Add Bitonic Sort (TheAlgorithms#460)

Author: domix80

DIRECTORY.md

@@ -149,6 +149,7 @@
     * [Ternary Search Min Max Recursive](https://github.com/TheAlgorithms/Rust/blob/master/src/searching/ternary_search_min_max_recursive.rs)
     * [Ternary Search Recursive](https://github.com/TheAlgorithms/Rust/blob/master/src/searching/ternary_search_recursive.rs)
   * Sorting
+    * [Bitonic Sort](https://github.com/TheAlgorithms/Rust/blob/master/src/sorting/bitonic_sort.rs)
     * [Bogo Sort](https://github.com/TheAlgorithms/Rust/blob/master/src/sorting/bogo_sort.rs)
     * [Bubble Sort](https://github.com/TheAlgorithms/Rust/blob/master/src/sorting/bubble_sort.rs)
     * [Bucket Sort](https://github.com/TheAlgorithms/Rust/blob/master/src/sorting/bucket_sort.rs)

src/sorting/bitonic_sort.rs

@@ -0,0 +1,53 @@
+pub fn comp_and_swap<T: Ord>(array: &mut [T], index1: i32, index2: i32, direction: i32) {
+    if (direction == 1 && array[index1 as usize] > array[index2 as usize])
+        || (direction == 0 && array[index1 as usize] < array[index2 as usize])
+    {
+        array.swap(index1 as usize, index2 as usize);
+    }
+}
+
+pub fn bitonic_merge<T: Ord>(array: &mut [T], low: i32, length: i32, direction: i32) {
+    if length > 1 {
+        let middle = length / 2;
+        for i in low..(low + middle) {
+            comp_and_swap(array, i, i + middle, direction);
+        }
+        bitonic_merge(array, low, middle, direction);
+        bitonic_merge(array, low + middle, middle, direction);
+    }
+}
+
+pub fn bitonic_sort<T: Ord>(array: &mut [T], low: i32, length: i32, direction: i32) {
+    if length > 1 {
+        let middle = length / 2;
+        bitonic_sort(array, low, middle, 1);
+        bitonic_sort(array, low + middle, middle, 0);
+        bitonic_merge(array, low, length, direction);
+    }
+}
+
+//Note that this program works only when size of input is a power of 2.
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn descending() {
+        //descending
+        let mut ve1 = vec![6, 5, 4, 3];
+        bitonic_sort(&mut ve1, 0, 4, 1);
+        for i in 0..ve1.len() - 1 {
+            assert!(ve1[i] <= ve1[i + 1]);
+        }
+    }
+
+    #[test]
+    fn ascending() {
+        //pre-sorted
+        let mut ve2 = vec![1, 2, 3, 4];
+        bitonic_sort(&mut ve2, 0, 4, 0);
+        for i in 0..ve2.len() - 1 {
+            assert!(ve2[i] >= ve2[i + 1]);
+        }
+    }
+}