Add Bitonic Sort Algorithm in R

sorting_algorithms/bitonic_sort.r

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+# Bitonic sort in R:
+
+# This implementation follows the style used across this repository: a single exported
+# function with a commented example at the bottom. Bitonic sort is typically defined
+# for sequences with length as a power of two. For convenience, we handle arbitrary
+# lengths by padding with +Inf (for ascending) or -Inf (for descending) and removing
+# the padding before returning.
+
+# Helper to perform the bitonic compare and swap across a segment
+.bitonic_compare_swap <- function(a, low, cnt, asc = TRUE) {
+  if (cnt <= 1) return(a)
+  k <- cnt %/% 2
+  idx <- seq.int(low, length.out = k)
+  partner <- idx + k
+  if (asc) {
+    swap <- a[idx] > a[partner]
+  } else {
+    swap <- a[idx] < a[partner]
+  }
+  if (any(swap)) {
+    tmp <- a[idx[swap]]
+    a[idx[swap]] <- a[partner[swap]]
+    a[partner[swap]] <- tmp
+  }
+  # Recurse on both halves
+  a <- .bitonic_compare_swap(a, low, k, asc)
+  a <- .bitonic_compare_swap(a, low + k, k, asc)
+  return(a)
+}
+
+# Core recursive builder of a bitonic sequence followed by merge
+.bitonic_sort_core <- function(a, low, cnt, asc = TRUE) {
+  if (cnt <= 1) return(a)
+  k <- cnt %/% 2
+  # Sort first half ascending, second half descending to form bitonic sequence
+  a <- .bitonic_sort_core(a, low, k, TRUE)
+  a <- .bitonic_sort_core(a, low + k, k, FALSE)
+  # Merge whole sequence in desired direction
+  a <- .bitonic_compare_swap(a, low, cnt, asc)
+  return(a)
+}
+
+# Public API: bitonic.sort
+# - elements.vec: numeric or comparable vector
+# - ascending: TRUE for increasing order, FALSE for decreasing
+bitonic.sort <- function(elements.vec, ascending = TRUE) {
+  n <- length(elements.vec)
+  if (n <= 1) return(elements.vec)
+
+  # Determine next power of two
+  next_pow2 <- 1L
+  while (next_pow2 < n) next_pow2 <- next_pow2 * 2L
+
+  # Pad to power of two for algorithmic convenience
+  pad_len <- next_pow2 - n
+  if (pad_len > 0) {
+    pad_val <- if (ascending) Inf else -Inf
+    a <- c(elements.vec, rep(pad_val, pad_len))
+  } else {
+    a <- elements.vec
+  }
+
+  a <- .bitonic_sort_core(a, 1L, length(a), asc = ascending)
+
+  # Trim padding if any
+  if (pad_len > 0) {
+    a <- a[seq_len(n)]
+  }
+  return(a)
+}
+
+# Example:
+# bitonic.sort(c(3, 7, 4, 8, 6, 2, 1, 5))
+# [1] 1 2 3 4 5 6 7 8
+# bitonic.sort(c(3, 7, 4, 8, 6, 2, 1, 5), ascending = FALSE)
+# [1] 8 7 6 5 4 3 2 1