src/hashmap.cc

// Copyright 2008 the V8 project authors. All rights reserved.
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#include "v8.h"

#include "hashmap.h"

namespace v8 {
namespace internal {

Allocator HashMap::DefaultAllocator;


HashMap::HashMap() {
  allocator_ = NULL;
  match_ = NULL;
}


HashMap::HashMap(MatchFun match,
                 Allocator* allocator,
                 uint32_t initial_capacity) {
  allocator_ = allocator;
  match_ = match;
  Initialize(initial_capacity);
}


HashMap::~HashMap() {
  if (allocator_) {
    allocator_->Delete(map_);
  }
}


HashMap::Entry* HashMap::Lookup(void* key, uint32_t hash, bool insert) {
  // Find a matching entry.
  Entry* p = Probe(key, hash);
  if (p->key != NULL) {
    return p;
  }

  // No entry found; insert one if necessary.
  if (insert) {
    p->key = key;
    p->value = NULL;
    p->hash = hash;
    occupancy_++;

    // Grow the map if we reached >= 80% occupancy.
    if (occupancy_ + occupancy_/4 >= capacity_) {
      Resize();
      p = Probe(key, hash);
    }

    return p;
  }

  // No entry found and none inserted.
  return NULL;
}


void HashMap::Remove(void* key, uint32_t hash) {
  // Lookup the entry for the key to remove.
  Entry* p = Probe(key, hash);
  if (p->key == NULL) {
    // Key not found nothing to remove.
    return;
  }

  // To remove an entry we need to ensure that it does not create an empty
  // entry that will cause the search for another entry to stop too soon. If all
  // the entries between the entry to remove and the next empty slot have their
  // initial position inside this interval, clearing the entry to remove will
  // not break the search. If, while searching for the next empty entry, an
  // entry is encountered which does not have its initial position between the
  // entry to remove and the position looked at, then this entry can be moved to
  // the place of the entry to remove without breaking the search for it. The
  // entry made vacant by this move is now the entry to remove and the process
  // starts over.
  // Algorithm from http://en.wikipedia.org/wiki/Open_addressing.

  // This guarantees loop termination as there is at least one empty entry so
  // eventually the removed entry will have an empty entry after it.
  ASSERT(occupancy_ < capacity_);

  // p is the candidate entry to clear. q is used to scan forwards.
  Entry* q = p;  // Start at the entry to remove.
  while (true) {
    // Move q to the next entry.
    q = q + 1;
    if (q == map_end()) {
      q = map_;
    }

    // All entries between p and q have their initial position between p and q
    // and the entry p can be cleared without breaking the search for these
    // entries.
    if (q->key == NULL) {
      break;
    }

    // Find the initial position for the entry at position q.
    Entry* r = map_ + (q->hash & (capacity_ - 1));

    // If the entry at position q has its initial position outside the range
    // between p and q it can be moved forward to position p and will still be
    // found. There is now a new candidate entry for clearing.
    if ((q > p && (r <= p || r > q)) ||
        (q < p && (r <= p && r > q))) {
      *p = *q;
      p = q;
    }
  }

  // Clear the entry which is allowed to en emptied.
  p->key = NULL;
  occupancy_--;
}


void HashMap::Clear() {
  // Mark all entries as empty.
  const Entry* end = map_end();
  for (Entry* p = map_; p < end; p++) {
    p->key = NULL;
  }
  occupancy_ = 0;
}


HashMap::Entry* HashMap::Start() const {
  return Next(map_ - 1);
}


HashMap::Entry* HashMap::Next(Entry* p) const {
  const Entry* end = map_end();
  ASSERT(map_ - 1 <= p && p < end);
  for (p++; p < end; p++) {
    if (p->key != NULL) {
      return p;
    }
  }
  return NULL;
}


HashMap::Entry* HashMap::Probe(void* key, uint32_t hash) {
  ASSERT(key != NULL);

  ASSERT(IsPowerOf2(capacity_));
  Entry* p = map_ + (hash & (capacity_ - 1));
  const Entry* end = map_end();
  ASSERT(map_ <= p && p < end);

  ASSERT(occupancy_ < capacity_);  // Guarantees loop termination.
  while (p->key != NULL && (hash != p->hash || !match_(key, p->key))) {
    p++;
    if (p >= end) {
      p = map_;
    }
  }

  return p;
}


void HashMap::Initialize(uint32_t capacity) {
  ASSERT(IsPowerOf2(capacity));
  map_ = reinterpret_cast<Entry*>(allocator_->New(capacity * sizeof(Entry)));
  if (map_ == NULL) {
    V8::FatalProcessOutOfMemory("HashMap::Initialize");
    return;
  }
  capacity_ = capacity;
  Clear();
}


void HashMap::Resize() {
  Entry* map = map_;
  uint32_t n = occupancy_;

  // Allocate larger map.
  Initialize(capacity_ * 2);

  // Rehash all current entries.
  for (Entry* p = map; n > 0; p++) {
    if (p->key != NULL) {
      Lookup(p->key, p->hash, true)->value = p->value;
      n--;
    }
  }

  // Delete old map.
  allocator_->Delete(map);
}


} }  // namespace v8::internal