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// The story is at // https://groups.google.com/forum/#!msg/scylladb-dev/sxqTHM9rSDQ/WqwF1AQDAQAJ This is the B+ version which satisfies several specific requirements to be suitable for row-cache usage. 1. Insert/Remove doesn't invalidate iterators 2. Elements should be LSA-compactable 3. Low overhead of data nodes (1 pointer) 4. External less-only comparator 5. As little actions on insert/delete as possible 6. Iterator walks the sorted keys The design, briefly is: There are 3 types of nodes: inner, leaf and data, inner and leaf keep build-in array of N keys and N(+1) nodes. Leaf nodes sit in a doubly linked list. Data nodes live separately from the leaf ones and keep pointers on them. Tree handler keeps pointers on root and left-most and right-most leaves. Nodes do _not_ keep pointers or references on the tree (except 3 of them, see below). changes in v9: - explicitly marked keys/kids indices with type aliases - marked the whole erase/clear stuff noexcept - disposers now accept object pointer instead of reference - clear tree in destructor - added more comments - style/readability review comments fixed Prior changes ** - Add noexcepts where possible - Restrict Less-comparator constraint -- it must be noexcept - Generalized node_id - Packed code for beging()/cbegin() ** - Unsigned indices everywhere - Cosmetics changes ** - Const iterators - C++20 concepts ** - The index_for() implmenetation is templatized the other way to make it possible for AVX key search specialization (further patching) ** - Insertion tries to push kids to siblings before split Before this change insertion into full node resulted into this node being split into two equal parts. This behaviour for random keys stress gives a tree with ~2/3 of nodes half-filled. With this change before splitting the full node try to push one element to each of the siblings (if they exist and not full). This slows the insertion a bit (but it's still way faster than the std::set), but gives 15% less total number of nodes. - Iterator method to reconstruct the data at the given position The helper creates a new data node, emplaces data into it and replaces the iterator's one with it. Needed to keep arrays of data in tree. - Milli-optimize erase() - Return back an iterator that will likely be not re-validated - Do not try to update ancestors separation key for leftmost kid This caused the clear()-like workload work poorly as compared to std:set. In particular the row_cache::invalidate() method does exactly this and this change improves its timing. - Perf test to measure drain speed - Helper call to collect tree counters ** - Fix corner case of iterator.emplace_before() - Clean heterogenous lookup API - Handle exceptions from nodes allocations - Explicitly mark places where the key is copied (for future) - Extend the tree.lower_bound() API to report back whether the bound hit the key or not - Addressed style/cleanness review comments Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>