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120 lines
5.8 KiB
C++
120 lines
5.8 KiB
C++
#pragma once
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#include <Compression/ICompressionCodec.h>
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namespace DB
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{
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/** DoubleDelta column codec implementation.
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*
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* Based on Gorilla paper: http://www.vldb.org/pvldb/vol8/p1816-teller.pdf, which was extended
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* to support 64bit types. The drawback is 1 extra bit for 32-byte wide deltas: 5-bit prefix
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* instead of 4-bit prefix.
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*
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* This codec is best used against monotonic integer sequences with constant (or almost contant)
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* stride, like event timestamp for some monitoring application.
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*
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* Given input sequence a: [a0, a1, ... an]:
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*
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* First, write number of items (sizeof(int32)*8 bits): n
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* Then write first item as is (sizeof(a[0])*8 bits): a[0]
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* Second item is written as delta (sizeof(a[0])*8 bits): a[1] - a[0]
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* Loop over remaining items and calculate double delta:
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* double_delta = a[i] - 2 * a[i - 1] + a[i - 2]
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* Write it in compact binary form with `BitWriter`
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* if double_delta == 0:
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* write 1bit: 0
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* else if -63 < double_delta < 64:
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* write 2 bit prefix: 10
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* write sign bit (1 if signed): x
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* write 7-1 bits of abs(double_delta - 1): xxxxxx
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* else if -255 < double_delta < 256:
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* write 3 bit prefix: 110
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* write sign bit (1 if signed): x
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* write 9-1 bits of abs(double_delta - 1): xxxxxxxx
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* else if -2047 < double_delta < 2048:
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* write 4 bit prefix: 1110
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* write sign bit (1 if signed): x
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* write 12-1 bits of abs(double_delta - 1): xxxxxxxxxxx
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* else if double_delta fits into 32-bit int:
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* write 5 bit prefix: 11110
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* write sign bit (1 if signed): x
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* write 32-1 bits of abs(double_delta - 1): xxxxxxxxxxx...
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* else
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* write 5 bit prefix: 11111
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* write sign bit (1 if signed): x
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* write 64-1 bits of abs(double_delta - 1): xxxxxxxxxxx...
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*
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* @example sequence of UInt8 values [1, 2, 3, 4, 5, 6, 7, 8, 9 10] is encoded as (codec header is ommited):
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*
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* .- 4-byte little-endian sequence length (10 == 0xa)
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* | .- 1 byte (sizeof(UInt8) a[0] : 0x01
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* | | .- 1 byte of delta: a[1] - a[0] = 2 - 1 = 1 : 0x01
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* | | | .- 8 zero bits since double delta for remaining 8 elements was 0 : 0x00
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* v_______________v___v___v___
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* \x0a\x00\x00\x00\x01\x01\x00
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*
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* @example sequence of Int16 values [-10, 10, -20, 20, -40, 40] is encoded as:
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*
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* .- 4-byte little endian sequence length = 6 : 0x00000006
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* | .- 2 bytes (sizeof(Int16) a[0] as UInt16 = -10 : 0xfff6
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* | | .- 2 bytes of delta: a[1] - a[0] = 10 - (-10) = 20 : 0x0014
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* | | | .- 4 encoded double deltas (see below)
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* v_______________ v______ v______ v______________________
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* \x06\x00\x00\x00\xf6\xff\x14\x00\xb8\xe2\x2e\xb1\xe4\x58
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*
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* 4 binary encoded double deltas (\xb8\xe2\x2e\xb1\xe4\x58):
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* double_delta (DD) = -20 - 2 * 10 + (-10) = -50
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* .- 2-bit prefix : 0b10
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* | .- sign-bit : 0b1
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* | |.- abs(DD - 1) = 49 : 0b110001
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* | ||
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* | || DD = 20 - 2 * (-20) + 10 = 70
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* | || .- 3-bit prefix : 0b110
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* | || | .- sign bit : 0b0
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* | || | |.- abs(DD - 1) = 69 : 0b1000101
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* | || | ||
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* | || | || DD = -40 - 2 * 20 + (-20) = -100
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* | || | || .- 3-bit prefix : 0b110
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* | || | || | .- sign-bit : 0b0
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* | || | || | |.- abs(DD - 1) = 99 : 0b1100011
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* | || | || | ||
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* | || | || | || DD = 40 - 2 * (-40) + 20 = 140
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* | || | || | || .- 3-bit prefix : 0b110
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* | || | || | || | .- sign bit : 0b0
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* | || | || | || | |.- abs(DD - 1) = 139 : 0b10001011
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* | || | || | || | ||
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* V_vv______V__vv________V____vv_______V__vv________,- padding bits
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* 10111000 11100010 00101110 10110001 11100100 01011000
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*
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* Please also see unit tests for:
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* * Examples on what output `BitWriter` produces on predefined input.
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* * Compatibility tests solidifying encoded binary output on set of predefined sequences.
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*/
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class CompressionCodecDoubleDelta : public ICompressionCodec
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{
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public:
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CompressionCodecDoubleDelta(UInt8 data_bytes_size_);
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uint8_t getMethodByte() const override;
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String getCodecDesc() const override;
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void useInfoAboutType(DataTypePtr data_type) override;
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protected:
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UInt32 doCompressData(const char * source, UInt32 source_size, char * dest) const override;
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void doDecompressData(const char * source, UInt32 source_size, char * dest, UInt32 uncompressed_size) const override;
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UInt32 getMaxCompressedDataSize(UInt32 uncompressed_size) const override;
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private:
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UInt8 data_bytes_size;
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};
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class CompressionCodecFactory;
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void registerCodecDoubleDelta(CompressionCodecFactory & factory);
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}
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