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simplified hash calculation for varchars in constant time #608

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simplified hash calculation for varchars in constant time #608

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12 changes: 7 additions & 5 deletions src/main/resources/com/nec/cyclone/cpp/cyclone/transfer-definitions.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -28,6 +28,7 @@
#include <string>
#include <type_traits>
#include <vector>
#include <iostream>

inline void set_valid_bit(uint64_t * buffer,
const size_t idx,
Expand Down Expand Up @@ -242,11 +243,12 @@ struct nullable_varchar_vector {
int64_t out = seed;
auto start = offsets[idx];
auto end = offsets[idx] + lengths[idx];
#pragma _NEC vector
#pragma _NEC ivdep
for (int x = start; x < end; x++) {
out = 31 * out + data[x];
}
auto median = (end+start) / 2;

// simplified hash in constant time:
// sample first, median and last element in varchar
out = 31*(31*(31*seed+data[start])+data[median])+data[end-1];

return out;
}

Expand Down