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xo-arena: comment + bugfix

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This commit is contained in:
Roland Conybeare 2026-01-08 14:39:09 -05:00
commit b5c4ea47f8
1 changed files with 37 additions and 2 deletions
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39
xo-arena/include/xo/arena/DArenaHashMap.hpp
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@ -60,6 +60,41 @@ namespace xo {
};
#endif
/** @class DArenaHashMapUtil
*
* @pre
*
* control
*
* <----------------- control_size(n_slot) ---------------->
* <-stub-> <----------- n_slot ----------> <group> <-stub->
* +--------+-------------------------------+-------+--------+
* | 0xF0 | empty / data / tombstone | wrap | 0xF0 |
* +--------+-------------------------------+-------+--------+
* ^ ^
* | ... | control_[stub+i] <--> slots_[i]
* slots v v
* +-------------------------------+
* | {k,v} pairs |
* +-------------------------------+
* <--- n_slot key-value pairs -->
*
* sizes:
* - stub before+after bookends. c_control_stub bytes (16)
* - group c_group_size. power of 2 (16 bytes)
* - n_slot hash table slots. power of 2 multiple of c_group_size.
*
* control bytes:
* - 0b1xxxxxxx sentinel bitmask
* - 0xf0 sentinel for before/after stubs (iterator bookends)
* - 0xff sentinel for empty slot.
* - 0xfe sentinel for tombstone
* - 0b0xxxxxxx high bit clear; remainder hold low 7 bits of hash
* - wrap duplicate first c_group_size bytes (after front stub)
* for SIMD convenience
*
* @endpre
**/
struct DArenaHashMapUtil {
using size_type = std::size_t;
using control_type = std::uint8_t;
@ -88,7 +123,7 @@ namespace xo {
/** control: true for sentinel values **/
static constexpr bool is_sentinel(control_type ctrl) {
return ctrl & c_sentinel_mask;
return c_sentinel_mask == (ctrl & c_sentinel_mask);
}
/** control; true for non-sentinel values **/
@ -254,7 +289,7 @@ namespace xo {
/* end stub: iterator bookend */
std::fill(this->control_.end() - c_control_stub,
this->control_.end(),
c_empty_slot);
c_iterator_bookend);
this->slots_.resize(n_slot_);
}