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unordered_map,unordered_set: Simplify excess list size computation #415

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Apr 10, 2024
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unordered_map,unordered_set: Simplify excess list size computation #415

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5 changes: 2 additions & 3 deletions src/stdgpu/impl/unordered_base_detail.cuh
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Original file line number Diff line number Diff line change
Expand Up @@ -16,7 +16,6 @@
#ifndef STDGPU_UNORDERED_BASE_DETAIL_H
#define STDGPU_UNORDERED_BASE_DETAIL_H

#include <algorithm>
#include <cmath>

#include <stdgpu/algorithm.h>
Expand All @@ -39,7 +38,7 @@ expected_collisions(const index_t bucket_count, const index_t capacity)
long double k = static_cast<long double>(bucket_count);
long double n = static_cast<long double>(capacity);
// NOLINTNEXTLINE(readability-magic-numbers,cppcoreguidelines-avoid-magic-numbers)
index_t result = static_cast<index_t>(n * (1.0L - std::pow(1.0L - (1.0L / k), n - 1.0L)));
index_t result = static_cast<index_t>(std::ceil(n * (1.0L - std::pow(1.0L - (1.0L / k), n - 1.0L))));

STDGPU_ENSURES(result >= 0);

Expand Down Expand Up @@ -1206,7 +1205,7 @@ unordered_base<Key, Value, KeyFromValue, Hash, KeyEqual, Allocator>::createDevic
// excess count is estimated by the expected collision count:
// - Conservatively lower the amount since entries falling into regular buckets are already included here
// - Increase amount by 1 since insertion expects a non-empty excess list also in case of no collision
index_t excess_count = std::max<index_t>(1, expected_collisions(bucket_count, capacity) * 2 / 3 + 1);
index_t excess_count = expected_collisions(bucket_count, capacity) * 2 / 3 + 1;

index_t total_count = bucket_count + excess_count;

Expand Down