--- /dev/null
+#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 character sets, and generally isn't optimized at all.
+//
+// It encodes about 0.01% denser than the reference encoder (averaged over
+// a real plocate corpus), probably since it has a slower but more precise
+// method for estimating the cost of a PFOR + varbyte block.
+
+#include "turbopfor-common.h"
+
+#include <assert.h>
+#include <limits.h>
+#include <stdint.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) {
+ val = val;
+ } 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 {
+ 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)
+{
+ if (x == 0) {
+ return 0;
+ } else {
+ return sizeof(Docid) * CHAR_BIT - __builtin_clz(x);
+ }
+}
+
+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;
+ }
+ }
+
+ // 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 count up the varbytes and indexes. (This is precise
+ // but very slow.)
+ unsigned cost = 1 + bytes_for_packed_bits(num, test_bit_width);
+ for (unsigned i = 0; i < num && cost < best_cost; ++i) {
+ unsigned val = in[i] >> test_bit_width;
+ if (val == 0) {
+ // Not stored, and then also no index.
+ } else if (val <= 176) {
+ cost += 2;
+ } else if (val <= 16560) {
+ cost += 3;
+ } else if (val <= 540848) {
+ cost += 4;
+ } else if (val <= 16777215) {
+ cost += 5;
+ } else {
+ cost += 6;
+ }
+ }
+ 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)
projects / plocate / blobdiff