src/java.desktop/share/classes/sun/java2d/marlin/MergeSort.java

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package sun.java2d.marlin;

/**
 * MergeSort adapted from (OpenJDK 8) java.util.Array.legacyMergeSort(Object[])
 * to swap two arrays at the same time (x & y)
 * and use external auxiliary storage for temporary arrays
 */
final class MergeSort {

    // insertion sort threshold
    public static final int INSERTION_SORT_THRESHOLD = 14;

    /**
     * Modified merge sort:
     * Input arrays are in both auxX/auxY (sorted: 0 to insertionSortIndex)
     *                     and x/y (unsorted: insertionSortIndex to toIndex)
     * Outputs are stored in x/y arrays
     */
    static void mergeSortNoCopy(final int[] x, final int[] y,
                                final int[] auxX, final int[] auxY,
                                final int toIndex,
                                final int insertionSortIndex)
    {
        if ((toIndex > x.length) || (toIndex > y.length)
                || (toIndex > auxX.length) || (toIndex > auxY.length)) {
            // explicit check to avoid bound checks within hot loops (below):
            throw new ArrayIndexOutOfBoundsException("bad arguments: toIndex="
                                                     + toIndex);
        }

        // sort second part only using merge / insertion sort
        // in auxiliary storage (auxX/auxY)
        mergeSort(x, y, x, auxX, y, auxY, insertionSortIndex, toIndex);

        // final pass to merge both
        // Merge sorted parts (auxX/auxY) into x/y arrays
        if ((insertionSortIndex == 0)
            || (auxX[insertionSortIndex - 1] <= auxX[insertionSortIndex])) {
            // 34 occurences
            // no initial left part or both sublists (auxX, auxY) are sorted:
            // copy back data into (x, y):
            System.arraycopy(auxX, 0, x, 0, toIndex);
            System.arraycopy(auxY, 0, y, 0, toIndex);
            return;
        }

        for (int i = 0, p = 0, q = insertionSortIndex; i < toIndex; i++) {
            if ((q >= toIndex) || ((p < insertionSortIndex)
                                   && (auxX[p] <= auxX[q]))) {
                x[i] = auxX[p];
                y[i] = auxY[p];
                p++;
            } else {
                x[i] = auxX[q];
                y[i] = auxY[q];
                q++;
            }
        }
    }

    /**
     * Src is the source array that starts at index 0
     * Dest is the (possibly larger) array destination with a possible offset
     * low is the index in dest to start sorting
     * high is the end index in dest to end sorting
     */
    private static void mergeSort(final int[] refX, final int[] refY,
                                  final int[] srcX, final int[] dstX,
                                  final int[] srcY, final int[] dstY,
                                  final int low, final int high)
    {
        final int length = high - low;

        /*
         * Tuning parameter: list size at or below which insertion sort
         * will be used in preference to mergesort.
         */
        if (length <= INSERTION_SORT_THRESHOLD) {
            // Insertion sort on smallest arrays
            dstX[low] = refX[low];
            dstY[low] = refY[low];

            for (int i = low + 1, j = low, x, y; i < high; j = i++) {
                x = refX[i];
                y = refY[i];

                while (dstX[j] > x) {
                    // swap element
                    dstX[j + 1] = dstX[j];
                    dstY[j + 1] = dstY[j];
                    if (j-- == low) {
                        break;
                    }
                }
                dstX[j + 1] = x;
                dstY[j + 1] = y;
            }
            return;
        }

        // Recursively sort halves of dest into src

        // note: use signed shift (not >>>) for performance
        // as indices are small enough to exceed Integer.MAX_VALUE
        final int mid = (low + high) >> 1;

        mergeSort(refX, refY, dstX, srcX, dstY, srcY, low, mid);
        mergeSort(refX, refY, dstX, srcX, dstY, srcY, mid, high);

        // If arrays are inverted ie all(A) > all(B) do swap A and B to dst
        if (srcX[high - 1] <= srcX[low]) {
            // 1561 occurences
            final int left = mid - low;
            final int right = high - mid;
            final int off = (left != right) ? 1 : 0;
            // swap parts:
            System.arraycopy(srcX, low, dstX, mid + off, left);
            System.arraycopy(srcX, mid, dstX, low, right);
            System.arraycopy(srcY, low, dstY, mid + off, left);
            System.arraycopy(srcY, mid, dstY, low, right);
            return;
        }

        // If arrays are already sorted, just copy from src to dest.  This is an
        // optimization that results in faster sorts for nearly ordered lists.
        if (srcX[mid - 1] <= srcX[mid]) {
            // 14 occurences
            System.arraycopy(srcX, low, dstX, low, length);
            System.arraycopy(srcY, low, dstY, low, length);
            return;
        }

        // Merge sorted halves (now in src) into dest
        for (int i = low, p = low, q = mid; i < high; i++) {
            if ((q >= high) || ((p < mid) && (srcX[p] <= srcX[q]))) {
                dstX[i] = srcX[p];
                dstY[i] = srcY[p];
                p++;
            } else {
                dstX[i] = srcX[q];
                dstY[i] = srcY[q];
                q++;
            }
        }
    }

    private MergeSort() {
    }
}