src/Map.js - main - githubmirror/immutable-js

src/Map.js
881 строка23 KB
Julien Deniau
perf(Map): index large hash-collision buckets for faster lookups
26 июн 2026, 00:52
26 июн 2026, 00:52e51d49f
import { Collection, KeyedCollection } from './Collection';
import { hash, hashCollisionKey } from './Hash';
import { Iterator, iteratorDone, iteratorValue } from './Iterator';
import { sortFactory } from './Operations';
import { OrderedMap } from './OrderedMap';
import {
  DELETE,
  MASK,
  MakeRef,
  NOT_SET,
  OwnerID,
  SHIFT,
  SIZE,
  SetRef,
} from './TrieUtils';
import { is } from './is';
import { asImmutable } from './methods/asImmutable';
import { asMutable } from './methods/asMutable';
import { deleteIn } from './methods/deleteIn';
import { merge, mergeWith } from './methods/merge';
import { mergeDeep, mergeDeepWith } from './methods/mergeDeep';
import { mergeDeepIn } from './methods/mergeDeepIn';
import { mergeIn } from './methods/mergeIn';
import { setIn } from './methods/setIn';
import { update } from './methods/update';
import { updateIn } from './methods/updateIn';
import { wasAltered } from './methods/wasAltered';
import { withMutations } from './methods/withMutations';
import { IS_MAP_SYMBOL, isMap } from './predicates/isMap';
import { isOrdered } from './predicates/isOrdered';
import arrCopy from './utils/arrCopy';
import assertNotInfinite from './utils/assertNotInfinite';

export class Map extends KeyedCollection {
  // @pragma Construction

  constructor(value) {
    // eslint-disable-next-line no-constructor-return
    return value === undefined || value === null
      ? emptyMap()
      : isMap(value) && !isOrdered(value)
        ? value
        : emptyMap().withMutations((map) => {
            const iter = KeyedCollection(value);
            assertNotInfinite(iter.size);
            iter.forEach((v, k) => map.set(k, v));
          });
  }

  toString() {
    return this.__toString('Map {', '}');
  }

  // @pragma Access

  get(k, notSetValue) {
    return this._root
      ? this._root.get(0, undefined, k, notSetValue)
      : notSetValue;
  }

  // @pragma Modification

  set(k, v) {
    return updateMap(this, k, v);
  }

  remove(k) {
    return updateMap(this, k, NOT_SET);
  }

  deleteAll(keys) {
    const collection = Collection(keys);

    if (collection.size === 0) {
      return this;
    }

    return this.withMutations((map) => {
      collection.forEach((key) => map.remove(key));
    });
  }

  clear() {
    if (this.size === 0) {
      return this;
    }
    if (this.__ownerID) {
      this.size = 0;
      this._root = null;
      this.__hash = undefined;
      this.__altered = true;
      return this;
    }
    return emptyMap();
  }

  // @pragma Composition

  sort(comparator) {
    // Late binding
    return OrderedMap(sortFactory(this, comparator));
  }

  sortBy(mapper, comparator) {
    // Late binding
    return OrderedMap(sortFactory(this, comparator, mapper));
  }

  map(mapper, context) {
    return this.withMutations((map) => {
      map.forEach((value, key) => {
        map.set(key, mapper.call(context, value, key, this));
      });
    });
  }

  // @pragma Mutability

  __iterator(type, reverse) {
    return new MapIterator(this, type, reverse);
  }

  __iterate(fn, reverse) {
    let iterations = 0;
    // eslint-disable-next-line @typescript-eslint/no-unused-expressions -- TODO enable eslint here
    this._root &&
      this._root.iterate((entry) => {
        iterations++;
        return fn(entry[1], entry[0], this);
      }, reverse);
    return iterations;
  }

  __ensureOwner(ownerID) {
    if (ownerID === this.__ownerID) {
      return this;
    }
    if (!ownerID) {
      if (this.size === 0) {
        return emptyMap();
      }
      this.__ownerID = ownerID;
      this.__altered = false;
      return this;
    }
    return makeMap(this.size, this._root, ownerID, this.__hash);
  }
}

Map.isMap = isMap;

const MapPrototype = Map.prototype;
MapPrototype[IS_MAP_SYMBOL] = true;
MapPrototype[DELETE] = MapPrototype.remove;
MapPrototype.removeAll = MapPrototype.deleteAll;
MapPrototype.setIn = setIn;
MapPrototype.removeIn = MapPrototype.deleteIn = deleteIn;
MapPrototype.update = update;
MapPrototype.updateIn = updateIn;
MapPrototype.merge = MapPrototype.concat = merge;
MapPrototype.mergeWith = mergeWith;
MapPrototype.mergeDeep = mergeDeep;
MapPrototype.mergeDeepWith = mergeDeepWith;
MapPrototype.mergeIn = mergeIn;
MapPrototype.mergeDeepIn = mergeDeepIn;
MapPrototype.withMutations = withMutations;
MapPrototype.wasAltered = wasAltered;
MapPrototype.asImmutable = asImmutable;
MapPrototype['@@transducer/init'] = MapPrototype.asMutable = asMutable;
MapPrototype['@@transducer/step'] = function (result, arr) {
  return result.set(arr[0], arr[1]);
};
MapPrototype['@@transducer/result'] = function (obj) {
  return obj.asImmutable();
};

// #pragma Trie Nodes

class ArrayMapNode {
  constructor(ownerID, entries) {
    this.ownerID = ownerID;
    this.entries = entries;
  }

  get(shift, keyHash, key, notSetValue) {
    const entries = this.entries;
    for (let ii = 0, len = entries.length; ii < len; ii++) {
      if (is(key, entries[ii][0])) {
        return entries[ii][1];
      }
    }
    return notSetValue;
  }

  update(ownerID, shift, keyHash, key, value, didChangeSize, didAlter) {
    const removed = value === NOT_SET;

    const entries = this.entries;
    let idx = 0;
    const len = entries.length;
    for (; idx < len; idx++) {
      if (is(key, entries[idx][0])) {
        break;
      }
    }
    const exists = idx < len;

    if (exists ? entries[idx][1] === value : removed) {
      return this;
    }

    SetRef(didAlter);
    // eslint-disable-next-line @typescript-eslint/no-unused-expressions -- TODO enable eslint here
    (removed || !exists) && SetRef(didChangeSize);

    if (removed && entries.length === 1) {
      return; // undefined
    }

    if (!exists && !removed && entries.length >= MAX_ARRAY_MAP_SIZE) {
      return createNodes(ownerID, entries, key, value);
    }

    const isEditable = ownerID && ownerID === this.ownerID;
    const newEntries = isEditable ? entries : arrCopy(entries);

    if (exists) {
      if (removed) {
        // eslint-disable-next-line @typescript-eslint/no-unused-expressions -- TODO enable eslint here
        idx === len - 1
          ? newEntries.pop()
          : (newEntries[idx] = newEntries.pop());
      } else {
        newEntries[idx] = [key, value];
      }
    } else {
      newEntries.push([key, value]);
    }

    if (isEditable) {
      this.entries = newEntries;
      return this;
    }

    return new ArrayMapNode(ownerID, newEntries);
  }
}

class BitmapIndexedNode {
  constructor(ownerID, bitmap, nodes) {
    this.ownerID = ownerID;
    this.bitmap = bitmap;
    this.nodes = nodes;
  }

  get(shift, keyHash, key, notSetValue) {
    if (keyHash === undefined) {
      keyHash = hash(key);
    }
    const bit = 1 << ((shift === 0 ? keyHash : keyHash >>> shift) & MASK);
    const bitmap = this.bitmap;
    return (bitmap & bit) === 0
      ? notSetValue
      : this.nodes[popCount(bitmap & (bit - 1))].get(
          shift + SHIFT,
          keyHash,
          key,
          notSetValue
        );
  }

  update(ownerID, shift, keyHash, key, value, didChangeSize, didAlter) {
    if (keyHash === undefined) {
      keyHash = hash(key);
    }
    const keyHashFrag = (shift === 0 ? keyHash : keyHash >>> shift) & MASK;
    const bit = 1 << keyHashFrag;
    const bitmap = this.bitmap;
    const exists = (bitmap & bit) !== 0;

    if (!exists && value === NOT_SET) {
      return this;
    }

    const idx = popCount(bitmap & (bit - 1));
    const nodes = this.nodes;
    const node = exists ? nodes[idx] : undefined;
    const newNode = updateNode(
      node,
      ownerID,
      shift + SHIFT,
      keyHash,
      key,
      value,
      didChangeSize,
      didAlter
    );

    if (newNode === node) {
      return this;
    }

    if (!exists && newNode && nodes.length >= MAX_BITMAP_INDEXED_SIZE) {
      return expandNodes(ownerID, nodes, bitmap, keyHashFrag, newNode);
    }

    if (
      exists &&
      !newNode &&
      nodes.length === 2 &&
      isLeafNode(nodes[idx ^ 1])
    ) {
      return nodes[idx ^ 1];
    }

    if (exists && newNode && nodes.length === 1 && isLeafNode(newNode)) {
      return newNode;
    }

    const isEditable = ownerID && ownerID === this.ownerID;
    const newBitmap = exists ? (newNode ? bitmap : bitmap ^ bit) : bitmap | bit;
    const newNodes = exists
      ? newNode
        ? setAt(nodes, idx, newNode, isEditable)
        : spliceOut(nodes, idx, isEditable)
      : spliceIn(nodes, idx, newNode, isEditable);

    if (isEditable) {
      this.bitmap = newBitmap;
      this.nodes = newNodes;
      return this;
    }

    return new BitmapIndexedNode(ownerID, newBitmap, newNodes);
  }
}

class HashArrayMapNode {
  constructor(ownerID, count, nodes) {
    this.ownerID = ownerID;
    this.count = count;
    this.nodes = nodes;
  }

  get(shift, keyHash, key, notSetValue) {
    if (keyHash === undefined) {
      keyHash = hash(key);
    }
    const idx = (shift === 0 ? keyHash : keyHash >>> shift) & MASK;
    const node = this.nodes[idx];
    return node
      ? node.get(shift + SHIFT, keyHash, key, notSetValue)
      : notSetValue;
  }

  update(ownerID, shift, keyHash, key, value, didChangeSize, didAlter) {
    if (keyHash === undefined) {
      keyHash = hash(key);
    }
    const idx = (shift === 0 ? keyHash : keyHash >>> shift) & MASK;
    const removed = value === NOT_SET;
    const nodes = this.nodes;
    const node = nodes[idx];

    if (removed && !node) {
      return this;
    }

    const newNode = updateNode(
      node,
      ownerID,
      shift + SHIFT,
      keyHash,
      key,
      value,
      didChangeSize,
      didAlter
    );
    if (newNode === node) {
      return this;
    }

    let newCount = this.count;
    if (!node) {
      newCount++;
    } else if (!newNode) {
      newCount--;
      if (newCount < MIN_HASH_ARRAY_MAP_SIZE) {
        return packNodes(ownerID, nodes, newCount, idx);
      }
    }

    const isEditable = ownerID && ownerID === this.ownerID;
    const newNodes = setAt(nodes, idx, newNode, isEditable);

    if (isEditable) {
      this.count = newCount;
      this.nodes = newNodes;
      return this;
    }

    return new HashArrayMapNode(ownerID, newCount, newNodes);
  }
}

/**
 * Trie leaf gathering entries whose keys all share the same 32-bit `hash()`.
 * The trie routes by hash, so colliding keys cannot be separated and land here
 * in a flat `entries` array, disambiguated by `is()`.
 *
 * To guard against hash-flooding DoS (CWE-407), large buckets build a secondary
 * index keyed by a per-process seeded hash (`hashCollisionKey`). `is()` still
 * decides equality, so the index can only ever narrow candidates, never lose a key.
 */
class HashCollisionNode {
  constructor(ownerID, keyHash, entries) {
    this.ownerID = ownerID;
    this.keyHash = keyHash;
    this.entries = entries;
    // Lazy `{ [secondaryHash]: number[] }`, built only past
    // MIN_HASH_COLLISION_INDEX_SIZE so small buckets keep their linear path.
    this._index = undefined;
  }

  // Returns the position of `key` in `this.entries`, or -1. Uses the secondary
  // index when present; builds it only when `buildIndex` is true (reads and
  // transient inserts, where the node is reused so the O(n) build amortizes).
  // Persistent inserts already pay an O(n) copy, so a throwaway index is skipped.
  _positionOf(key, buildIndex) {
    const entries = this.entries;
    let index = this._index;
    if (
      index === undefined &&
      buildIndex &&
      entries.length >= MIN_HASH_COLLISION_INDEX_SIZE
    ) {
      index = this._buildIndex();
    }
    if (index !== undefined) {
      const positions = index[hashCollisionKey(key)];
      if (positions !== undefined) {
        for (let jj = 0; jj < positions.length; jj++) {
          const ii = positions[jj];
          if (is(key, entries[ii][0])) {
            return ii;
          }
        }
      }
      return -1;
    }
    for (let ii = 0, len = entries.length; ii < len; ii++) {
      if (is(key, entries[ii][0])) {
        return ii;
      }
    }
    return -1;
  }

  // Builds and memoizes the secondary index. A plain object, not `Map` — which
  // in this module resolves to the *Immutable* Map, not the native one.
  _buildIndex() {
    const index = Object.create(null);
    const entries = this.entries;
    for (let ii = 0, len = entries.length; ii < len; ii++) {
      const secondaryHash = hashCollisionKey(entries[ii][0]);
      const positions = index[secondaryHash];
      if (positions !== undefined) {
        positions.push(ii);
      } else {
        index[secondaryHash] = [ii];
      }
    }
    this._index = index;
    return index;
  }

  get(shift, keyHash, key, notSetValue) {
    const idx = this._positionOf(key, true);
    return idx === -1 ? notSetValue : this.entries[idx][1];
  }

  update(ownerID, shift, keyHash, key, value, didChangeSize, didAlter) {
    if (keyHash === undefined) {
      keyHash = hash(key);
    }

    const removed = value === NOT_SET;

    if (keyHash !== this.keyHash) {
      if (removed) {
        return this;
      }
      SetRef(didAlter);
      SetRef(didChangeSize);
      return mergeIntoNode(this, ownerID, shift, keyHash, [key, value]);
    }

    const entries = this.entries;
    const len = entries.length;
    const isEditable = ownerID && ownerID === this.ownerID;
    const foundIdx = this._positionOf(key, isEditable);
    const idx = foundIdx === -1 ? len : foundIdx;
    const exists = foundIdx !== -1;

    if (exists ? entries[idx][1] === value : removed) {
      return this;
    }

    SetRef(didAlter);
    // eslint-disable-next-line @typescript-eslint/no-unused-expressions -- TODO enable eslint here
    (removed || !exists) && SetRef(didChangeSize);

    if (removed && len === 2) {
      return new ValueNode(ownerID, this.keyHash, entries[idx ^ 1]);
    }

    const newEntries = isEditable ? entries : arrCopy(entries);

    if (exists) {
      if (removed) {
        // eslint-disable-next-line @typescript-eslint/no-unused-expressions -- TODO enable eslint here
        idx === len - 1
          ? newEntries.pop()
          : (newEntries[idx] = newEntries.pop());
        // The swap-pop reshuffles positions; drop the stale index (rebuilt lazily).
        if (isEditable) {
          this._index = undefined;
        }
      } else {
        // Same key, same position: the index stays valid.
        newEntries[idx] = [key, value];
      }
    } else {
      newEntries.push([key, value]);
      // Keep the index in sync on the transient insert path. Persistent inserts
      // return a fresh node below whose index rebuilds lazily, so skip them.
      if (isEditable && this._index !== undefined) {
        const secondaryHash = hashCollisionKey(key);
        const positions = this._index[secondaryHash];
        if (positions !== undefined) {
          positions.push(len);
        } else {
          this._index[secondaryHash] = [len];
        }
      }
    }

    if (isEditable) {
      this.entries = newEntries;
      return this;
    }

    return new HashCollisionNode(ownerID, this.keyHash, newEntries);
  }
}

class ValueNode {
  constructor(ownerID, keyHash, entry) {
    this.ownerID = ownerID;
    this.keyHash = keyHash;
    this.entry = entry;
  }

  get(shift, keyHash, key, notSetValue) {
    return is(key, this.entry[0]) ? this.entry[1] : notSetValue;
  }

  update(ownerID, shift, keyHash, key, value, didChangeSize, didAlter) {
    const removed = value === NOT_SET;
    const keyMatch = is(key, this.entry[0]);
    if (keyMatch ? value === this.entry[1] : removed) {
      return this;
    }

    SetRef(didAlter);

    if (removed) {
      SetRef(didChangeSize);
      return; // undefined
    }

    if (keyMatch) {
      if (ownerID && ownerID === this.ownerID) {
        this.entry[1] = value;
        return this;
      }
      return new ValueNode(ownerID, this.keyHash, [key, value]);
    }

    SetRef(didChangeSize);
    return mergeIntoNode(this, ownerID, shift, hash(key), [key, value]);
  }
}

// #pragma Iterators

ArrayMapNode.prototype.iterate = HashCollisionNode.prototype.iterate =
  function (fn, reverse) {
    const entries = this.entries;
    for (let ii = 0, maxIndex = entries.length - 1; ii <= maxIndex; ii++) {
      if (fn(entries[reverse ? maxIndex - ii : ii]) === false) {
        return false;
      }
    }
  };

BitmapIndexedNode.prototype.iterate = HashArrayMapNode.prototype.iterate =
  function (fn, reverse) {
    const nodes = this.nodes;
    for (let ii = 0, maxIndex = nodes.length - 1; ii <= maxIndex; ii++) {
      const node = nodes[reverse ? maxIndex - ii : ii];
      if (node && node.iterate(fn, reverse) === false) {
        return false;
      }
    }
  };

// eslint-disable-next-line @typescript-eslint/no-unused-vars
ValueNode.prototype.iterate = function (fn, reverse) {
  return fn(this.entry);
};

class MapIterator extends Iterator {
  constructor(map, type, reverse) {
    this._type = type;
    this._reverse = reverse;
    this._stack = map._root && mapIteratorFrame(map._root);
  }

  next() {
    const type = this._type;
    let stack = this._stack;
    while (stack) {
      const node = stack.node;
      const index = stack.index++;
      let maxIndex;
      if (node.entry) {
        if (index === 0) {
          return mapIteratorValue(type, node.entry);
        }
      } else if (node.entries) {
        maxIndex = node.entries.length - 1;
        if (index <= maxIndex) {
          return mapIteratorValue(
            type,
            node.entries[this._reverse ? maxIndex - index : index]
          );
        }
      } else {
        maxIndex = node.nodes.length - 1;
        if (index <= maxIndex) {
          const subNode = node.nodes[this._reverse ? maxIndex - index : index];
          if (subNode) {
            if (subNode.entry) {
              return mapIteratorValue(type, subNode.entry);
            }
            stack = this._stack = mapIteratorFrame(subNode, stack);
          }
          continue;
        }
      }
      stack = this._stack = this._stack.__prev;
    }
    return iteratorDone();
  }
}

function mapIteratorValue(type, entry) {
  return iteratorValue(type, entry[0], entry[1]);
}

function mapIteratorFrame(node, prev) {
  return {
    node: node,
    index: 0,
    __prev: prev,
  };
}

function makeMap(size, root, ownerID, hash) {
  const map = Object.create(MapPrototype);
  map.size = size;
  map._root = root;
  map.__ownerID = ownerID;
  map.__hash = hash;
  map.__altered = false;
  return map;
}

let EMPTY_MAP;
export function emptyMap() {
  return EMPTY_MAP || (EMPTY_MAP = makeMap(0));
}

function updateMap(map, k, v) {
  let newRoot;
  let newSize;
  if (!map._root) {
    if (v === NOT_SET) {
      return map;
    }
    newSize = 1;
    newRoot = new ArrayMapNode(map.__ownerID, [[k, v]]);
  } else {
    const didChangeSize = MakeRef();
    const didAlter = MakeRef();
    newRoot = updateNode(
      map._root,
      map.__ownerID,
      0,
      undefined,
      k,
      v,
      didChangeSize,
      didAlter
    );
    if (!didAlter.value) {
      return map;
    }
    newSize = map.size + (didChangeSize.value ? (v === NOT_SET ? -1 : 1) : 0);
  }
  if (map.__ownerID) {
    map.size = newSize;
    map._root = newRoot;
    map.__hash = undefined;
    map.__altered = true;
    return map;
  }
  return newRoot ? makeMap(newSize, newRoot) : emptyMap();
}

function updateNode(
  node,
  ownerID,
  shift,
  keyHash,
  key,
  value,
  didChangeSize,
  didAlter
) {
  if (!node) {
    if (value === NOT_SET) {
      return node;
    }
    SetRef(didAlter);
    SetRef(didChangeSize);
    return new ValueNode(ownerID, keyHash, [key, value]);
  }
  return node.update(
    ownerID,
    shift,
    keyHash,
    key,
    value,
    didChangeSize,
    didAlter
  );
}

function isLeafNode(node) {
  return (
    node.constructor === ValueNode || node.constructor === HashCollisionNode
  );
}

function mergeIntoNode(node, ownerID, shift, keyHash, entry) {
  if (node.keyHash === keyHash) {
    return new HashCollisionNode(ownerID, keyHash, [node.entry, entry]);
  }

  const idx1 = (shift === 0 ? node.keyHash : node.keyHash >>> shift) & MASK;
  const idx2 = (shift === 0 ? keyHash : keyHash >>> shift) & MASK;

  let newNode;
  const nodes =
    idx1 === idx2
      ? [mergeIntoNode(node, ownerID, shift + SHIFT, keyHash, entry)]
      : ((newNode = new ValueNode(ownerID, keyHash, entry)),
        idx1 < idx2 ? [node, newNode] : [newNode, node]);

  return new BitmapIndexedNode(ownerID, (1 << idx1) | (1 << idx2), nodes);
}

function createNodes(ownerID, entries, key, value) {
  if (!ownerID) {
    ownerID = new OwnerID();
  }
  let node = new ValueNode(ownerID, hash(key), [key, value]);
  for (let ii = 0; ii < entries.length; ii++) {
    const entry = entries[ii];
    node = node.update(ownerID, 0, undefined, entry[0], entry[1]);
  }
  return node;
}

function packNodes(ownerID, nodes, count, excluding) {
  let bitmap = 0;
  let packedII = 0;
  const packedNodes = new Array(count);
  for (let ii = 0, bit = 1, len = nodes.length; ii < len; ii++, bit <<= 1) {
    const node = nodes[ii];
    if (node !== undefined && ii !== excluding) {
      bitmap |= bit;
      packedNodes[packedII++] = node;
    }
  }
  return new BitmapIndexedNode(ownerID, bitmap, packedNodes);
}

function expandNodes(ownerID, nodes, bitmap, including, node) {
  let count = 0;
  const expandedNodes = new Array(SIZE);
  for (let ii = 0; bitmap !== 0; ii++, bitmap >>>= 1) {
    expandedNodes[ii] = bitmap & 1 ? nodes[count++] : undefined;
  }
  expandedNodes[including] = node;
  return new HashArrayMapNode(ownerID, count + 1, expandedNodes);
}

function popCount(x) {
  x -= (x >> 1) & 0x55555555;
  x = (x & 0x33333333) + ((x >> 2) & 0x33333333);
  x = (x + (x >> 4)) & 0x0f0f0f0f;
  x += x >> 8;
  x += x >> 16;
  return x & 0x7f;
}

function setAt(array, idx, val, canEdit) {
  const newArray = canEdit ? array : arrCopy(array);
  newArray[idx] = val;
  return newArray;
}

function spliceIn(array, idx, val, canEdit) {
  const newLen = array.length + 1;
  if (canEdit && idx + 1 === newLen) {
    array[idx] = val;
    return array;
  }
  const newArray = new Array(newLen);
  let after = 0;
  for (let ii = 0; ii < newLen; ii++) {
    if (ii === idx) {
      newArray[ii] = val;
      after = -1;
    } else {
      newArray[ii] = array[ii + after];
    }
  }
  return newArray;
}

function spliceOut(array, idx, canEdit) {
  const newLen = array.length - 1;
  if (canEdit && idx === newLen) {
    array.pop();
    return array;
  }
  const newArray = new Array(newLen);
  let after = 0;
  for (let ii = 0; ii < newLen; ii++) {
    if (ii === idx) {
      after = 1;
    }
    newArray[ii] = array[ii + after];
  }
  return newArray;
}

const MAX_ARRAY_MAP_SIZE = SIZE / 4;
const MAX_BITMAP_INDEXED_SIZE = SIZE / 2;
const MIN_HASH_ARRAY_MAP_SIZE = SIZE / 4;

// Above this many colliding entries, a `HashCollisionNode` builds a seeded
// secondary index instead of scanning linearly. Kept small so the rare,
// naturally-occurring collision buckets stay overhead-free, while adversarial
// hash-flooding (thousands of keys sharing one hash) degrades gracefully.
const MIN_HASH_COLLISION_INDEX_SIZE = 16;