table.h

//===-- Resizable Monotonic HashTable ---------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_LIBC_SRC___SUPPORT_HASHTABLE_TABLE_H
#define LLVM_LIBC_SRC___SUPPORT_HASHTABLE_TABLE_H

#include "hdr/types/ENTRY.h"
#include "src/__support/CPP/bit.h" // bit_ceil
#include "src/__support/CPP/new.h"
#include "src/__support/HashTable/bitmask.h"
#include "src/__support/hash.h"
#include "src/__support/macros/attributes.h"
#include "src/__support/macros/config.h"
#include "src/__support/macros/optimization.h"
#include "src/__support/memory_size.h"
#include "src/string/memory_utils/inline_strcmp.h"
#include "src/string/string_utils.h"
#include <stddef.h>
#include <stdint.h>

namespace LIBC_NAMESPACE_DECL {
namespace internal {

LIBC_INLINE uint8_t secondary_hash(uint64_t hash) {
  // top 7 bits of the hash.
  return static_cast<uint8_t>(hash >> 57);
}

// Probe sequence based on triangular numbers, which is guaranteed (since our
// table size is a power of two) to visit every group of elements exactly once.
//
// A triangular probe has us jump by 1 more group every time. So first we
// jump by 1 group (meaning we just continue our linear scan), then 2 groups
// (skipping over 1 group), then 3 groups (skipping over 2 groups), and so on.
//
// If we set sizeof(Group) to be one unit:
//               T[k] = sum {1 + 2 + ... + k} = k * (k + 1) / 2
// It is provable that T[k] mod 2^m generates a permutation of
//                0, 1, 2, 3, ..., 2^m - 2, 2^m - 1
// Detailed proof is available at:
// https://fgiesen.wordpress.com/2015/02/22/triangular-numbers-mod-2n/
struct ProbeSequence {
  size_t position;
  size_t stride;
  size_t entries_mask;

  LIBC_INLINE size_t next() {
    position += stride;
    position &= entries_mask;
    stride += sizeof(Group);
    return position;
  }
};

// The number of entries is at least group width: we do not
// need to do the fixup when we set the control bytes.
// The number of entries is at least 8: we don't have to worry
// about special sizes when check the fullness of the table.
LIBC_INLINE size_t capacity_to_entries(size_t cap) {
  if (8 >= sizeof(Group) && cap < 8)
    return 8;
  if (16 >= sizeof(Group) && cap < 15)
    return 16;
  if (cap < sizeof(Group))
    cap = sizeof(Group);
  // overflow is always checked in allocate()
  return cpp::bit_ceil(cap * 8 / 7);
}

// The heap memory layout for N buckets HashTable is as follows:
//
//             =======================
//             |   N * Entry         |
//             ======================= <- align boundary
//             |   Header            |
//             ======================= <- align boundary (for fast resize)
//             |   (N + 1) * Byte    |
//             =======================
//
// The trailing group part is to make sure we can always load
// a whole group of control bytes.

struct HashTable {
  HashState state;
  size_t entries_mask;    // number of buckets - 1
  size_t available_slots; // less than capacity
private:
  // How many entries are there in the table.
  LIBC_INLINE size_t num_of_entries() const { return entries_mask + 1; }

  // How many entries can we store in the table before resizing.
  LIBC_INLINE size_t full_capacity() const { return num_of_entries() / 8 * 7; }

  // The alignment of the whole memory area is the maximum of the alignment
  // among the following types:
  // - HashTable
  // - ENTRY
  // - Group
  LIBC_INLINE constexpr static size_t table_alignment() {
    size_t left_align = alignof(HashTable) > alignof(ENTRY) ? alignof(HashTable)
                                                            : alignof(ENTRY);
    return left_align > alignof(Group) ? left_align : alignof(Group);
  }

  LIBC_INLINE bool is_full() const { return available_slots == 0; }

  LIBC_INLINE size_t offset_from_entries() const {
    size_t entries_size = num_of_entries() * sizeof(ENTRY);
    return entries_size +
           SafeMemSize::offset_to(entries_size, table_alignment());
  }

  LIBC_INLINE constexpr static size_t offset_to_groups() {
    size_t header_size = sizeof(HashTable);
    return header_size + SafeMemSize::offset_to(header_size, table_alignment());
  }

  LIBC_INLINE ENTRY &entry(size_t i) {
    return reinterpret_cast<ENTRY *>(this)[-i - 1];
  }

  LIBC_INLINE const ENTRY &entry(size_t i) const {
    return reinterpret_cast<const ENTRY *>(this)[-i - 1];
  }

  LIBC_INLINE uint8_t &control(size_t i) {
    uint8_t *ptr = reinterpret_cast<uint8_t *>(this) + offset_to_groups();
    return ptr[i];
  }

  LIBC_INLINE const uint8_t &control(size_t i) const {
    const uint8_t *ptr =
        reinterpret_cast<const uint8_t *>(this) + offset_to_groups();
    return ptr[i];
  }

  // We duplicate a group of control bytes to the end. Thus, it is possible that
  // we need to set two control bytes at the same time.
  LIBC_INLINE void set_ctrl(size_t index, uint8_t value) {
    size_t index2 = ((index - sizeof(Group)) & entries_mask) + sizeof(Group);
    control(index) = value;
    control(index2) = value;
  }

  LIBC_INLINE size_t find(const char *key, uint64_t primary) {
    uint8_t secondary = secondary_hash(primary);
    ProbeSequence sequence{static_cast<size_t>(primary), 0, entries_mask};
    while (true) {
      size_t pos = sequence.next();
      Group ctrls = Group::load(&control(pos));
      IteratableBitMask masks = ctrls.match_byte(secondary);
      for (size_t i : masks) {
        size_t index = (pos + i) & entries_mask;
        ENTRY &entry = this->entry(index);
        auto comp = [](char l, char r) -> int { return l - r; };
        if (LIBC_LIKELY(entry.key != nullptr &&
                        inline_strcmp(entry.key, key, comp) == 0))
          return index;
      }
      BitMask available = ctrls.mask_available();
      // Since there is no deletion, the first time we find an available slot
      // it is also ready to be used as an insertion point. Therefore, we also
      // return the first available slot we find. If such entry is empty, the
      // key will be nullptr.
      if (LIBC_LIKELY(available.any_bit_set())) {
        size_t index =
            (pos + available.lowest_set_bit_nonzero()) & entries_mask;
        return index;
      }
    }
  }

  LIBC_INLINE uint64_t oneshot_hash(const char *key) const {
    LIBC_NAMESPACE::internal::HashState hasher = state;
    hasher.update(key, internal::string_length(key));
    return hasher.finish();
  }

  // A fast insertion routine without checking if a key already exists.
  // Nor does the routine check if the table is full.
  // This is only to be used in grow() where we insert all existing entries
  // into a new table. Hence, the requirements are naturally satisfied.
  LIBC_INLINE ENTRY *unsafe_insert(ENTRY item) {
    uint64_t primary = oneshot_hash(item.key);
    uint8_t secondary = secondary_hash(primary);
    ProbeSequence sequence{static_cast<size_t>(primary), 0, entries_mask};
    while (true) {
      size_t pos = sequence.next();
      Group ctrls = Group::load(&control(pos));
      BitMask available = ctrls.mask_available();
      if (available.any_bit_set()) {
        size_t index =
            (pos + available.lowest_set_bit_nonzero()) & entries_mask;
        set_ctrl(index, secondary);
        entry(index).key = item.key;
        entry(index).data = item.data;
        available_slots--;
        return &entry(index);
      }
    }
  }

  LIBC_INLINE HashTable *grow() const {
    size_t hint = full_capacity() + 1;
    HashState state = this->state;
    // migrate to a new random state
    state.update(&hint, sizeof(hint));
    HashTable *new_table = allocate(hint, state.finish());
    // It is safe to call unsafe_insert() because we know that:
    // - the new table has enough capacity to hold all the entries
    // - there is no duplicate key in the old table
    if (new_table != nullptr)
      for (ENTRY e : *this)
        new_table->unsafe_insert(e);
    return new_table;
  }

  LIBC_INLINE static ENTRY *insert(HashTable *&table, ENTRY item,
                                   uint64_t primary) {
    auto index = table->find(item.key, primary);
    auto slot = &table->entry(index);
    // SVr4 and POSIX.1-2001 specify that action is significant only for
    // unsuccessful searches, so that an ENTER should not do anything
    // for a successful search.
    if (slot->key != nullptr)
      return slot;

    // if table of full, we try to grow the table
    if (table->is_full()) {
      HashTable *new_table = table->grow();
      // allocation failed, return nullptr to indicate failure
      if (new_table == nullptr)
        return nullptr;
      // resized sccuessfully: clean up the old table and use the new one
      deallocate(table);
      table = new_table;
      // it is still valid to use the fastpath insertion.
      return table->unsafe_insert(item);
    }

    table->set_ctrl(index, secondary_hash(primary));
    slot->key = item.key;
    slot->data = item.data;
    table->available_slots--;
    return slot;
  }

public:
  LIBC_INLINE static void deallocate(HashTable *table) {
    if (table) {
      void *ptr =
          reinterpret_cast<uint8_t *>(table) - table->offset_from_entries();
      operator delete(ptr, std::align_val_t{table_alignment()});
    }
  }

  LIBC_INLINE static HashTable *allocate(size_t capacity, uint64_t randomness) {
    // check if capacity_to_entries overflows MAX_MEM_SIZE
    if (capacity > size_t{1} << (8 * sizeof(size_t) - 1 - 3))
      return nullptr;
    SafeMemSize entries{capacity_to_entries(capacity)};
    SafeMemSize entries_size = entries * SafeMemSize{sizeof(ENTRY)};
    SafeMemSize align_boundary = entries_size.align_up(table_alignment());
    SafeMemSize ctrl_sizes = entries + SafeMemSize{sizeof(Group)};
    SafeMemSize header_size{offset_to_groups()};
    SafeMemSize total_size =
        (align_boundary + header_size + ctrl_sizes).align_up(table_alignment());
    if (!total_size.valid())
      return nullptr;
    AllocChecker ac;

    void *mem = operator new(total_size, std::align_val_t{table_alignment()},
                             ac);

    HashTable *table = reinterpret_cast<HashTable *>(
        static_cast<uint8_t *>(mem) + align_boundary);
    if (ac) {
      table->entries_mask = entries - 1u;
      table->available_slots = entries / 8 * 7;
      table->state = HashState{randomness};
      __builtin_memset(&table->control(0), 0x80, ctrl_sizes);
      __builtin_memset(mem, 0, table->offset_from_entries());
    }
    return table;
  }

  struct FullTableIterator {
    size_t current_offset;
    size_t remaining;
    IteratableBitMask current_mask;
    const HashTable &table;

    // It is fine to use remaining to represent the iterator:
    // - this comparison only happens with the same table
    // - hashtable will not be mutated during the iteration
    LIBC_INLINE bool operator==(const FullTableIterator &other) const {
      return remaining == other.remaining;
    }
    LIBC_INLINE bool operator!=(const FullTableIterator &other) const {
      return remaining != other.remaining;
    }

    LIBC_INLINE FullTableIterator &operator++() {
      this->ensure_valid_group();
      current_mask.remove_lowest_bit();
      remaining--;
      return *this;
    }
    LIBC_INLINE const ENTRY &operator*() {
      this->ensure_valid_group();
      return table.entry(
          (current_offset + current_mask.lowest_set_bit_nonzero()) &
          table.entries_mask);
    }

  private:
    LIBC_INLINE void ensure_valid_group() {
      while (!current_mask.any_bit_set()) {
        current_offset += sizeof(Group);
        // It is ensured that the load will only happen at aligned boundaries.
        current_mask =
            Group::load_aligned(&table.control(current_offset)).occupied();
      }
    }
  };

  using value_type = ENTRY;
  using iterator = FullTableIterator;
  iterator begin() const {
    return {0, full_capacity() - available_slots,
            Group::load_aligned(&control(0)).occupied(), *this};
  }
  iterator end() const { return {0, 0, {BitMask{0}}, *this}; }

  LIBC_INLINE ENTRY *find(const char *key) {
    uint64_t primary = oneshot_hash(key);
    ENTRY &entry = this->entry(find(key, primary));
    if (entry.key == nullptr)
      return nullptr;
    return &entry;
  }

  LIBC_INLINE static ENTRY *insert(HashTable *&table, ENTRY item) {
    uint64_t primary = table->oneshot_hash(item.key);
    return insert(table, item, primary);
  }
};
} // namespace internal
} // namespace LIBC_NAMESPACE_DECL

#endif // LLVM_LIBC_SRC___SUPPORT_HASHTABLE_TABLE_H