105 lines
4 KiB
C++
105 lines
4 KiB
C++
/** @file DArenaHashMapUtil.hpp
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*
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* @author Roland Conybeare, Jan 2026
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**/
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namespace xo {
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namespace mm {
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/** @class DArenaHashMapUtil
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*
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* @pre
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*
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* control
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*
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* <----------------- control_size(n_slot) ---------------->
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* <-stub-> <----------- n_slot ----------> <group> <-stub->
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* +--------+-------------------------------+-------+--------+
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* | 0xF0 | empty / data / tombstone | wrap | 0xF0 |
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* +--------+-------------------------------+-------+--------+
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* ^ ^
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* | ... | control_[stub+i] <--> slots_[i]
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* slots v v
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* +-------------------------------+
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* | {k,v} pairs |
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* +-------------------------------+
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* <--- n_slot key-value pairs -->
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*
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* sizes:
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* - stub before+after bookends. c_control_stub bytes (16)
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* - group c_group_size. power of 2 (16 bytes)
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* - n_slot hash table slots. power of 2 multiple of c_group_size.
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*
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* control bytes:
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* - 0b1xxxxxxx sentinel bitmask
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* - 0xf0 sentinel for before/after stubs (iterator bookends)
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* - 0xff sentinel for empty slot.
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* - 0xfe sentinel for tombstone
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* - 0b0xxxxxxx high bit clear; remainder hold low 7 bits of hash
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* - wrap duplicate first c_group_size bytes (after front stub)
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* for SIMD convenience
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*
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* @endpre
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**/
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struct DArenaHashMapUtil {
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using size_type = std::size_t;
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using control_type = std::uint8_t;
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/** control: mask for sentinel states **/
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static constexpr uint8_t c_sentinel_mask = 0xF0;
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/** control: sentinel for empty slot **/
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static constexpr uint8_t c_empty_slot = 0xFF;
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/** control: tombstone for deleted slot **/
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static constexpr uint8_t c_tombstone = 0xFE;
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/** control: bookends around control array,
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* for iterator edge support
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**/
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static constexpr uint8_t c_iterator_bookend = 0xF0;
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/** group size **/
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static constexpr size_type c_group_size = 16;
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/** max load factor **/
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static constexpr float c_max_load_factor = 0.875;
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/** Iterator sentinel at begin/end of control array.
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* Load-bearing for bidirectional iterator implementation
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**/
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static constexpr size_type c_control_stub = c_group_size; //c_group_size;
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/** control: true for sentinel values **/
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static constexpr bool is_sentinel(control_type ctrl) {
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return c_sentinel_mask == (ctrl & c_sentinel_mask);
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}
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/** control; true for non-sentinel values **/
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static constexpr bool is_data(control_type ctrl) {
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return 0 == (ctrl & c_sentinel_mask);
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}
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/** control: compute size of control array for swiss hash map with @p n_slot cells **/
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static constexpr size_type control_size(size_type n_slot) {
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return n_slot + c_group_size + 2 * c_control_stub;
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}
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/** find smallest multiple k : k * c_group_size >= n **/
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static size_type lub_group_mult(size_t n) {
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return (n + c_group_size - 1) / c_group_size;
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}
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/** find smallest x such that 2^x >= n. Return {x, 2^x} **/
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static std::pair<size_type, size_type> lub_exp2(size_t n) {
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size_type ngx = 0;
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size_type ng = 1;
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while (ng < n) {
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++ngx;
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ng *= 2;
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}
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return std::make_pair(ngx, ng);;
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}
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};
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} /*namespace mm*/
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} /*namespace xo*/
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/* end DArenaHashMapUtil.hpp */
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