/*
 * Copyright 2015 Facebook, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/**
 * @author Philip Pronin (philipp@fb.com)
 *
 * Based on the paper by Sebastiano Vigna,
 * "Quasi-succinct indices" (arxiv:1206.4300).
 */

#ifndef FOLLY_EXPERIMENTAL_ELIAS_FANO_CODING_H
#define FOLLY_EXPERIMENTAL_ELIAS_FANO_CODING_H

#include <cstdlib>
#include <limits>
#include <type_traits>

#include <folly/Bits.h>
#include <folly/Likely.h>
#include <folly/Portability.h>
#include <folly/Range.h>
#include <folly/experimental/Instructions.h>
#include <folly/experimental/Select64.h>
#include <glog/logging.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

namespace folly { namespace compression {

template <class Pointer>
struct EliasFanoCompressedListBase {
  EliasFanoCompressedListBase() = default;

  template <class OtherPointer>
  EliasFanoCompressedListBase(
    const EliasFanoCompressedListBase<OtherPointer>& other)
      : size(other.size),
        numLowerBits(other.numLowerBits),
        data(other.data),
        skipPointers(reinterpret_cast<Pointer>(other.skipPointers)),
        forwardPointers(reinterpret_cast<Pointer>(other.forwardPointers)),
        lower(reinterpret_cast<Pointer>(other.lower)),
        upper(reinterpret_cast<Pointer>(other.upper)) {
  }

  void free() {
    ::free(const_cast<unsigned char*>(data.data()));
  }

  size_t upperSize() const {
    return data.end() - upper;
  }

  size_t size = 0;
  uint8_t numLowerBits = 0;

  // WARNING: EliasFanoCompressedList has no ownership of data. The 7
  // bytes following the last byte should be readable.
  folly::Range<Pointer> data;

  Pointer skipPointers = nullptr;
  Pointer forwardPointers = nullptr;
  Pointer lower = nullptr;
  Pointer upper = nullptr;
};

typedef EliasFanoCompressedListBase<const uint8_t*> EliasFanoCompressedList;
typedef EliasFanoCompressedListBase<uint8_t*> MutableEliasFanoCompressedList;

template <class Value,
          class SkipValue = size_t,
          size_t kSkipQuantum = 0,     // 0 = disabled
          size_t kForwardQuantum = 0>  // 0 = disabled
struct EliasFanoEncoderV2 {
  static_assert(std::is_integral<Value>::value &&
                std::is_unsigned<Value>::value,
                "Value should be unsigned integral");

  typedef EliasFanoCompressedList CompressedList;

  typedef Value ValueType;
  typedef SkipValue SkipValueType;
  struct Layout;

  static constexpr size_t skipQuantum = kSkipQuantum;
  static constexpr size_t forwardQuantum = kForwardQuantum;

  static uint8_t defaultNumLowerBits(size_t upperBound, size_t size) {
    if (size == 0 || upperBound < size) {
      return 0;
    }
    // floor(log(upperBound / size));
    return folly::findLastSet(upperBound / size) - 1;
  }

  // Requires: input range (begin, end) is sorted (encoding
  // crashes if it's not).
  // 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) {
    if (begin == end) {
      return EliasFanoCompressedList();
    }
    EliasFanoEncoderV2 encoder(end - begin, *(end - 1));
    for (; begin != end; ++begin) {
      encoder.add(*begin);
    }
    return encoder.finish();
  }

  explicit EliasFanoEncoderV2(const MutableEliasFanoCompressedList& result)
      : lower_(result.lower),
        upper_(result.upper),
        skipPointers_(reinterpret_cast<SkipValueType*>(
                        result.skipPointers)),
        forwardPointers_(reinterpret_cast<SkipValueType*>(
                           result.forwardPointers)),
        result_(result) {
    memset(result.data.data(), 0, result.data.size());
  }

  EliasFanoEncoderV2(size_t size, ValueType upperBound)
      : EliasFanoEncoderV2(
            Layout::fromUpperBoundAndSize(upperBound, size).allocList()) {
  }

  void add(ValueType value) {
    CHECK_GE(value, lastValue_);

    const auto numLowerBits = result_.numLowerBits;
    const ValueType upperBits = value >> numLowerBits;

    // Upper sequence consists of upperBits 0-bits and (size_ + 1) 1-bits.
    const size_t pos = upperBits + size_;
    upper_[pos / 8] |= 1U << (pos % 8);
    // Append numLowerBits bits to lower sequence.
    if (numLowerBits != 0) {
      const ValueType lowerBits = value & ((ValueType(1) << numLowerBits) - 1);
      writeBits56(lower_, size_ * numLowerBits, numLowerBits, lowerBits);
    }

    /* static */ if (skipQuantum != 0) {
      while ((skipPointersSize_ + 1) * skipQuantum <= upperBits) {
        // Store the number of preceding 1-bits.
        skipPointers_[skipPointersSize_++] = size_;
      }
    }

    /* static */ if (forwardQuantum != 0) {
      if ((size_ + 1) % forwardQuantum == 0) {
        const auto pos = size_ / forwardQuantum;
        // Store the number of preceding 0-bits.
        forwardPointers_[pos] = upperBits;
      }
    }

    lastValue_ = value;
    ++size_;
  }

  const EliasFanoCompressedList& finish() const {
    CHECK_EQ(size_, result_.size);
    return result_;
  }

 private:
  // Writes value (with len up to 56 bits) to data starting at pos-th bit.
  static void writeBits56(unsigned char* data, size_t pos,
                          uint8_t len, uint64_t value) {
    DCHECK_LE(uint32_t(len), 56);
    DCHECK_EQ(0, value & ~((uint64_t(1) << len) - 1));
    unsigned char* const ptr = data + (pos / 8);
    uint64_t ptrv = folly::loadUnaligned<uint64_t>(ptr);
    ptrv |= value << (pos % 8);
    folly::storeUnaligned<uint64_t>(ptr, ptrv);
  }

  unsigned char* lower_ = nullptr;
  unsigned char* upper_ = nullptr;
  SkipValueType* skipPointers_ = nullptr;
  SkipValueType* forwardPointers_ = nullptr;

  ValueType lastValue_ = 0;
  size_t size_ = 0;
  size_t skipPointersSize_ = 0;

  EliasFanoCompressedList result_;
};

template <class Value,
          class SkipValue,
          size_t kSkipQuantum,
          size_t kForwardQuantum>
struct EliasFanoEncoderV2<Value,
                          SkipValue,
                          kSkipQuantum,
                          kForwardQuantum>::Layout {
  static Layout fromUpperBoundAndSize(size_t upperBound, size_t size) {
    // numLowerBits can be at most 56 because of detail::writeBits56.
    const uint8_t numLowerBits = std::min(defaultNumLowerBits(upperBound,
                                                              size),
                                          uint8_t(56));
    // *** Upper bits.
    // Upper bits are stored using unary delta encoding.
    // For example, (3 5 5 9) will be encoded as 1000011001000_2.
    const size_t upperSizeBits =
      (upperBound >> numLowerBits) +  // Number of 0-bits to be stored.
      size;                           // 1-bits.
    const size_t upper = (upperSizeBits + 7) / 8;

    // *** Validity checks.
    // Shift by numLowerBits must be valid.
    CHECK_LT(numLowerBits, 8 * sizeof(Value));
    CHECK_LT(size, std::numeric_limits<SkipValueType>::max());
    CHECK_LT(upperBound >> numLowerBits,
             std::numeric_limits<SkipValueType>::max());

    return fromInternalSizes(numLowerBits, upper, size);
  }

  static Layout fromInternalSizes(uint8_t numLowerBits,
                                  size_t upper,
                                  size_t size) {
    Layout layout;
    layout.size = size;
    layout.numLowerBits = numLowerBits;

    layout.lower = (numLowerBits * size + 7) / 8;
    layout.upper = upper;

    // *** Skip pointers.
    // Store (1-indexed) position of every skipQuantum-th
    // 0-bit in upper bits sequence.
    /* static */ if (skipQuantum != 0) {
      // 8 * upper is used here instead of upperSizeBits, as that is
      // more serialization-friendly way (upperSizeBits doesn't need
      // to be known by this function, unlike upper).

      // '?: 1' is a workaround for false 'division by zero'
      // compile-time error.
      size_t numSkipPointers = (8 * upper - size) / (skipQuantum ?: 1);
      layout.skipPointers = numSkipPointers * sizeof(SkipValueType);
    }

    // *** Forward pointers.
    // Store (1-indexed) position of every forwardQuantum-th
    // 1-bit in upper bits sequence.
    /* static */ if (forwardQuantum != 0) {
      size_t numForwardPointers = size / (forwardQuantum ?: 1);
      layout.forwardPointers = numForwardPointers * sizeof(SkipValueType);
    }

    return layout;
  }

  size_t bytes() const {
    return lower + upper + skipPointers + forwardPointers;
  }

  template <typename Range>
  EliasFanoCompressedListBase<typename Range::iterator>
  openList(Range& buf) const {
    EliasFanoCompressedListBase<typename Range::iterator> result;
    result.size = size;
    result.numLowerBits = numLowerBits;
    result.data = buf.subpiece(0, bytes());

    auto advance = [&] (size_t n) {
      auto begin = buf.data();
      buf.advance(n);
      return begin;
    };

    result.skipPointers = advance(skipPointers);
    result.forwardPointers = advance(forwardPointers);
    result.lower = advance(lower);
    result.upper = advance(upper);

    return result;
  }

  MutableEliasFanoCompressedList allocList() const {
    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 7 bytes, but do not include them in size
    // of returned value.
    if (size > 0) {
      buf = static_cast<uint8_t*>(malloc(bytes() + 7));
    }
    folly::MutableByteRange bufRange(buf, bytes());
    return openList(bufRange);
  }

  size_t size = 0;
  uint8_t numLowerBits = 0;

  // Sizes in bytes.
  size_t lower = 0;
  size_t upper = 0;
  size_t skipPointers = 0;
  size_t forwardPointers = 0;
};

namespace detail {

template <class Encoder, class Instructions>
class UpperBitsReader {
  typedef typename Encoder::SkipValueType SkipValueType;
 public:
  typedef typename Encoder::ValueType ValueType;

  explicit UpperBitsReader(const EliasFanoCompressedList& list)
    : forwardPointers_(list.forwardPointers),
      skipPointers_(list.skipPointers),
      start_(list.upper) {
    reset();
  }

  void reset() {
    block_ = start_ != nullptr ? folly::loadUnaligned<block_t>(start_) : 0;
    outer_ = 0;
    inner_ = -1;
    position_ = -1;
    value_ = 0;
  }

  size_t position() const { return position_; }
  ValueType value() const { return value_; }

  ValueType next() {
    // Skip to the first non-zero block.
    while (block_ == 0) {
      outer_ += sizeof(block_t);
      block_ = folly::loadUnaligned<block_t>(start_ + outer_);
    }

    ++position_;
    inner_ = Instructions::ctz(block_);
    block_ = Instructions::blsr(block_);

    return setValue();
  }

  ValueType skip(size_t n) {
    DCHECK_GT(n, 0);

    position_ += n;  // n 1-bits will be read.

    // Use forward pointer.
    if (Encoder::forwardQuantum > 0 && n > Encoder::forwardQuantum) {
      // Workaround to avoid 'division by zero' compile-time error.
      constexpr size_t q = Encoder::forwardQuantum ?: 1;

      const size_t steps = position_ / q;
      const size_t dest =
        folly::loadUnaligned<SkipValueType>(
            forwardPointers_ + (steps - 1) * sizeof(SkipValueType));

      reposition(dest + steps * q);
      n = position_ + 1 - steps * q;  // n is > 0.
      // Correct inner_ will be set at the end.
    }

    size_t cnt;
    // Find necessary block.
    while ((cnt = Instructions::popcount(block_)) < n) {
      n -= cnt;
      outer_ += sizeof(block_t);
      block_ = folly::loadUnaligned<block_t>(start_ + outer_);
    }

    // Skip to the n-th one in the block.
    DCHECK_GT(n, 0);
    inner_ = select64<Instructions>(block_, n - 1);
    block_ &= (block_t(-1) << inner_) << 1;

    return setValue();
  }

  // Skip to the first element that is >= v and located *after* the current
  // one (so even if current value equals v, position will be increased by 1).
  ValueType skipToNext(ValueType v) {
    DCHECK_GE(v, value_);

    // Use skip pointer.
    if (Encoder::skipQuantum > 0 && v >= value_ + Encoder::skipQuantum) {
      // Workaround to avoid 'division by zero' compile-time error.
      constexpr size_t q = Encoder::skipQuantum ?: 1;

      const size_t steps = v / q;
      const size_t dest =
        folly::loadUnaligned<SkipValueType>(
            skipPointers_ + (steps - 1) * sizeof(SkipValueType));

      reposition(dest + q * steps);
      position_ = dest - 1;

      // Correct inner_ and value_ will be set during the next()
      // call at the end.

      // NOTE: Corresponding block of lower bits sequence may be
      // prefetched here (via __builtin_prefetch), but experiments
      // didn't show any significant improvements.
    }

    // Skip by blocks.
    size_t cnt;
    size_t skip = v - (8 * outer_ - position_ - 1);

    constexpr size_t kBitsPerBlock = 8 * sizeof(block_t);
    while ((cnt = Instructions::popcount(~block_)) < skip) {
      skip -= cnt;
      position_ += kBitsPerBlock - cnt;
      outer_ += sizeof(block_t);
      block_ = folly::loadUnaligned<block_t>(start_ + outer_);
    }

    if (LIKELY(skip)) {
      auto inner = select64<Instructions>(~block_, skip - 1);
      position_ += inner - skip + 1;
      block_ &= block_t(-1) << inner;
    }

    next();
    return value_;
  }

  ValueType jump(size_t n) {
    if (Encoder::forwardQuantum == 0 || n <= Encoder::forwardQuantum) {
      reset();
    } else {
      position_ = -1;  // Avoid reading the head, skip() will reposition.
    }
    return skip(n);
  }

  ValueType jumpToNext(ValueType v) {
    if (Encoder::skipQuantum == 0 || v < Encoder::skipQuantum) {
      reset();
    } else {
      value_ = 0;  // Avoid reading the head, skipToNext() will reposition.
    }
    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;
  }

 private:
  ValueType setValue() {
    value_ = static_cast<ValueType>(8 * outer_ + inner_ - position_);
    return value_;
  }

  void reposition(size_t dest) {
    outer_ = dest / 8;
    block_ = folly::loadUnaligned<block_t>(start_ + outer_);
    block_ &= ~((block_t(1) << (dest % 8)) - 1);
  }

  typedef uint64_t block_t;
  const unsigned char* const forwardPointers_;
  const unsigned char* const skipPointers_;
  const unsigned char* const start_;
  block_t block_;
  size_t outer_;  // Outer offset: number of consumed bytes in upper.
  size_t inner_;  // Inner offset: (bit) position in current block.
  size_t position_;  // Index of current value (= #reads - 1).
  ValueType value_;
};

}  // 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,
          bool kUnchecked = false>
class EliasFanoReader {
 public:
  typedef Encoder EncoderType;
  typedef typename Encoder::ValueType ValueType;

  explicit EliasFanoReader(const EliasFanoCompressedList& list)
      : size_(list.size),
        lower_(list.lower),
        upper_(list),
        lowerMask_((ValueType(1) << list.numLowerBits) - 1),
        numLowerBits_(list.numLowerBits) {
    DCHECK(Instructions::supported());
    // To avoid extra branching during skipTo() while reading
    // upper sequence we need to know the last element.
    // 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;
    }
    ValueType lastUpperValue = 8 * list.upperSize() - size_;
    auto it = list.upper + list.upperSize() - 1;
    DCHECK_NE(*it, 0);
    lastUpperValue -= 8 - folly::findLastSet(*it);
    lastValue_ = readLowerPart(size_ - 1) | (lastUpperValue << numLowerBits_);
  }

  void reset() {
    upper_.reset();
    value_ = 0;
  }

  bool next() {
    if (!kUnchecked && UNLIKELY(position() + 1 >= size_)) {
      return setDone();
    }
    upper_.next();
    value_ = readLowerPart(upper_.position()) |
             (upper_.value() << numLowerBits_);
    return true;
  }

  bool skip(size_t n) {
    CHECK_GT(n, 0);

    if (kUnchecked || LIKELY(position() + n < size_)) {
      if (LIKELY(n < kLinearScanThreshold)) {
        for (size_t i = 0; i < n; ++i) upper_.next();
      } else {
        upper_.skip(n);
      }
      value_ = readLowerPart(upper_.position()) |
        (upper_.value() << numLowerBits_);
      return true;
    }

    return setDone();
  }

  bool skipTo(ValueType value) {
    DCHECK_GE(value, value_);
    if (value <= value_) {
      return true;
    } else if (!kUnchecked && value > lastValue_) {
      return setDone();
    }

    size_t upperValue = (value >> numLowerBits_);
    size_t upperSkip = upperValue - upper_.value();
    // The average density of ones in upper bits is 1/2.
    // LIKELY here seems to make things worse, even for small skips.
    if (upperSkip < 2 * kLinearScanThreshold) {
      do {
        upper_.next();
      } while (UNLIKELY(upper_.value() < upperValue));
    } else {
      upper_.skipToNext(upperValue);
    }

    iterateTo(value);
    return true;
  }

  bool jump(size_t n) {
    if (LIKELY(n - 1 < size_)) {  // n > 0 && n <= size_
      value_ = readLowerPart(n - 1) | (upper_.jump(n) << numLowerBits_);
      return true;
    } else if (n == 0) {
      reset();
      return true;
    }
    return setDone();
  }

  bool jumpTo(ValueType value) {
    if (value <= 0) {
      reset();
      return true;
    } else if (!kUnchecked && value > lastValue_) {
      return setDone();
    }

    upper_.jumpToNext(value >> numLowerBits_);
    iterateTo(value);
    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(); }
  ValueType value() const { return value_; }

 private:
  bool setDone() {
    value_ = std::numeric_limits<ValueType>::max();
    upper_.setDone(size_);
    return false;
  }

  ValueType readLowerPart(size_t i) const {
    DCHECK_LT(i, size_);
    const size_t pos = i * numLowerBits_;
    const unsigned char* ptr = lower_ + (pos / 8);
    const uint64_t ptrv = folly::loadUnaligned<uint64_t>(ptr);
    return lowerMask_ & (ptrv >> (pos % 8));
  }

  void iterateTo(ValueType value) {
    while (true) {
      value_ = readLowerPart(upper_.position()) |
        (upper_.value() << numLowerBits_);
      if (LIKELY(value_ >= value)) break;
      upper_.next();
    }
  }

  constexpr static size_t kLinearScanThreshold = 8;

  size_t size_;
  const uint8_t* lower_;
  detail::UpperBitsReader<Encoder, Instructions> upper_;
  const ValueType lowerMask_;
  ValueType value_ = 0;
  ValueType lastValue_;
  uint8_t numLowerBits_;
};

}}  // namespaces

#endif  // FOLLY_EXPERIMENTAL_ELIAS_FANO_CODING_H
