Use hash map for event handler file descriptor map

runtime/bin/hashmap.cc

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#include "bin/hashmap.h"
+
+#include "bin/builtin.h"
+#include "platform/utils.h"
+
+HashMap::HashMap(MatchFun match,
+                 uint32_t initial_capacity) {
+  match_ = match;
+  Initialize(initial_capacity);
+}
+
+
+HashMap::~HashMap() {
+  free(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.

[Ivan Posva, 2012/01/17 07:57:41]

Personally I find it really hard to follow these algorithms when using p, q and
similar names. Rather than having the explanation in a comment, can you use
descriptive names?

[Søren Gjesse, 2012/01/17 09:28:04]

Changed names p, q and r to candidate, next and start. Updated the comments as
well.

+  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.

[Ivan Posva, 2012/01/17 07:57:41]

This comment does not parse.

[Søren Gjesse, 2012/01/17 09:28:04]

Rephrased.

+  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(dart::Utils::IsPowerOfTwo(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(dart::Utils::IsPowerOfTwo(capacity));
+  map_ = reinterpret_cast<Entry*>(malloc(capacity * sizeof(Entry)));
+  if (map_ == NULL) {
+    // TODO(sgjesse): Handle out of memory.
+    FATAL("Cannot allocate memory for hashmap");
+    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.
+  free(map);
+}