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Commit c1f059de authored by Giuseppe Ottaviano's avatar Giuseppe Ottaviano Committed by Sara Golemon
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Implement previousValue on EliasFanoReader 

Summary: It is often useful to retrieve the value preceding the current value
in an Elias-Fano iterator, for example when the list represents
adjacent ranges. This diff implements a new method `previousValue` in
`EliasFanoReader`.

It also adds a new `kUnchecked` boolean template argument to
`EliasFanoReader` which skips bounds checking.

Reviewed By: @philippv

Differential Revision: D2155049
parent 31c71948
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Inline Side-by-side
Showing with 78 additions and 23 deletions
folly/experimental/EliasFanoCoding.h
View file @ c1f059de
......@@ -24,26 +24,26 @@
#ifndef FOLLY_EXPERIMENTAL_ELIAS_FANO_CODING_H
#define FOLLY_EXPERIMENTAL_ELIAS_FANO_CODING_H
#ifndef __GNUC__
#error EliasFanoCoding.h requires GCC
#endif
#if !FOLLY_X64
#error EliasFanoCoding.h requires x86_64
#endif
#include <cstdlib>
#include <limits>
#include <type_traits>
#include <boost/noncopyable.hpp>
#include <glog/logging.h>
#include <folly/Bits.h>
#include <folly/CpuId.h>
#include <folly/Likely.h>
#include <folly/Portability.h>
#include <folly/Range.h>
#include <folly/experimental/Select64.h>
#ifndef __GNUC__
#error EliasFanoCoding.h requires GCC
#endif
#if !FOLLY_X64
#error EliasFanoCoding.h requires x86_64
#endif
#if __BYTE_ORDER__ != __ORDER_LITTLE_ENDIAN__
#error EliasFanoCoding.h requires little endianness
#endif
......@@ -118,9 +118,9 @@ struct EliasFanoEncoderV2 {
// Requires: input range (begin, end) is sorted (encoding
// crashes if it's not).
// WARNING: encode() mallocates lower, upper, skipPointers
// and forwardPointers. As EliasFanoCompressedList has
// no ownership of them, you need to call free() explicitly.
// WARNING: encode() mallocates EliasFanoCompressedList::data. As
// EliasFanoCompressedList has no ownership of it, you need to call
// free() explicitly.
template <class RandomAccessIterator>
static EliasFanoCompressedList encode(RandomAccessIterator begin,
RandomAccessIterator end) {
......@@ -142,6 +142,7 @@ struct EliasFanoEncoderV2 {
forwardPointers_(reinterpret_cast<SkipValueType*>(
result.forwardPointers)),
result_(result) {
memset(result.data.data(), 0, result.data.size());
}
EliasFanoEncoderV2(size_t size, ValueType upperBound)
......@@ -308,11 +309,11 @@ struct EliasFanoEncoderV2<Value,
uint8_t* buf = nullptr;
// WARNING: Current read/write logic assumes that the 7 bytes
// following the last byte of lower and upper sequences are
// readable (stored value doesn't matter and won't be changed),
// so we allocate additional 7B, but do not include them in size
// readable (stored value doesn't matter and won't be changed), so
// we allocate additional 7 bytes, but do not include them in size
// of returned value.
if (size > 0) {
buf = static_cast<uint8_t*>(calloc(bytes() + 7, 1));
buf = static_cast<uint8_t*>(malloc(bytes() + 7));
}
folly::MutableByteRange bufRange(buf, bytes());
return openList(bufRange);
......@@ -348,6 +349,10 @@ struct Default {
DCHECK_GT(value, 0);
return __builtin_ctzll(value);
}
static inline int clz(uint64_t value) {
DCHECK_GT(value, 0);
return __builtin_clzll(value);
}
static inline uint64_t blsr(uint64_t value) {
return value & (value - 1);
}
......@@ -522,6 +527,24 @@ class UpperBitsReader {
return skipToNext(v);
}
ValueType previousValue() const {
DCHECK_NE(position(), -1);
DCHECK_GT(position(), 0);
size_t outer = outer_;
block_t block = folly::loadUnaligned<block_t>(start_ + outer);
block &= (block_t(1) << inner_) - 1;
while (UNLIKELY(block == 0)) {
DCHECK_GE(outer, sizeof(block_t));
outer -= sizeof(block_t);
block = folly::loadUnaligned<block_t>(start_ + outer);
}
auto inner = 8 * sizeof(block_t) - 1 - Instructions::clz(block);
return static_cast<ValueType>(8 * outer + inner - (position_ - 1));
}
void setDone(size_t endPos) {
position_ = endPos;
}
......@@ -538,7 +561,7 @@ class UpperBitsReader {
block_ &= ~((block_t(1) << (dest % 8)) - 1);
}
typedef unsigned long long block_t;
typedef uint64_t block_t;
const unsigned char* const forwardPointers_;
const unsigned char* const skipPointers_;
const unsigned char* const start_;
......@@ -551,9 +574,13 @@ class UpperBitsReader {
} // namespace detail
// If kUnchecked = true the caller must guarantee that all the
// operations return valid elements, i.e., they would never return
// false if checked.
template <class Encoder,
class Instructions = instructions::Default>
class EliasFanoReader : private boost::noncopyable {
class Instructions = instructions::Default,
bool kUnchecked = false>
class EliasFanoReader {
public:
typedef Encoder EncoderType;
typedef typename Encoder::ValueType ValueType;
......@@ -567,7 +594,9 @@ class EliasFanoReader : private boost::noncopyable {
DCHECK(Instructions::supported());
// To avoid extra branching during skipTo() while reading
// upper sequence we need to know the last element.
if (UNLIKELY(list.size == 0)) {
// If kUnchecked == true, we do not check that skipTo() is called
// within the bounds, so we can avoid initializing lastValue_.
if (kUnchecked || UNLIKELY(list.size == 0)) {
lastValue_ = 0;
return;
}
......@@ -584,7 +613,7 @@ class EliasFanoReader : private boost::noncopyable {
}
bool next() {
if (UNLIKELY(position() + 1 >= size_)) {
if (!kUnchecked && UNLIKELY(position() + 1 >= size_)) {
return setDone();
}
upper_.next();
......@@ -596,7 +625,7 @@ class EliasFanoReader : private boost::noncopyable {
bool skip(size_t n) {
CHECK_GT(n, 0);
if (LIKELY(position() + n < size_)) {
if (kUnchecked || LIKELY(position() + n < size_)) {
if (LIKELY(n < kLinearScanThreshold)) {
for (size_t i = 0; i < n; ++i) upper_.next();
} else {
......@@ -614,7 +643,7 @@ class EliasFanoReader : private boost::noncopyable {
DCHECK_GE(value, value_);
if (value <= value_) {
return true;
} else if (value > lastValue_) {
} else if (!kUnchecked && value > lastValue_) {
return setDone();
}
......@@ -649,7 +678,7 @@ class EliasFanoReader : private boost::noncopyable {
if (value <= 0) {
reset();
return true;
} else if (value > lastValue_) {
} else if (!kUnchecked && value > lastValue_) {
return setDone();
}
......@@ -658,6 +687,13 @@ class EliasFanoReader : private boost::noncopyable {
return true;
}
ValueType previousValue() const {
DCHECK_GT(position(), 0);
DCHECK_LT(position(), size());
return readLowerPart(upper_.position() - 1) |
(upper_.previousValue() << numLowerBits_);
}
size_t size() const { return size_; }
size_t position() const { return upper_.position(); }
......
folly/experimental/test/CodingTestUtils.h
View file @ c1f059de
......@@ -73,6 +73,21 @@ inline std::vector<uint32_t> loadList(const std::string& filename) {
return result;
}
// Test previousValue only if Reader has it.
template <class... Args>
void maybeTestPreviousValue(Args&&...) { }
// Make all the arguments template because if the types are not exact,
// the above overload will be picked (for example i could be size_t or
// ssize_t).
template <class Vector, class Reader, class Index>
auto maybeTestPreviousValue(const Vector& data, Reader& reader, Index i)
-> decltype(reader.previousValue(), void()) {
if (i != 0) {
EXPECT_EQ(reader.previousValue(), data[i - 1]);
}
}
template <class Reader, class List>
void testNext(const std::vector<uint32_t>& data, const List& list) {
Reader reader(list);
......@@ -81,6 +96,7 @@ void testNext(const std::vector<uint32_t>& data, const List& list) {
EXPECT_TRUE(reader.next());
EXPECT_EQ(reader.value(), data[i]);
EXPECT_EQ(reader.position(), i);
maybeTestPreviousValue(data, reader, i);
}
EXPECT_FALSE(reader.next());
EXPECT_EQ(reader.value(), std::numeric_limits<uint32_t>::max());
......@@ -97,6 +113,7 @@ void testSkip(const std::vector<uint32_t>& data, const List& list,
EXPECT_TRUE(reader.skip(skipStep));
EXPECT_EQ(reader.value(), data[i]);
EXPECT_EQ(reader.position(), i);
maybeTestPreviousValue(data, reader, i);
}
EXPECT_FALSE(reader.skip(skipStep));
EXPECT_EQ(reader.value(), std::numeric_limits<uint32_t>::max());
......@@ -134,6 +151,7 @@ void testSkipTo(const std::vector<uint32_t>& data, const List& list,
EXPECT_TRUE(reader.skipTo(value));
EXPECT_EQ(reader.value(), *it);
value = reader.value() + delta;
maybeTestPreviousValue(data, reader, std::distance(data.begin(), it));
}
EXPECT_EQ(reader.value(), std::numeric_limits<uint32_t>::max());
EXPECT_EQ(reader.position(), reader.size());
......@@ -177,6 +195,7 @@ void testJump(const std::vector<uint32_t>& data, const List& list) {
EXPECT_TRUE(reader.jump(i + 1));
EXPECT_EQ(reader.value(), data[i]);
EXPECT_EQ(reader.position(), i);
maybeTestPreviousValue(data, reader, i);
}
EXPECT_FALSE(reader.jump(data.size() + 1));
EXPECT_EQ(reader.value(), std::numeric_limits<uint32_t>::max());
......
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