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117 lines
5.8 KiB
C++
117 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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/** Gorilla column codec implementation.
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*
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* Based on Gorilla paper: http://www.vldb.org/pvldb/vol8/p1816-teller.pdf
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*
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* This codec is best used against monotonic floating sequences, like CPU usage percentage
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* or any other gauge.
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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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* Loop over remaining items and calculate xor_diff:
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* xor_diff = a[i] ^ a[i - 1] (e.g. 00000011'10110100)
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* Write it in compact binary form with `BitWriter`
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* if xor_diff == 0:
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* write 1 bit: 0
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* else:
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* calculate leading zero bits (lzb)
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* and trailing zero bits (tzb) of xor_diff,
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* compare to lzb and tzb of previous xor_diff
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* (X = sizeof(a[i]) * 8, e.g. X = 16, lzb = 6, tzb = 2)
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* if lzb >= prev_lzb && tzb >= prev_tzb:
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* (e.g. prev_lzb=4, prev_tzb=1)
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* write 2 bit prefix: 0b10
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* write xor_diff >> prev_tzb (X - prev_lzb - prev_tzb bits):0b00111011010
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* (where X = sizeof(a[i]) * 8, e.g. 16)
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* else:
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* write 2 bit prefix: 0b11
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* write 5 bits of lzb: 0b00110
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* write 6 bits of (X - lzb - tzb)=(16-6-2)=8: 0b001000
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* write (X - lzb - tzb) non-zero bits of xor_diff: 0b11101101
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* prev_lzb = lzb
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* prev_tzb = tzb
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*
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* @example sequence of Float32 values [0.1, 0.1, 0.11, 0.2, 0.1] is encoded as:
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*
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* .- 4-byte little endian sequence length: 5 : 0x00000005
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* | .- 4 byte (sizeof(Float32) a[0] as UInt32 : -10 : 0xcdcccc3d
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* | | .- 4 encoded xor diffs (see below)
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* v_______________ v______________ v__________________________________________________
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* \x05\x00\x00\x00\xcd\xcc\xcc\x3d\x6a\x5a\xd8\xb6\x3c\xcd\x75\xb1\x6c\x77\x00\x00\x00
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*
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* 4 binary encoded xor diffs (\x6a\x5a\xd8\xb6\x3c\xcd\x75\xb1\x6c\x77\x00\x00\x00):
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*
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* ...........................................
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* a[i-1] = 00111101110011001100110011001101
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* a[i] = 00111101110011001100110011001101
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* xor_diff = 00000000000000000000000000000000
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* .- 1-bit prefix : 0b0
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* |
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* | ...........................................
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* | a[i-1] = 00111101110011001100110011001101
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* ! a[i] = 00111101111000010100011110101110
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* | xor_diff = 00000000001011011000101101100011
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* | lzb = 10
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* | tzb = 0
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* |.- 2-bit prefix : 0b11
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* || .- lzb (10) : 0b1010
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* || | .- data length (32-10-0): 22 : 0b010110
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* || | | .- data : 0b1011011000101101100011
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* || | | |
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* || | | | ...........................................
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* || | | | a[i-1] = 00111101111000010100011110101110
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* || | | | a[i] = 00111110010011001100110011001101
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* || | | | xor_diff = 00000011101011011000101101100011
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* || | | | .- 2-bit prefix : 0b11
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* || | | | | .- lzb = 6 : 0b00110
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* || | | | | | .- data length = (32 - 6) = 26 : 0b011010
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* || | | | | | | .- data : 0b11101011011000101101100011
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* || | | | | | | |
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* || | | | | | | | ...........................................
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* || | | | | | | | a[i-1] = 00111110010011001100110011001101
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* || | | | | | | | a[i] = 00111101110011001100110011001101
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* || | | | | | | | xor_diff = 00000011100000000000000000000000
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* || | | | | | | | .- 2-bit prefix : 0b10
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* || | | | | | | | | .- data : 0b11100000000000000000000000
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* VV_v____ v_____v________________________V_v_____v______v____________________________V_v_____________________________
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* 01101010 01011010 11011000 10110110 00111100 11001101 01110101 10110001 01101100 01110111 00000000 00000000 00000000
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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 CompressionCodecGorilla : public ICompressionCodec
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{
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public:
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CompressionCodecGorilla(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 registerCodecGorilla(CompressionCodecFactory & factory);
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}
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