blockswapping

Blockswapping is an in-place data manipulation technique used to rotate or reorder contiguous blocks of elements in arrays or memory buffers. The core idea is to partition the sequence into adjacent blocks and swap their contents in place, repeating with smaller subproblems until the desired arrangement is achieved. This approach avoids extra storage and is widely cited in the context of array rotation.

In the common left-rotation task, the sequence of length n is rotated left by d positions. The algorithm repeatedly partitions the current region into blocks of lengths d and n−d and performs block swaps that move elements toward their final positions. If d equals n−d, a single swap completes the rotation; if one block is shorter, smaller swaps reduce the remaining problem size. The process can be implemented recursively or iteratively and is typically optimized to minimize data movement.

Blockswapping is related to other in-place rotation methods, such as the reversal algorithm and the juggling algorithm, each with different performance trade-offs. The block swap method guarantees linear time and constant extra space, but the constant factors depend on the data layout and implementation.

Applications include in-place rotation of arrays, memory buffers, or strings in constrained environments, cryptographic preprocessing where in-place data transformations are required, and low-level data rearrangements in embedded systems or performance-critical software.

Limitations include potential cache inefficiency for very large data or irregular block sizes, and the need for careful implementation to avoid overwriting data during swaps. See also: array rotation, in-place algorithms, reversal algorithm, juggling algorithm.