ClickHouse/src/Functions/randomStringUTF8.cpp

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#include <Columns/ColumnString.h>
#include <DataTypes/DataTypeString.h>
#include <Functions/FunctionFactory.h>
#include <Functions/FunctionHelpers.h>
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#include <Functions/IFunction.h>
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#include <pcg_random.hpp>
#include <Common/UTF8Helpers.h>
#include <Common/randomSeed.h>
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#include <base/defines.h>
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namespace DB
{
namespace ErrorCodes
{
extern const int ILLEGAL_TYPE_OF_ARGUMENT;
extern const int TOO_LARGE_STRING_SIZE;
}
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namespace
{
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/* Generate string with a UTF-8 encoded text.
* Take a single argument - length of result string in Unicode code points.
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* ATTENTION: Method generate only assignable code points (excluded 4-13 planes).
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* See https://en.wikipedia.org/wiki/Plane_(Unicode) */
class FunctionRandomStringUTF8 : public IFunction
{
public:
static constexpr auto name = "randomStringUTF8";
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static FunctionPtr create(ContextPtr) { return std::make_shared<FunctionRandomStringUTF8>(); }
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String getName() const override { return name; }
bool isVariadic() const override { return false; }
size_t getNumberOfArguments() const override { return 1; }
DataTypePtr getReturnTypeImpl(const DataTypes & arguments) const override
{
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if (!isNumber(*arguments[0]))
throw Exception(ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT, "First argument of function {} must have numeric type", getName());
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return std::make_shared<DataTypeString>();
}
bool isDeterministic() const override { return false; }
bool isDeterministicInScopeOfQuery() const override { return false; }
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bool isSuitableForShortCircuitArgumentsExecution(const DataTypesWithConstInfo & /*arguments*/) const override { return false; }
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ColumnPtr executeImpl(const ColumnsWithTypeAndName & arguments, const DataTypePtr &, size_t input_rows_count) const override
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{
auto col_to = ColumnString::create();
ColumnString::Chars & data_to = col_to->getChars();
ColumnString::Offsets & offsets_to = col_to->getOffsets();
if (input_rows_count == 0)
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return col_to;
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offsets_to.resize(input_rows_count);
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const IColumn & length_column = *arguments[0].column;
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size_t summary_utf8_len = 0;
for (size_t row_num = 0; row_num < input_rows_count; ++row_num)
{
size_t utf8_len = length_column.getUInt(row_num);
summary_utf8_len += utf8_len;
}
/* As we generate only assigned planes, the mathematical expectation of the number of bytes
* per generated code point ~= 3.85. So, reserving for coefficient 4 will not be an overhead
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*/
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if (summary_utf8_len > (1 << 29))
throw Exception(ErrorCodes::TOO_LARGE_STRING_SIZE, "Too large string size in function {}", getName());
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size_t size_in_bytes_with_margin = summary_utf8_len * 4 + input_rows_count;
data_to.resize(size_in_bytes_with_margin);
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pcg64_fast rng(randomSeed()); // TODO It is inefficient. We should use SIMD PRNG instead.
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const auto generate_code_point = [](UInt32 rand) -> UInt32 {
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/// We want to generate number in [0x0, 0x70000) and shift it if need
/// Generate highest byte in [0, 6]
/// https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
UInt32 code_point = (rand >> 16) * 7u;
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code_point &= ~0xFFFF;
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code_point |= rand & 0xFFFF; // and other bytes obtaining in a simple way
if (code_point >= 0x40000)
{
code_point += 0xa0000; // shift if it is in 14-16 plane
return code_point;
}
if (0xD7FF < code_point && code_point < 0xE000) // this range will not be valid in isValidUTF8
{
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/// The distribution will be slightly non-uniform but we don't care.
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return 0u;
}
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return code_point;
};
IColumn::Offset offset = 0;
for (size_t row_num = 0; row_num < input_rows_count; ++row_num)
{
size_t utf8_len = length_column.getUInt(row_num);
auto * pos = data_to.data() + offset;
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size_t last_writen_bytes = 0;
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size_t i = 0;
for (; i < utf8_len; i += 2)
{
UInt64 rand = rng();
UInt32 code_point1 = generate_code_point(static_cast<UInt32>(rand));
UInt32 code_point2 = generate_code_point(static_cast<UInt32>(rand >> 32u));
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/// We have padding in column buffers that we can overwrite.
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size_t length1 = UTF8::convertCodePointToUTF8(code_point1, pos, sizeof(int));
assert(length1 <= 4);
pos += length1;
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size_t length2 = UTF8::convertCodePointToUTF8(code_point2, pos, sizeof(int));
assert(length2 <= 4);
last_writen_bytes = length2;
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pos += last_writen_bytes;
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}
offset = pos - data_to.data() + 1;
if (i > utf8_len)
{
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offset -= last_writen_bytes;
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}
offsets_to[row_num] = offset;
}
/// Put zero bytes in between.
auto * pos = data_to.data();
for (size_t row_num = 0; row_num < input_rows_count; ++row_num)
pos[offsets_to[row_num] - 1] = 0;
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return col_to;
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}
};
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}
REGISTER_FUNCTION(RandomStringUTF8)
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{
factory.registerFunction<FunctionRandomStringUTF8>();
}
}