mergesort: add macros for typed sort of linked lists

Add the macros DECLARE_LIST_SORT and DEFINE_LIST_SORT for building
type-specific functions for sorting linked lists.  The generated
function expects a typed comparison function.

The programmer provides full type information (no void pointers).  This
allows the compiler to check whether the comparison function matches the
list type.  It can also inline the "next" pointer accessor functions and
even the comparison function to get rid of the calling overhead.

Also provide a DECLARE_LIST_SORT_DEBUG macro that allows executing
custom code whenever the accessor functions are used.  It's intended to
be used by test-mergesort, which counts these operations.

Signed-off-by: René Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

mergesort.h

@@ -14,4 +14,105 @@ void *llist_mergesort(void *list,
 		      void (*set_next_fn)(void *, void *),
 		      int (*compare_fn)(const void *, const void *));
 
+/* Combine two sorted lists.  Take from `list` on equality. */
+#define DEFINE_LIST_MERGE_INTERNAL(name, type)				\
+static type *name##__merge(type *list, type *other,			\
+			   int (*compare_fn)(const type *, const type *))\
+{									\
+	type *result = list, *tail;					\
+	int prefer_list = compare_fn(list, other) <= 0;			\
+									\
+	if (!prefer_list) {						\
+		result = other;						\
+		SWAP(list, other);					\
+	}								\
+	for (;;) {							\
+		do {							\
+			tail = list;					\
+			list = name##__get_next(list);			\
+			if (!list) {					\
+				name##__set_next(tail, other);		\
+				return result;				\
+			}						\
+		} while (compare_fn(list, other) < prefer_list);	\
+		name##__set_next(tail, other);				\
+		prefer_list ^= 1;					\
+		SWAP(list, other);					\
+	}								\
+}
+
+/*
+ * Perform an iterative mergesort using an array of sublists.
+ *
+ * n is the number of items.
+ * ranks[i] is undefined if n & 2^i == 0, and assumed empty.
+ * ranks[i] contains a sublist of length 2^i otherwise.
+ *
+ * The number of bits in a void pointer limits the number of objects
+ * that can be created, and thus the number of array elements necessary
+ * to be able to sort any valid list.
+ *
+ * Adding an item to this array is like incrementing a binary number;
+ * positional values for set bits correspond to sublist lengths.
+ */
+#define DEFINE_LIST_SORT_INTERNAL(scope, name, type)			\
+scope void name(type **listp,						\
+		int (*compare_fn)(const type *, const type *))		\
+{									\
+	type *list = *listp;						\
+	type *ranks[bitsizeof(type *)];					\
+	size_t n = 0;							\
+									\
+	if (!list)							\
+		return;							\
+									\
+	for (;;) {							\
+		int i;							\
+		size_t m;						\
+		type *next = name##__get_next(list);			\
+		if (next)						\
+			name##__set_next(list, NULL);			\
+		for (i = 0, m = n;; i++, m >>= 1) {			\
+			if (m & 1) {					\
+				list = name##__merge(ranks[i], list,	\
+						    compare_fn);	\
+			} else if (next) {				\
+				break;					\
+			} else if (!m) {				\
+				*listp = list;				\
+				return;					\
+			}						\
+		}							\
+		n++;							\
+		ranks[i] = list;					\
+		list = next;						\
+	}								\
+}
+
+#define DECLARE_LIST_SORT(scope, name, type)			\
+scope void name(type **listp,					\
+		int (*compare_fn)(const type *, const type *))
+
+#define DEFINE_LIST_SORT_DEBUG(scope, name, type, next_member,	\
+			       on_get_next, on_set_next)	\
+								\
+static inline type *name##__get_next(const type *elem)		\
+{								\
+	on_get_next;						\
+	return elem->next_member;				\
+}								\
+								\
+static inline void name##__set_next(type *elem, type *next)	\
+{								\
+	on_set_next;						\
+	elem->next_member = next;				\
+}								\
+								\
+DEFINE_LIST_MERGE_INTERNAL(name, type)				\
+DEFINE_LIST_SORT_INTERNAL(scope, name, type)			\
+DECLARE_LIST_SORT(scope, name, type)
+
+#define DEFINE_LIST_SORT(scope, name, type, next_member) \
+DEFINE_LIST_SORT_DEBUG(scope, name, type, next_member, (void)0, (void)0)
+
 #endif