Merge tag 'v0.1' of https://github.com/svpv/qsort

Submodules dou't work with gear(1), so I have to merge this unrelated
repo.  Only qsort.h is kept.

qsort.h

@@ -0,0 +1,186 @@
+/*
+ * Copyright (c) 2013, 2017 Alexey Tourbin
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+/*
+ * This is a traditional Quicksort implementation which mostly follows
+ * [Sedgewick 1978].  Sorting is performed entirely on array indices,
+ * while actual access to the array elements is abstracted out with the
+ * user-provided `LESS` and `SWAP` primitives.
+ *
+ * Synopsis:
+ *	QSORT(N, LESS, SWAP);
+ * where
+ *	N - the number of elements in A[];
+ *	LESS(i, j) - compares A[i] to A[j];
+ *	SWAP(i, j) - exchanges A[i] with A[j].
+ */
+
+#ifndef QSORT_H
+#define QSORT_H
+
+/* Sort 3 elements. */
+#define Q_SORT3(q_a1, q_a2, q_a3, Q_LESS, Q_SWAP) \
+do {					\
+    if (Q_LESS(q_a2, q_a1)) {		\
+	if (Q_LESS(q_a3, q_a2))		\
+	    Q_SWAP(q_a1, q_a3);		\
+	else {				\
+	    Q_SWAP(q_a1, q_a2);		\
+	    if (Q_LESS(q_a3, q_a2))	\
+		Q_SWAP(q_a2, q_a3);	\
+	}				\
+    }					\
+    else if (Q_LESS(q_a3, q_a2)) {	\
+	Q_SWAP(q_a2, q_a3);		\
+	if (Q_LESS(q_a2, q_a1))		\
+	    Q_SWAP(q_a1, q_a2);		\
+    }					\
+} while (0)
+
+/* Partition [q_l,q_r] around a pivot.  After partitioning,
+ * [q_l,q_j] are the elements that are less than or equal to the pivot,
+ * while [q_i,q_r] are the elements greater than or equal to the pivot. */
+#define Q_PARTITION(q_l, q_r, q_i, q_j, Q_UINT, Q_LESS, Q_SWAP)		\
+do {									\
+    /* The middle element, not to be confused with the median. */	\
+    Q_UINT q_m = q_l + ((q_r - q_l) >> 1);				\
+    /* Reorder the second, the middle, and the last items.		\
+     * As [Edelkamp Weiss 2016] explain, using the second element	\
+     * instead of the first one helps avoid bad behaviour for		\
+     * decreasingly sorted arrays.  This method is used in recent	\
+     * versions of gcc's std::sort, see gcc bug 58437#c13, although	\
+     * the details are somewhat different (cf. #c14). */		\
+    Q_SORT3(q_l + 1, q_m, q_r, Q_LESS, Q_SWAP);				\
+    /* Place the median at the beginning. */				\
+    Q_SWAP(q_l, q_m);							\
+    /* Partition [q_l+2, q_r-1] around the median which is in q_l.	\
+     * q_i and q_j are initially off by one, they get decremented	\
+     * in the do-while loops. */					\
+    q_i = q_l + 1; q_j = q_r;						\
+    while (1) {								\
+	do q_i++; while (Q_LESS(q_i, q_l));				\
+	do q_j--; while (Q_LESS(q_l, q_j));				\
+	if (q_i >= q_j) break; /* Sedgewick says "until j < i" */	\
+	Q_SWAP(q_i, q_j);						\
+    }									\
+    /* Compensate for the i==j case. */					\
+    q_i = q_j + 1;							\
+    /* Put the median to its final place. */				\
+    Q_SWAP(q_l, q_j);							\
+    /* The median is not part of the left subfile. */			\
+    q_j--;								\
+} while (0)
+
+/* Insertion sort is applied to small subfiles - this is contrary to
+ * Sedgewick's suggestion to run a separate insertion sort pass after
+ * the partitioning is done.  The reason I don't like a separate pass
+ * is that it triggers extra comparisons, because it can't see that the
+ * medians are already in their final positions and need not be rechecked.
+ * Since I do not assume that comparisons are cheap, I also do not try
+ * to eliminate the (q_j > q_l) boundary check. */
+#define Q_INSERTION_SORT(q_l, q_r, Q_UINT, Q_LESS, Q_SWAP)		\
+do {									\
+    Q_UINT q_i, q_j;							\
+    /* For each item starting with the second... */			\
+    for (q_i = q_l + 1; q_i <= q_r; q_i++)				\
+    /* move it down the array so that the first part is sorted. */	\
+    for (q_j = q_i; q_j > q_l && (Q_LESS(q_j, q_j - 1)); q_j--)		\
+	Q_SWAP(q_j, q_j - 1);						\
+} while (0)
+
+/* When the size of [q_l,q_r], i.e. q_r-q_l+1, is greater than or equal to
+ * Q_THRESH, the algorithm performs recursive partitioning.  When the size
+ * drops below Q_THRESH, the algorithm switches to insertion sort.
+ * The minimum valid value is probably 5 (with 5 items, the second and
+ * the middle items, the middle itself being rounded down, are distinct. */
+#define Q_THRESH 16
+
+/* The main loop. */
+#define Q_LOOP(Q_UINT, Q_N, Q_LESS, Q_SWAP)				\
+do {									\
+    Q_UINT q_l = 0;							\
+    Q_UINT q_r = (Q_N) - 1;						\
+    Q_UINT q_sp = 0; /* the number of frames pushed to the stack */	\
+    struct { Q_UINT q_l, q_r; }						\
+	/* On 32-bit platforms, to sort a "char[3GB+]" array,		\
+	 * it may take full 32 stack frames.  On 64-bit CPUs,		\
+	 * though, the address space is limited to 48 bits.		\
+	 * The usage is further reduced if Q_N has a 32-bit type. */	\
+	q_st[sizeof(Q_UINT) > 4 && sizeof(Q_N) > 4 ? 48 : 32];		\
+    while (1) {								\
+	if (q_r - q_l + 1 >= Q_THRESH) {				\
+	    Q_UINT q_i, q_j;						\
+	    Q_PARTITION(q_l, q_r, q_i, q_j, Q_UINT, Q_LESS, Q_SWAP);	\
+	    /* Now have two subfiles: [q_l,q_j] and [q_i,q_r].		\
+	     * Dealing with them depends on which one is bigger. */	\
+	    if (q_j - q_l >= q_r - q_i)					\
+		Q_SUBFILES(q_l, q_j, q_i, q_r);				\
+	    else							\
+		Q_SUBFILES(q_i, q_r, q_l, q_j);				\
+	}								\
+	else {								\
+	    Q_INSERTION_SORT(q_l, q_r, Q_UINT, Q_LESS, Q_SWAP);		\
+	    /* Pop subfiles from the stack, until it gets empty. */	\
+	    if (q_sp == 0) break;					\
+	    q_sp--;							\
+	    q_l = q_st[q_sp].q_l;					\
+	    q_r = q_st[q_sp].q_r;					\
+	}								\
+    }									\
+} while (0)
+
+/* The missing part: dealing with subfiles.
+ * Assumes that the first subfile is not smaller than the second. */
+#define Q_SUBFILES(q_l1, q_r1, q_l2, q_r2)				\
+do {									\
+    /* If the second subfile is only a single element, it needs		\
+     * no further processing.  The first subfile will be processed	\
+     * on the next iteration (both subfiles cannot be only a single	\
+     * element, due to Q_THRESH). */					\
+    if (q_l2 == q_r2) {							\
+	q_l = q_l1;							\
+	q_r = q_r1;							\
+    }									\
+    else {								\
+	/* Otherwise, both subfiles need processing.			\
+	 * Push the larger subfile onto the stack. */			\
+	q_st[q_sp].q_l = q_l1;						\
+	q_st[q_sp].q_r = q_r1;						\
+	q_sp++;								\
+	/* Process the smaller subfile on the next iteration. */	\
+	q_l = q_l2;							\
+	q_r = q_r2;							\
+    }									\
+} while (0)
+
+/* And now, ladies and gentlemen, may I proudly present to you... */
+#define QSORT(Q_N, Q_LESS, Q_SWAP)					\
+do {									\
+    if ((Q_N) > 1)							\
+	/* We could check sizeof(Q_N) and use "unsigned", but at least	\
+	 * on x86_64, this has the performance penalty of up to 5%. */	\
+	Q_LOOP(unsigned long, Q_N, Q_LESS, Q_SWAP);			\
+} while (0)
+
+#endif
+
+/* ex:set ts=8 sts=4 sw=4 noet: */