Add Hamiltonian Path in R

documentation/fast_fourier_transform.md

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+# Fast Fourier Transform (FFT)
+
+This file documents the recursive Cooley-Tukey FFT implementation added to `R/mathematics/fast_fourier_transform.r`.
+
+## Description
+
+The `fft_recursive` function computes the discrete Fourier transform (DFT) of a numeric or complex vector using a divide-and-conquer Cooley-Tukey algorithm. If the input length is not a power of two, it is zero-padded to the next power of two.
+
+## Usage
+
+In an R session:
+
+source('mathematics/fast_fourier_transform.r')
+fft_recursive(c(0, 1, 2, 3))
+
+From the command line with Rscript:
+
+Rscript -e "source('R/mathematics/fast_fourier_transform.r'); print(fft_recursive(c(0,1,2,3)))"
+
+## Complexity
+
+Time complexity: O(n log n) for inputs with length a power of two; otherwise dominated by padding to next power of two.
+
+Space complexity: O(n) additional space for recursive calls.
+
+## Notes
+
+- The function returns a complex vector of the same length (after padding) as the input.
+- This implementation is primarily educational; production code should prefer the optimized `fft` function available in base R.

documentation/hamiltonian_path.md

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+# Hamiltonian Path (Backtracking)
+
+This document describes the Hamiltonian Path backtracking implementation in `R/graph_algorithms/hamiltonian_path.r`.
+
+## Description
+
+The `hamiltonianPath` function searches for a Hamiltonian Path in an undirected graph represented by an adjacency matrix. It uses backtracking to attempt to build a path that visits every vertex exactly once.
+
+## Usage
+
+In an R session:
+
+source('graph_algorithms/hamiltonian_path.r')
+
+From command line using Rscript:
+
+Rscript -e "source('R/graph_algorithms/hamiltonian_path.r')"
+
+## Complexity
+
+- Time complexity: O(n!) in the worst case (backtracking over permutations).
+- Space complexity: O(n) for path storage and recursion.
+
+## Notes
+
+- The implementation assumes an undirected graph given as an adjacency matrix with 0/1 entries.
+- For production use on larger graphs, consider heuristics or approximation algorithms; the problem is NP-complete.

graph_algorithms/hamiltonian_path.r

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+# Hamiltonian Path (Backtracking)
+#
+# This implementation searches for a Hamiltonian Path in an undirected graph
+# represented by an adjacency matrix. It uses backtracking to try all possible
+# vertex sequences. The implementation follows the style used in other
+# algorithms in the `R/graph_algorithms` folder.
+#
+# Time Complexity: O(n!) in the worst case (backtracking over permutations)
+# Space Complexity: O(n) for the path and recursion stack
+#
+# Input: adjacency matrix `graph` (n x n)
+# Output: prints a Hamiltonian path if found and returns TRUE, otherwise prints
+#         a message and returns FALSE
+
+# Function to check if vertex v can be added to path at position pos
+isSafe <- function(v, graph, path, pos) {
+  # Check adjacency between current vertex and previous vertex
+  if (graph[path[pos - 1], v] == 0)
+    return(FALSE)
+
+  # Check if vertex is already in path
+  if (v %in% path)
+    return(FALSE)
+
+  return(TRUE)
+}
+
+# Recursive function to find Hamiltonian path
+hamiltonianUtil <- function(graph, path, pos) {
+  n <- nrow(graph)
+
+  # Base case: if all vertices are included in the path
+  if (pos > n)
+    return(TRUE)
+
+  for (v in 1:n) {
+    if (isSafe(v, graph, path, pos)) {
+      path[pos] <- v
+
+      if (hamiltonianUtil(graph, path, pos + 1))
+        return(TRUE)
+
+      # Backtrack
+      path[pos] <- -1
+    }
+  }
+  return(FALSE)
+}
+
+# Main function to find Hamiltonian path
+hamiltonianPath <- function(graph) {
+  n <- nrow(graph)
+
+  for (start in 1:n) {
+    path <- rep(-1, n)
+    path[1] <- start
+
+    if (hamiltonianUtil(graph, path, 2)) {
+      cat("Hamiltonian Path found:\n")
+      print(path)
+      return(TRUE)
+    }
+  }
+
+  cat("No Hamiltonian Path found.\n")
+  return(FALSE)
+}
+
+# Example usage and test
+cat("=== Hamiltonian Path (Backtracking) ===\n")
+
+graph <- matrix(c(
+  0, 1, 1, 0,
+  1, 0, 1, 1,
+  1, 1, 0, 1,
+  0, 1, 1, 0
+), nrow = 4, byrow = TRUE)
+
+cat("Adjacency matrix:\n")
+print(graph)
+
+cat("\nSearching for Hamiltonian Path...\n")
+hamiltonianPath(graph)

mathematics/fast_fourier_transform.r

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+# Fast Fourier Transform (Cooley-Tukey recursive implementation)
+#
+# This implementation accepts a numeric or complex vector and returns
+# its discrete Fourier transform as a complex vector. If the input length
+# is not a power of two, the vector is zero-padded to the next power of two.
+#
+# Usage:
+#   source('mathematics/fast_fourier_transform.r')
+#   x <- c(0,1,2,3)
+#   fft_result <- fft_recursive(x)
+#   print(fft_result)
+
+next_power_of_two <- function(n) {
+  if (n <= 0) return(1)
+  p <- 1
+  while (p < n) p <- p * 2
+  p
+}
+
+fft_recursive <- function(x) {
+  # Ensure input is complex
+  x <- as.complex(x)
+  N <- length(x)
+
+  # Pad to next power of two if necessary
+  M <- next_power_of_two(N)
+  if (M != N) {
+    x <- c(x, rep(0+0i, M - N))
+    N <- M
+  }
+
+  if (N == 1) return(x)
+
+  even <- fft_recursive(x[seq(1, N, by = 2)])
+  odd  <- fft_recursive(x[seq(2, N, by = 2)])
+
+  factor <- exp(-2i * pi * (0:(N/2 - 1)) / N)
+  T <- factor * odd
+
+  c(even + T, even - T)
+}
+
+# Example usage when run directly with Rscript
+if (identical(Sys.getenv("R_SCRIPT_NAME"), "") && interactive()) {
+  # Running in interactive R session - show sample
+  x <- c(0, 1, 2, 3)
+  cat("Input:\n")
+  print(x)
+  cat("FFT result:\n")
+  print(fft_recursive(x))
+}
+
+# When running via Rscript, users can call: Rscript -e "source('R/mathematics/fast_fourier_transform.r'); print(fft_recursive(c(0,1,2,3)))"