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MINOR: [C++] Fix the wrong comment in #45104 #45111

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Dec 26, 2024
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MINOR: [C++] Fix the wrong comment in #45104 #45111

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9 changes: 7 additions & 2 deletions cpp/src/arrow/compute/key_map_internal.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -281,13 +281,18 @@ void SwissTable::early_filter_imp(const int num_keys, const uint32_t* hashes,
// When we reach this limit, we need to break processing of any further rows and resize.
//
uint64_t SwissTable::num_groups_for_resize() const {
// Resize small hash tables when 50% full (up to 32KB).
// Resize large hash tables when 75% full.
// Consider N = 9 (aka 2 ^ 9 = 512 blocks) as small.
// When N = 9, a slot id takes N + 3 = 12 bits, rounded up to 16 bits. This is also the
// number of bits needed for a key id. Since each slot stores a status byte and a key
// id, then a slot takes 1 byte + 16 bits = 3 bytes. Therefore a block of 8 slots takes
// 24 bytes. The threshold of a small hash table ends up being 24 bytes * 512 = 12 KB.
constexpr int log_blocks_small_ = 9;
uint64_t num_slots = 1ULL << (log_blocks_ + 3);
if (log_blocks_ <= log_blocks_small_) {
// Resize small hash tables when 50% full.
return num_slots / 2;
} else {
// Resize large hash tables when 75% full.
return num_slots * 3 / 4;
}
}
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