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plocate/turbopfor-encode.h
Steinar H. Gunderson a286a78f06 Fix a typo.
2021-02-13 01:30:27 +01:00

407 lines
11 KiB
C++

#ifndef _TURBOPFOR_ENCODE_H
#define _TURBOPFOR_ENCODE_H
// Much like turbopfor.h (and shares all of the same caveats), except this is
// for encoding. It is _much_ slower than the reference implementation, but we
// encode only during build, and most time in build is spent in other things
// than encoding posting lists, so it only costs ~5-10% overall. Does not use
// any special instruction sets, and generally isn't optimized at all.
//
// It encodes roughly about as dense as the reference encoder.
#include "turbopfor-common.h"
#include <algorithm>
#include <assert.h>
#ifdef HAS_ENDIAN_H
#include <endian.h>
#endif
#include <limits.h>
#include <stdint.h>
#include <string.h>
template<class Docid>
void write_le(Docid val, void *out)
{
if constexpr (sizeof(Docid) == 8) {
val = htole64(val);
} else if constexpr (sizeof(Docid) == 4) {
val = htole32(val);
} else if constexpr (sizeof(Docid) == 2) {
val = htole16(val);
} else if constexpr (sizeof(Docid) == 1) {
// No change.
} else {
assert(false);
}
memcpy(out, &val, sizeof(val));
}
// Corresponds to read_baseval.
template<class Docid>
unsigned char *write_baseval(Docid in, unsigned char *out)
{
if (in < 128) {
*out = in;
return out + 1;
} else if (in < 0x4000) {
out[0] = (in >> 8) | 0x80;
out[1] = in & 0xff;
return out + 2;
} else if (in < 0x200000) {
out[0] = (in >> 16) | 0xc0;
out[1] = in & 0xff;
out[2] = (in >> 8) & 0xff;
return out + 3;
} else if (in < 0x10000000) {
out[0] = (in >> 24) | 0xe0;
out[1] = (in >> 16) & 0xff;
out[2] = (in >> 8) & 0xff;
out[3] = in & 0xff;
return out + 4;
} else {
assert(false); // Not implemented.
}
}
// Writes a varbyte-encoded exception.
template<class Docid>
unsigned char *write_vb(Docid val, unsigned char *out)
{
if (val <= 176) {
*out++ = val;
return out;
} else if (val <= 16560) {
val -= 177;
*out++ = (val >> 8) + 177;
*out++ = val & 0xff;
return out;
} else if (val <= 540848) {
val -= 16561;
*out = (val >> 16) + 241;
write_le<uint16_t>(val & 0xffff, out + 1);
return out + 3;
} else if (val <= 16777215) {
*out = 249;
write_le<uint32_t>(val, out + 1);
return out + 4;
} else {
*out = 250;
write_le<uint32_t>(val, out + 1);
return out + 5;
}
}
template<class Docid>
inline unsigned num_bits(Docid x)
{
#ifdef __GNUC__
if (x == 0) {
return 0;
} else {
return sizeof(Docid) * CHAR_BIT - __builtin_clz(x);
}
#else
for (int i = sizeof(Docid) * CHAR_BIT; i-- > 0;) {
if (x & (Docid{ 1 } << i)) {
return i;
}
}
return 0;
#endif
}
struct BitWriter {
public:
BitWriter(unsigned char *out, unsigned bits)
: out(out), bits(bits) {}
void write(uint32_t val)
{
cur_val |= val << bits_used;
write_le<uint32_t>(cur_val, out);
bits_used += bits;
cur_val >>= (bits_used / 8) * 8;
out += bits_used / 8;
bits_used %= 8;
}
private:
unsigned char *out;
const unsigned bits;
unsigned bits_used = 0;
unsigned cur_val = 0;
};
template<unsigned NumStreams>
struct InterleavedBitWriter {
public:
InterleavedBitWriter(unsigned char *out, unsigned bits)
: out(out), bits(bits) {}
void write(uint32_t val)
{
cur_val |= uint64_t(val) << bits_used;
if (bits_used + bits >= 32) {
write_le<uint32_t>(cur_val & 0xffffffff, out);
out += Stride;
cur_val >>= 32;
bits_used -= 32; // Underflow, but will be fixed below.
}
write_le<uint32_t>(cur_val, out);
bits_used += bits;
}
private:
static constexpr unsigned Stride = NumStreams * sizeof(uint32_t);
unsigned char *out;
const unsigned bits;
unsigned bits_used = 0;
uint64_t cur_val = 0;
};
// Bitpacks a set of values (making sure the top bits are lopped off).
// If interleaved is set, makes SSE2-compatible interleaving (this is
// only allowed for full blocks).
template<class Docid>
unsigned char *encode_bitmap(const Docid *in, unsigned num, unsigned bit_width, bool interleaved, unsigned char *out)
{
unsigned mask = mask_for_bits(bit_width);
if (interleaved) {
InterleavedBitWriter<4> bs0(out + 0 * sizeof(uint32_t), bit_width);
InterleavedBitWriter<4> bs1(out + 1 * sizeof(uint32_t), bit_width);
InterleavedBitWriter<4> bs2(out + 2 * sizeof(uint32_t), bit_width);
InterleavedBitWriter<4> bs3(out + 3 * sizeof(uint32_t), bit_width);
assert(num % 4 == 0);
for (unsigned i = 0; i < num / 4; ++i) {
bs0.write(in[i * 4 + 0] & mask);
bs1.write(in[i * 4 + 1] & mask);
bs2.write(in[i * 4 + 2] & mask);
bs3.write(in[i * 4 + 3] & mask);
}
} else {
BitWriter bs(out, bit_width);
for (unsigned i = 0; i < num; ++i) {
bs.write(in[i] & mask);
}
}
return out + bytes_for_packed_bits(num, bit_width);
}
// See decode_for() for the format.
template<class Docid>
unsigned char *encode_for(const Docid *in, unsigned num, unsigned bit_width, bool interleaved, unsigned char *out)
{
return encode_bitmap(in, num, bit_width, interleaved, out);
}
// See decode_pfor_bitmap() for the format.
template<class Docid>
unsigned char *encode_pfor_bitmap(const Docid *in, unsigned num, unsigned bit_width, unsigned exception_bit_width, bool interleaved, unsigned char *out)
{
*out++ = exception_bit_width;
// Bitmap of exceptions.
{
BitWriter bs(out, 1);
for (unsigned i = 0; i < num; ++i) {
bs.write((in[i] >> bit_width) != 0);
}
out += bytes_for_packed_bits(num, 1);
}
// Exceptions.
{
BitWriter bs(out, exception_bit_width);
unsigned num_exceptions = 0;
for (unsigned i = 0; i < num; ++i) {
if ((in[i] >> bit_width) != 0) {
bs.write(in[i] >> bit_width);
++num_exceptions;
}
}
out += bytes_for_packed_bits(num_exceptions, exception_bit_width);
}
// Base values.
out = encode_bitmap(in, num, bit_width, interleaved, out);
return out;
}
// See decode_pfor_vb() for the format.
template<class Docid>
unsigned char *encode_pfor_vb(const Docid *in, unsigned num, unsigned bit_width, bool interleaved, unsigned char *out)
{
unsigned num_exceptions = 0;
for (unsigned i = 0; i < num; ++i) {
if ((in[i] >> bit_width) != 0) {
++num_exceptions;
}
}
*out++ = num_exceptions;
// Base values.
out = encode_bitmap(in, num, bit_width, interleaved, out);
// Exceptions.
for (unsigned i = 0; i < num; ++i) {
unsigned val = in[i] >> bit_width;
if (val != 0) {
out = write_vb(val, out);
}
}
// Exception indexes.
for (unsigned i = 0; i < num; ++i) {
unsigned val = in[i] >> bit_width;
if (val != 0) {
*out++ = i;
}
}
return out;
}
// Find out which block type would be the smallest for the given data.
template<class Docid>
BlockType decide_block_type(const Docid *in, unsigned num, unsigned *bit_width, unsigned *exception_bit_width)
{
// Check if the block is constant.
bool constant = true;
for (unsigned i = 1; i < num; ++i) {
if (in[i] != in[0]) {
constant = false;
break;
}
}
if (constant) {
*bit_width = num_bits(in[0]);
return BlockType::CONSTANT;
}
// Build up a histogram of bit sizes.
unsigned histogram[sizeof(Docid) * CHAR_BIT + 1] = { 0 };
unsigned max_bits = 0;
for (unsigned i = 0; i < num; ++i) {
unsigned bits = num_bits(in[i]);
++histogram[bits];
max_bits = std::max(max_bits, bits);
}
// Straight-up FOR.
unsigned best_cost = bytes_for_packed_bits(num, max_bits);
unsigned best_bit_width = max_bits;
// Try PFOR with bitmap exceptions.
const unsigned bitmap_cost = bytes_for_packed_bits(num, 1);
unsigned num_exceptions = 0;
for (unsigned exception_bit_width = 1; exception_bit_width <= max_bits; ++exception_bit_width) {
unsigned test_bit_width = max_bits - exception_bit_width;
num_exceptions += histogram[test_bit_width + 1];
// 1 byte for signaling exception bit width, then the bitmap,
// then the base values, then the exceptions.
unsigned cost = 1 + bitmap_cost + bytes_for_packed_bits(num, test_bit_width) +
bytes_for_packed_bits(num_exceptions, exception_bit_width);
if (cost < best_cost) {
best_cost = cost;
best_bit_width = test_bit_width;
}
}
// Make the histogram cumulative; histogram[n] now means the the number of
// elements with n bits or more.
for (unsigned i = max_bits; i > 0; --i) {
histogram[i - 1] += histogram[i];
}
// Try PFOR with varbyte exceptions.
bool best_is_varbyte = false;
for (unsigned test_bit_width = 0; test_bit_width < max_bits; ++test_bit_width) {
// 1 byte for signaling number of exceptions, plus the base values,
// and then we estimate up the varbytes and indexes using histogram
// indexes. This isn't exact, but it only helps ~0.1% on the total
// plocate.db size.
unsigned cost = 1 + bytes_for_packed_bits(num, test_bit_width);
if (cost >= best_cost) {
break;
}
if (test_bit_width + 1 <= max_bits) {
cost += 2 * histogram[test_bit_width + 1]; // > 0.
if (test_bit_width + 7 <= max_bits) {
cost += histogram[test_bit_width + 7]; // > 176, very roughly.
if (test_bit_width + 14 <= max_bits) {
cost += histogram[test_bit_width + 14]; // > 16560, very roughly.
if (test_bit_width + 19 <= max_bits) {
cost += histogram[test_bit_width + 19]; // > 540848, very roughly.
if (test_bit_width + 24 <= max_bits) {
cost += histogram[test_bit_width + 24]; // > 16777215, very roughly.
}
}
}
}
}
if (cost < best_cost) {
best_cost = cost;
best_bit_width = test_bit_width;
best_is_varbyte = true;
}
}
// TODO: Consider the last-resort option of just raw storage (255).
if (best_is_varbyte) {
*bit_width = best_bit_width;
return BlockType::PFOR_VB;
} else if (best_bit_width == max_bits) {
*bit_width = max_bits;
return BlockType::FOR;
} else {
*bit_width = best_bit_width;
*exception_bit_width = max_bits - best_bit_width;
return BlockType::PFOR_BITMAP;
}
}
// The basic entry point. Takes one block of integers (which already must
// be delta-minus-1-encoded) and packs it into TurboPFor format.
// interleaved corresponds to the interleaved parameter in decode_pfor_delta1()
// or the “128v” infix in the reference code's function names; such formats
// are much faster to decode, so for full blocks, you probably want it.
// The interleaved flag isn't stored anywhere; it's implicit whether you
// want to use it for full blocks or not.
//
// The first value must already be written using write_baseval() (so the delta
// coding starts from the second value). Returns the end of the string.
// May write 4 bytes past the end.
template<unsigned BlockSize, class Docid>
unsigned char *encode_pfor_single_block(const Docid *in, unsigned num, bool interleaved, unsigned char *out)
{
assert(num > 0);
if (interleaved) {
assert(num == BlockSize);
}
unsigned bit_width, exception_bit_width;
BlockType block_type = decide_block_type(in, num, &bit_width, &exception_bit_width);
*out++ = (block_type << 6) | bit_width;
switch (block_type) {
case BlockType::CONSTANT: {
unsigned bit_width = num_bits(in[0]);
write_le<Docid>(in[0], out);
return out + div_round_up(bit_width, 8);
}
case BlockType::FOR:
return encode_for(in, num, bit_width, interleaved, out);
case BlockType::PFOR_BITMAP:
return encode_pfor_bitmap(in, num, bit_width, exception_bit_width, interleaved, out);
case BlockType::PFOR_VB:
return encode_pfor_vb(in, num, bit_width, interleaved, out);
default:
assert(false);
}
}
#endif // !defined(_TURBOPFOR_ENCODE_H)