feat: Add Rabin–Karp string search algorithm in R (#166)

Author: sgindeed

searches/rabin.karp.string.search.R

@@ -0,0 +1,76 @@
+# Rabin–Karp String Search Algorithm in R
+# Author: sgindeed
+# Description: Finds all occurrences of a pattern in a given text using a rolling hash technique.
+
+# Ask user for input
+text <- readline(prompt = "Enter the text: ")
+pattern <- readline(prompt = "Enter the pattern to search: ")
+
+# Convert both to lowercase for case-insensitive matching
+text <- tolower(text)
+pattern <- tolower(pattern)
+
+# Get lengths
+n <- nchar(text)
+m <- nchar(pattern)
+
+# Handle empty or invalid inputs
+if (m == 0) {
+  cat("Empty pattern. Nothing to search.\n")
+  quit(save = "no")
+}
+
+if (m > n) {
+  cat("Pattern is longer than text. Pattern not found in text.\n")
+  quit(save = "no")
+}
+
+# Constants
+base <- 256   # Number of possible characters
+mod <- 101    # A prime number for hashing
+
+# Initialize variables
+p_hash <- 0  # hash for pattern
+t_hash <- 0  # hash for text window
+h <- 1       # base^(m-1)
+matches <- c()
+
+# Compute (base^(m-1)) % mod safely
+for (i in seq_len(m - 1)) {
+  h <- (h * base) %% mod
+}
+
+# Convert characters to ASCII values
+pattern_chars <- utf8ToInt(pattern)
+text_chars <- utf8ToInt(text)
+
+# Compute initial hash values for pattern and first window of text
+for (i in 1:m) {
+  p_hash <- (base * p_hash + pattern_chars[i]) %% mod
+  t_hash <- (base * t_hash + text_chars[i]) %% mod
+}
+
+# Rabin–Karp main search
+for (i in 0:(n - m)) {
+  # If hash matches, verify actual substring
+  if (p_hash == t_hash) {
+    if (substr(text, i + 1, i + m) == pattern) {
+      matches <- c(matches, i + 1)
+    }
+  }
+  
+  # Slide window: remove first char, add next char
+  if (i < n - m) {
+    t_hash <- (base * (t_hash - text_chars[i + 1] * h) + text_chars[i + m + 1]) %% mod
+    if (t_hash < 0) {
+      t_hash <- t_hash + mod
+    }
+  }
+}
+
+# Display results
+if (length(matches) > 0) {
+  cat("Pattern found at positions:", paste(matches, collapse = ", "), "\n")
+} else {
+  cat("Pattern not found in the given text.\n")
+}