text: #include <Functions/IFunction.h>
  #include <Functions/FunctionFactory.h>
  #include <Functions/FunctionHelpers.h>
  #include <Columns/ColumnString.h>
  #include <DataTypes/DataTypeString.h>
  #include <pcg_random.hpp>
  #include <Common/randomSeed.h>
  
  
  namespace DB
  {
  
  namespace ErrorCodes
  {
      extern const int NUMBER_OF_ARGUMENTS_DOESNT_MATCH;
      extern const int ILLEGAL_TYPE_OF_ARGUMENT;
      extern const int TOO_LARGE_STRING_SIZE;
  }
  
  namespace
  {
  
  /** Generate random string of specified length with printable ASCII characters, almost uniformly distributed.
    * First argument is length, other optional arguments are ignored and used to prevent common subexpression elimination to get different values.
    */
  class FunctionRandomPrintableASCII : public IFunction
  {
  public:
      static constexpr auto name = "randomPrintableASCII";
      static FunctionPtr create(ContextConstPtr) { return std::make_shared<FunctionRandomPrintableASCII>(); }
  
      String getName() const override
      {
          return name;
      }
  
      bool isVariadic() const override { return true; }
      size_t getNumberOfArguments() const override { return 0; }
  
      DataTypePtr getReturnTypeImpl(const DataTypes & arguments) const override
      {
          if (arguments.empty())
              throw Exception("Function " + getName() + " requires at least one argument: the size of resulting string",
                  ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH);
  
          if (arguments.size() > 2)
              throw Exception("Function " + getName() + " requires at most two arguments: the size of resulting string and optional disambiguation tag",
                  ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH);
  
          const IDataType & length_type = *arguments[0];
          if (!isNumber(length_type))
              throw Exception("First argument of function " + getName() + " must have numeric type", ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
  
          return std::make_shared<DataTypeString>();
      }
  
      bool isDeterministic() const override { return false; }
      bool isDeterministicInScopeOfQuery() const override { return false; }
  
      ColumnPtr executeImpl(const ColumnsWithTypeAndName & arguments, const DataTypePtr &, size_t input_rows_count) const override
      {
          auto col_to = ColumnString::create();
          ColumnString::Chars & data_to = col_to->getChars();
          ColumnString::Offsets & offsets_to = col_to->getOffsets();
          offsets_to.resize(input_rows_count);
  
          pcg64_fast rng(randomSeed());
  
          const IColumn & length_column = *arguments[0].column;
  
          IColumn::Offset offset = 0;
          for (size_t row_num = 0; row_num < input_rows_count; ++row_num)
          {
              size_t length = length_column.getUInt(row_num);
              if (length > (1 << 30))
                  throw Exception("Too large string size in function " + getName(), ErrorCodes::TOO_LARGE_STRING_SIZE);
  
              IColumn::Offset next_offset = offset + length + 1;
              data_to.resize(next_offset);
              offsets_to[row_num] = next_offset;
  
              auto * data_to_ptr = data_to.data();    /// avoid assert on array indexing after end
              for (size_t pos = offset, end = offset + length; pos < end; pos += 4)    /// We have padding in column buffers that we can overwrite.
              {
                  UInt64 rand = rng();
  
                  UInt16 rand1 = rand;
                  UInt16 rand2 = rand >> 16;
                  UInt16 rand3 = rand >> 32;
                  UInt16 rand4 = rand >> 48;
  
                  /// Printable characters are from range [32; 126].
                  /// https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
  
                  data_to_ptr[pos + 0] = 32 + ((rand1 * 95) >> 16);
                  data_to_ptr[pos + 1] = 32 + ((rand2 * 95) >> 16);
                  data_to_ptr[pos + 2] = 32 + ((rand3 * 95) >> 16);
                  data_to_ptr[pos + 3] = 32 + ((rand4 * 95) >> 16);
  
                  /// NOTE gcc failed to vectorize this code (aliasing of char?)
                  /// TODO Implement SIMD optimizations from Danila Kutenin.
              }
  
              data_to[offset + length] = 0;
  
              offset = next_offset;
          }
  
          return col_to;
      }
  };
  
  }
  
  void registerFunctionRandomPrintableASCII(FunctionFactory & factory)
  {
      factory.registerFunction<FunctionRandomPrintableASCII>();
  }
  
  }
  