_hash = hash(pattern);
+    let pat_hash = compute_hash(pattern);
+    let mut radix_pow = 1;
 
-    // Pre-calculate BASE^(n-1)
-    let mut pow_rem: u16 = 1;
+    // Compute RADIX^(n-1) % MOD
     for _ in 0..pattern.len() - 1 {
-        pow_rem *= BASE;
-        pow_rem %= MODULUS;
+        radix_pow = (radix_pow * RADIX) % MOD;
     }
 
     let mut rolling_hash = 0;
-    let mut ret = vec![];
-    for i in 0..=target.len() - pattern.len() {
+    let mut result = vec![];
+    for i in 0..=text.len() - pattern.len() {
         rolling_hash = if i == 0 {
-            hash(&target[0..pattern.len()])
+            compute_hash(&text[0..pattern.len()])
         } else {
-            recalculate_hash(target, i - 1, i + pattern.len() - 1, rolling_hash, pow_rem)
+            update_hash(text, i - 1, i + pattern.len() - 1, rolling_hash, radix_pow)
         };
-        if rolling_hash == pattern_hash && pattern[..] == target[i..i + pattern.len()] {
-            ret.push(i);
+        if rolling_hash == pat_hash && pattern[..] == text[i..i + pattern.len()] {
+            result.push(i);
         }
     }
-    ret
+    result
 }
 
-// hash(s) is defined as BASE^(n-1) * s_0 + BASE^(n-2) * s_1 + ... + BASE^0 * s_(n-1)
-fn hash(s: &str) -> u16 {
-    let mut res: u16 = 0;
-    for &c in s.as_bytes().iter() {
-        res = (res * BASE % MODULUS + c as u16) % MODULUS;
-    }
-    res
+/// Calculates the hash of a string using the Rabin-Karp formula.
+///
+/// # Arguments
+/// * `s` - The string to calculate the hash for.
+///
+/// # Returns
+/// The hash value of the string modulo `MOD`.
+fn compute_hash(s: &str) -> usize {
+    let mut hash_val = 0;
+    for &byte in s.as_bytes().iter() {
+        hash_val = (hash_val * RADIX + byte as usize) % MOD;
+    }
+    hash_val
 }
 
-// new_hash = (old_hash - BASE^(n-1) * s_(i-n)) * BASE + s_i
-fn recalculate_hash(
+/// Updates the rolling hash when shifting the search window.
+///
+/// # Arguments
+/// * `s` - The full text where the search is performed.
+/// * `old_idx` - The index of the character that is leaving the window.
+/// * `new_idx` - The index of the new character entering the window.
+/// * `old_hash` - The hash of the previous substring.
+/// * `radix_pow` - The precomputed value of RADIX^(n-1) % MOD.
+///
+/// # Returns
+/// The updated hash for the new substring.
+fn update_hash(
     s: &str,
-    old_index: usize,
-    new_index: usize,
-    old_hash: u16,
-    pow_rem: u16,
-) -> u16 {
+    old_idx: usize,
+    new_idx: usize,
+    old_hash: usize,
+    radix_pow: usize,
+) -> usize {
     let mut new_hash = old_hash;
-    let (old_ch, new_ch) = (
-        s.as_bytes()[old_index] as u16,
-        s.as_bytes()[new_index] as u16,
-    );
-    new_hash = (new_hash + MODULUS - pow_rem * old_ch % MODULUS) % MODULUS;
-    new_hash = (new_hash * BASE + new_ch) % MODULUS;
+    let old_char = s.as_bytes()[old_idx] as usize;
+    let new_char = s.as_bytes()[new_idx] as usize;
+    new_hash = (new_hash + MOD - (old_char * radix_pow % MOD)) % MOD;
+    new_hash = (new_hash * RADIX + new_char) % MOD;
     new_hash
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
-    #[test]
-    fn hi_hash() {
-        let hash_result = hash("hi");
-        assert_eq!(hash_result, 65);
-    }
-
-    #[test]
-    fn abr_hash() {
-        let hash_result = hash("abr");
-        assert_eq!(hash_result, 4);
-    }
-
-    #[test]
-    fn bra_hash() {
-        let hash_result = hash("bra");
-        assert_eq!(hash_result, 30);
-    }
-
-    // Attribution to @pgimalac for his tests from Knuth-Morris-Pratt
-    #[test]
-    fn each_letter_matches() {
-        let index = rabin_karp("aaa", "a");
-        assert_eq!(index, vec![0, 1, 2]);
-    }
-
-    #[test]
-    fn a_few_separate_matches() {
-        let index = rabin_karp("abababa", "ab");
-        assert_eq!(index, vec![0, 2, 4]);
-    }
-
-    #[test]
-    fn one_match() {
-        let index = rabin_karp("ABC ABCDAB ABCDABCDABDE", "ABCDABD");
-        assert_eq!(index, vec![15]);
-    }
-
-    #[test]
-    fn lots_of_matches() {
-        let index = rabin_karp("aaabaabaaaaa", "aa");
-        assert_eq!(index, vec![0, 1, 4, 7, 8, 9, 10]);
-    }
-
-    #[test]
-    fn lots_of_intricate_matches() {
-        let index = rabin_karp("ababababa", "aba");
-        assert_eq!(index, vec![0, 2, 4, 6]);
-    }
-
-    #[test]
-    fn not_found0() {
-        let index = rabin_karp("abcde", "f");
-        assert_eq!(index, vec![]);
-    }
-
-    #[test]
-    fn not_found1() {
-        let index = rabin_karp("abcde", "ac");
-        assert_eq!(index, vec![]);
-    }
-
-    #[test]
-    fn not_found2() {
-        let index = rabin_karp("ababab", "bababa");
-        assert_eq!(index, vec![]);
+    macro_rules! test_cases {
+        ($($name:ident: $inputs:expr,)*) => {
+            $(
+                #[test]
+                fn $name() {
+                    let (text, pattern, expected) = $inputs;
+                    assert_eq!(rabin_karp(text, pattern), expected);
+                }
+            )*
+        };
     }
 
-    #[test]
-    fn empty_string() {
-        let index = rabin_karp("", "abcdef");
-        assert_eq!(index, vec![]);
+    test_cases! {
+        single_match_at_start: ("hello world", "hello", vec![0]),
+        single_match_at_end: ("hello world", "world", vec![6]),
+        single_match_in_middle: ("abc def ghi", "def", vec![4]),
+        multiple_matches: ("ababcabc", "abc", vec![2, 5]),
+        overlapping_matches: ("aaaaa", "aaa", vec![0, 1, 2]),
+        no_match: ("abcdefg", "xyz", vec![]),
+        pattern_is_entire_string: ("abc", "abc", vec![0]),
+        target_is_multiple_patterns: ("abcabcabc", "abc", vec![0, 3, 6]),
+        empty_text: ("", "abc", vec![]),
+        empty_pattern: ("abc", "", vec![]),
+        empty_text_and_pattern: ("", "", vec![]),
+        pattern_larger_than_text: ("abc", "abcd", vec![]),
+        large_text_small_pattern: (&("a".repeat(1000) + "b"), "b", vec![1000]),
+        single_char_match: ("a", "a", vec![0]),
+        single_char_no_match: ("a", "b", vec![]),
+        large_pattern_no_match: ("abc", "defghi", vec![]),
+        repeating_chars: ("aaaaaa", "aa", vec![0, 1, 2, 3, 4]),
+        special_characters: ("abc$def@ghi", "$def@", vec![3]),
+        numeric_and_alphabetic_mix: ("abc123abc456", "123abc", vec![3]),
+        case_sensitivity: ("AbcAbc", "abc", vec![]),
     }
 }