Mastering Algorithms with C

Implementation and Analysis of Merge Sort

Merge sort works fundamentally by recursively dividing an unsorted set of elements into single-element divisions and merging the divisions repeatedly until a single set is reproduced. In the implementation presented here, data initially contains the unsorted set of size elements stored in a single block of contiguous storage. Since merging is not performed in place, mgsort allocates additional storage for the merges. Before mgsort returns, the final merged set is copied back into data.

As we have seen, an important part of merge sort is the process of merging two sorted sets into a single sorted one. This task is performed by the function merge (see Example 12.5), which merges the sets defined from position i to j and from j + 1 to k in data into a single sorted one from i to k.

Initially, ipos and jpos point to the beginning of each sorted set. Merging continues as long as there are still elements in at least one of the sets. While this is true, we proceed as follows. If one set has no elements remaining to be merged, we place all elements remaining in the other set into the merged set. Otherwise, we look at which set contains the next element that should be placed in the merged set to keep it properly ordered, place that element in the merged set, and increment ipos or jpos to the next element depending on from which set the element came (see Figure 12.4).

Figure 12.4. Merging two sorted sets

Now we look at how the recursion proceeds in