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Since the distances used in this algorithm are small integers (not more than the size of the U set, in fact), there is no good reason to use float arithmetic for them. Use short ints instead: they're smaller, faster, and require no special portability assumptions. Per testing by Greg Stark, which disclosed that the code got into an infinite loop on VAX for lack of IEEE-style float infinities. We don't really care all that much whether Postgres can run on a VAX anymore, but there seems sufficient reason to change this code anyway. In passing, make a few other small adjustments to make the code match usual Postgres coding style a bit better.
This directory contains a general purpose data structures, for use anywhere in the backend: binaryheap.c - a binary heap hyperloglog.c - a streaming cardinality estimator pairingheap.c - a pairing heap rbtree.c - a red-black tree ilist.c - single and double-linked lists. stringinfo.c - an extensible string type Aside from the inherent characteristics of the data structures, there are a few practical differences between the binary heap and the pairing heap. The binary heap is fully allocated at creation, and cannot be expanded beyond the allocated size. The pairing heap on the other hand has no inherent maximum size, but the caller needs to allocate each element being stored in the heap, while the binary heap works with plain Datums or pointers. The linked-lists in ilist.c can be embedded directly into other structs, as opposed to the List interface in nodes/pg_list.h.