diff --git a/src/gleam_stdlib.mjs b/src/gleam_stdlib.mjs
index 60ab625b..e53596b6 100644
--- a/src/gleam_stdlib.mjs
+++ b/src/gleam_stdlib.mjs
@@ -15,10 +15,10 @@ import {
 } from "./gleam/regex.mjs";
 import { DecodeError } from "./gleam/dynamic.mjs";
 import { Some, None } from "./gleam/option.mjs";
-
-const HASHCODE_CACHE = new WeakMap();
+import PMap from "./persistent-hash-map.mjs"
 
 const Nil = undefined;
+const NOT_FOUND = {}
 
 export function identity(x) {
   return x;
@@ -244,7 +244,7 @@ export function trim_right(string) {
 }
 
 export function bit_string_from_string(string) {
-  return new toBitString([stringBits(string)]);
+  return toBitString([stringBits(string)]);
 }
 
 export function bit_string_concat(bit_strings) {
@@ -395,71 +395,8 @@ export function regex_scan(regex, string) {
   return List.fromArray(matches);
 }
 
-class Map {
-  static #hashcode_cache = new WeakMap();
-
-  static hash(value) {
-    let existing = this.#hashcode_cache.get(value);
-    if (existing) {
-      return existing;
-    } else if (value instanceof Object) {
-      let hashcode = inspect(value);
-      HASHCODE_CACHE.set(value, hashcode);
-      return hashcode;
-    } else {
-      return value.toString();
-    }
-  }
-
-  constructor() {
-    this.entries = new globalThis.Map();
-  }
-
-  get size() {
-    return this.entries.size;
-  }
-
-  inspect() {
-    let entries = [...this.entries.values()]
-      .map((pair) => inspect(pair))
-      .join(", ");
-    return `map.from_list([${entries}])`;
-  }
-
-  copy() {
-    let map = new Map();
-    map.entries = new globalThis.Map(this.entries);
-    return map;
-  }
-
-  toList() {
-    return List.fromArray([...this.entries.values()]);
-  }
-
-  insert(k, v) {
-    let map = this.copy();
-    map.entries.set(Map.hash(k), [k, v]);
-    return map;
-  }
-
-  delete(k) {
-    let map = this.copy();
-    map.entries.delete(Map.hash(k));
-    return map;
-  }
-
-  get(key) {
-    let code = Map.hash(key);
-    if (this.entries.has(code)) {
-      return new Ok(this.entries.get(code)[1]);
-    } else {
-      return new Error(Nil);
-    }
-  }
-}
-
 export function new_map() {
-  return new Map();
+  return PMap.new();
 }
 
 export function map_size(map) {
@@ -467,19 +404,23 @@ export function map_size(map) {
 }
 
 export function map_to_list(map) {
-  return map.toList();
+  return List.fromArray(map.entries());
 }
 
-export function map_remove(k, map) {
-  return map.delete(k);
+export function map_remove(key, map) {
+  return map.delete(key);
 }
 
 export function map_get(map, key) {
-  return map.get(key);
+  const value = map.get(key, NOT_FOUND);
+  if (value === NOT_FOUND) {
+    return new Error(Nil);
+  }
+  return new Ok(value);
 }
 
 export function map_insert(key, value, map) {
-  return map.insert(key, value);
+  return map.set(key, value);
 }
 
 function unsafe_percent_decode(string) {
@@ -610,7 +551,7 @@ export function classify_dynamic(data) {
     return `Tuple of ${data.length} elements`;
   } else if (BitString.isBitString(data)) {
     return "BitString";
-  } else if (data instanceof Map) {
+  } else if (data instanceof PMap) {
     return "Map";
   } else if (typeof data === "number") {
     return "Float";
@@ -680,13 +621,17 @@ export function decode_result(data) {
 }
 
 export function decode_map(data) {
-  if (data instanceof Map) {
+  if(data instanceof PMap) {
     return new Ok(data)
   }
-  if (typeof data === 'object' && data !== null && Object.getPrototypeOf(data) == Object.getPrototypeOf({})) {
-    return new Ok(new Map(Object.entries(data)))
+  if (data instanceof Map) {
+    return new Ok(PMap.fromMap(data));
+  }
+  const proto = Object.getPrototypeOf(data);
+  if (proto === Object.prototype || proto === null) {
+    return new Ok(PMap.fromObject(data));
   }
-  return  decoder_error("Map", data);
+  return decoder_error("Map", data);
 }
 
 export function decode_option(data, decoder) {
@@ -703,8 +648,8 @@ export function decode_option(data, decoder) {
 
 export function decode_field(value, name) {
   let error = () => decoder_error_no_classify("field", "nothing");
-  if (value instanceof Map) {
-    let entry = value.get(name);
+  if (value instanceof PMap) {
+    let entry = map_get(value, name);
     return entry.isOk() ? entry : error();
   }
   try {
diff --git a/src/persistent-hash-map.mjs b/src/persistent-hash-map.mjs
new file mode 100644
index 00000000..f454c7db
--- /dev/null
+++ b/src/persistent-hash-map.mjs
@@ -0,0 +1,955 @@
+/**
+ * This file uses jsdoc to annotate types.
+ * These types can be checked using the typescript compiler with "checkjs" option.
+ */
+
+import { isEqual } from "./gleam.mjs";
+
+const referenceMap = new WeakMap();
+const tempDataView = new DataView(new ArrayBuffer(8));
+let referenceUID = 0;
+/**
+ * hash the object by reference using a weak map and incrementing uid
+ * @param {any} o
+ * @returns {number}
+ */
+function hashByReference(o) {
+  const known = referenceMap.get(o);
+  if (known !== undefined) {
+    return known;
+  }
+  const hash = referenceUID++;
+  if (referenceUID === 0x7fffffff) {
+    referenceUID = 0;
+  }
+  referenceMap.set(o, hash);
+  return hash;
+}
+/**
+ * merge two hashes in an order sensitive way
+ * @param {number} a
+ * @param {number} b
+ * @returns {number}
+ */
+function hashMerge(a, b) {
+  return (a ^ (b + 0x9e3779b9 + (a << 6) + (a >> 2))) | 0;
+}
+/**
+ * standard string hash popularised by java
+ * @param {string} s
+ * @returns {number}
+ */
+function hashString(s) {
+  let hash = 0;
+  const len = s.length;
+  for (let i = 0; i < len; i++) {
+    hash = (Math.imul(31, hash) + s.charCodeAt(i)) | 0;
+  }
+  return hash;
+}
+/**
+ * hash a number by converting to two integers and do some jumbling
+ * @param {number} n
+ * @returns {number}
+ */
+function hashNumber(n) {
+  tempDataView.setFloat64(0, n);
+  const i = tempDataView.getInt32(0);
+  const j = tempDataView.getInt32(4);
+  return Math.imul(0x45d9f3b, (i >> 16) ^ i) ^ j;
+}
+/**
+ * hash a BigInt by converting it to a string and hashing that
+ * @param {BigInt} n
+ * @returns {number}
+ */
+function hashBigInt(n) {
+  return hashString(n.toString());
+}
+/**
+ * hash any js object
+ * @param {any} o
+ * @returns {number}
+ */
+function hashObject(o) {
+  const proto = Object.getPrototypeOf(o);
+  if (proto !== null && typeof proto.hashCode === "function") {
+    try {
+      const code = o.hashCode(o);
+      if (typeof code === "number") {
+        return code
+      }
+    } catch {}
+  }
+  if (o instanceof Promise || o instanceof WeakSet || o instanceof WeakMap) {
+    return hashByReference(o);
+  }
+  if (o instanceof Date) {
+    return hashNumber(o.getTime());
+  }
+  let h = 0;
+  if (o instanceof ArrayBuffer) {
+    o = new Uint8Array(o);
+  }
+  if (Array.isArray(o) || o instanceof Uint8Array) {
+    for (let i = 0; i < o.length; i++) {
+      h = (Math.imul(31, h) + getHash(o[i])) | 0;
+    }
+  } else if (o instanceof Set) {
+    o.forEach((v) => {
+      h = (h + getHash(v)) | 0;
+    });
+  } else if (o instanceof Map) {
+    o.forEach((v, k) => {
+      h = (h + hashMerge(getHash(v), getHash(k))) | 0;
+    });
+  } else {
+    const keys = Object.keys(o);
+    for (let i = 0; i < keys.length; i++) {
+      const k = keys[i];
+      const v = o[k];
+      h = (h + hashMerge(getHash(v), hashString(k))) | 0;
+    }
+  }
+  return h;
+}
+/**
+ * hash any js value
+ * @param {any} u
+ * @returns {number}
+ */
+export function getHash(u) {
+  if (u === null) return 0x42108422;
+  if (u === undefined) return 0x42108423;
+  if (u === true) return 0x42108421;
+  if (u === false) return 0x42108420;
+  switch (typeof u) {
+    case "number":
+      return hashNumber(u);
+    case "string":
+      return hashString(u);
+    case "bigint":
+      return hashBigInt(u);
+    case "object":
+      return hashObject(u);
+    case "symbol":
+      return hashByReference(u);
+    case "function":
+      return hashByReference(u);
+    default:
+      return 0; // should be unreachable
+  }
+}
+/**
+ * @template K,V
+ * @typedef {ArrayNode<K,V> | IndexNode<K,V> | CollisionNode<K,V>} Node
+ */
+/**
+ * @template K,V
+ * @typedef {{ type: typeof ENTRY, k: K, v: V }} Entry
+ */
+/**
+ * @template K,V
+ * @typedef {{ type: typeof ARRAY_NODE, size: number, array: (undefined | Entry<K,V> | Node<K,V>)[] }} ArrayNode
+ */
+/**
+ * @template K,V
+ * @typedef {{ type: typeof INDEX_NODE, bitmap: number, array: (Entry<K,V> | Node<K,V>)[] }} IndexNode
+ */
+/**
+ * @template K,V
+ * @typedef {{ type: typeof COLLISION_NODE, hash: number, array: Entry<K, V>[] }} CollisionNode
+ */
+/**
+ * @typedef {{ val: boolean }} Flag
+ */
+const SHIFT = 5; // number of bits you need to shift by to get the next bucket
+const BUCKET_SIZE = Math.pow(2, SHIFT);
+const MASK = BUCKET_SIZE - 1; // used to zero out all bits not in the bucket
+const MAX_INDEX_NODE = BUCKET_SIZE / 2; // when does index node grow into array node
+const MIN_ARRAY_NODE = BUCKET_SIZE / 4; // when does array node shrink to index node
+const ENTRY = 0;
+const ARRAY_NODE = 1;
+const INDEX_NODE = 2;
+const COLLISION_NODE = 3;
+/** @type {IndexNode<any,any>} */
+const EMPTY = {
+  type: INDEX_NODE,
+  bitmap: 0,
+  array: [],
+};
+/**
+ * Mask the hash to get only the bucket corresponding to shift
+ * @param {number} hash
+ * @param {number} shift
+ * @returns {number}
+ */
+function mask(hash, shift) {
+  return (hash >>> shift) & MASK;
+}
+/**
+ * Set only the Nth bit where N is the masked hash
+ * @param {number} hash
+ * @param {number} shift
+ * @returns {number}
+ */
+function bitpos(hash, shift) {
+  return 1 << mask(hash, shift);
+}
+/**
+ * Count the number of 1 bits in a number
+ * @param {number} x
+ * @returns {number}
+ */
+function bitcount(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;
+}
+/**
+ * Calculate the array index of an item in a bitmap index node
+ * @param {number} bitmap
+ * @param {number} bit
+ * @returns {number}
+ */
+function index(bitmap, bit) {
+  return bitcount(bitmap & (bit - 1));
+}
+/**
+ * Efficiently copy an array and set one value at an index
+ * @template T
+ * @param {T[]} arr
+ * @param {number} at
+ * @param {T} val
+ * @returns {T[]}
+ */
+function cloneAndSet(arr, at, val) {
+  const len = arr.length;
+  const out = new Array(len);
+  for (let i = 0; i < len; ++i) {
+    out[i] = arr[i];
+  }
+  out[at] = val;
+  return out;
+}
+/**
+ * Efficiently copy an array and insert one value at an index
+ * @template T
+ * @param {T[]} arr
+ * @param {number} at
+ * @param {T} val
+ * @returns {T[]}
+ */
+function spliceIn(arr, at, val) {
+  const len = arr.length;
+  const out = new Array(len + 1);
+  let i = 0;
+  let g = 0;
+  while (i < at) {
+    out[g++] = arr[i++];
+  }
+  out[g++] = val;
+  while (i < len) {
+    out[g++] = arr[i++];
+  }
+  return out;
+}
+/**
+ * Efficiently copy an array and remove one value at an index
+ * @template T
+ * @param {T[]} arr
+ * @param {number} at
+ * @returns {T[]}
+ */
+function spliceOut(arr, at) {
+  const len = arr.length;
+  const out = new Array(len - 1);
+  let i = 0;
+  let g = 0;
+  while (i < at) {
+    out[g++] = arr[i++];
+  }
+  ++i;
+  while (i < len) {
+    out[g++] = arr[i++];
+  }
+  return out;
+}
+/**
+ * Create a new node containing two entries
+ * @template K,V
+ * @param {number} shift
+ * @param {K} key1
+ * @param {V} val1
+ * @param {number} key2hash
+ * @param {K} key2
+ * @param {V} val2
+ * @returns {Node<K,V>}
+ */
+function createNode(shift, key1, val1, key2hash, key2, val2) {
+  const key1hash = getHash(key1);
+  if (key1hash === key2hash) {
+    return {
+      type: COLLISION_NODE,
+      hash: key1hash,
+      array: [
+        { type: ENTRY, k: key1, v: val1 },
+        { type: ENTRY, k: key2, v: val2 },
+      ],
+    };
+  }
+  const addedLeaf = { val: false };
+  return assoc(
+    assocIndex(EMPTY, shift, key1hash, key1, val1, addedLeaf),
+    shift,
+    key2hash,
+    key2,
+    val2,
+    addedLeaf
+  );
+}
+/**
+ * @template T,K,V
+ * @callback AssocFunction
+ * @param {T} root
+ * @param {number} shift
+ * @param {number} hash
+ * @param {K} key
+ * @param {V} val
+ * @param {Flag} addedLeaf
+ * @returns {Node<K,V>}
+ */
+/**
+ * Associate a node with a new entry, creating a new node
+ * @template T,K,V
+ * @type {AssocFunction<Node<K,V>,K,V>}
+ */
+function assoc(root, shift, hash, key, val, addedLeaf) {
+  switch (root.type) {
+    case ARRAY_NODE:
+      return assocArray(root, shift, hash, key, val, addedLeaf);
+    case INDEX_NODE:
+      return assocIndex(root, shift, hash, key, val, addedLeaf);
+    case COLLISION_NODE:
+      return assocCollision(root, shift, hash, key, val, addedLeaf);
+  }
+}
+/**
+ * @template T,K,V
+ * @type {AssocFunction<ArrayNode<K,V>,K,V>}
+ */
+function assocArray(root, shift, hash, key, val, addedLeaf) {
+  const idx = mask(hash, shift);
+  const node = root.array[idx];
+  // if the corresponding index is empty set the index to a newly created node
+  if (node === undefined) {
+    addedLeaf.val = true;
+    return {
+      type: ARRAY_NODE,
+      size: root.size + 1,
+      array: cloneAndSet(root.array, idx, { type: ENTRY, k: key, v: val }),
+    };
+  }
+  if (node.type === ENTRY) {
+    // if keys are equal replace the entry
+    if (isEqual(key, node.k)) {
+      if (val === node.v) {
+        return root;
+      }
+      return {
+        type: ARRAY_NODE,
+        size: root.size,
+        array: cloneAndSet(root.array, idx, {
+          type: ENTRY,
+          k: key,
+          v: val,
+        }),
+      };
+    }
+    // otherwise upgrade the entry to a node and insert
+    addedLeaf.val = true;
+    return {
+      type: ARRAY_NODE,
+      size: root.size,
+      array: cloneAndSet(
+        root.array,
+        idx,
+        createNode(shift + SHIFT, node.k, node.v, hash, key, val)
+      ),
+    };
+  }
+  // otherwise call assoc on the child node
+  const n = assoc(node, shift + SHIFT, hash, key, val, addedLeaf);
+  // if the child node hasn't changed just return the old root
+  if (n === node) {
+    return root;
+  }
+  // otherwise set the index to the new node
+  return {
+    type: ARRAY_NODE,
+    size: root.size,
+    array: cloneAndSet(root.array, idx, n),
+  };
+}
+/**
+ * @template T,K,V
+ * @type {AssocFunction<IndexNode<K,V>,K,V>}
+ */
+function assocIndex(root, shift, hash, key, val, addedLeaf) {
+  const bit = bitpos(hash, shift);
+  const idx = index(root.bitmap, bit);
+  // if there is already a item at this hash index..
+  if ((root.bitmap & bit) !== 0) {
+    // if there is a node at the index (not an entry), call assoc on the child node
+    const node = root.array[idx];
+    if (node.type !== ENTRY) {
+      const n = assoc(node, shift + SHIFT, hash, key, val, addedLeaf);
+      if (n === node) {
+        return root;
+      }
+      return {
+        type: INDEX_NODE,
+        bitmap: root.bitmap,
+        array: cloneAndSet(root.array, idx, n),
+      };
+    }
+    // otherwise there is an entry at the index
+    // if the keys are equal replace the entry with the updated value
+    const nodeKey = node.k;
+    if (isEqual(key, nodeKey)) {
+      if (val === node.v) {
+        return root;
+      }
+      return {
+        type: INDEX_NODE,
+        bitmap: root.bitmap,
+        array: cloneAndSet(root.array, idx, {
+          type: ENTRY,
+          k: key,
+          v: val,
+        }),
+      };
+    }
+    // if the keys are not equal, replace the entry with a new child node
+    addedLeaf.val = true;
+    return {
+      type: INDEX_NODE,
+      bitmap: root.bitmap,
+      array: cloneAndSet(
+        root.array,
+        idx,
+        createNode(shift + SHIFT, nodeKey, node.v, hash, key, val)
+      ),
+    };
+  } else {
+    // else there is currently no item at the hash index
+    const n = root.array.length;
+    // if the number of nodes is at the maximum, expand this node into an array node
+    if (n >= MAX_INDEX_NODE) {
+      // create a 32 length array for the new array node (one for each bit in the hash)
+      const nodes = new Array(32);
+      // create and insert a node for the new entry
+      const jdx = mask(hash, shift);
+      nodes[jdx] = assocIndex(EMPTY, shift + SHIFT, hash, key, val, addedLeaf);
+      let j = 0;
+      let bitmap = root.bitmap;
+      // place each item in the index node into the correct spot in the array node
+      // loop through all 32 bits / array positions
+      for (let i = 0; i < 32; i++) {
+        if ((bitmap & 1) !== 0) {
+          const node = root.array[j++];
+          nodes[i] = node;
+        }
+        // shift the bitmap to process the next bit
+        bitmap = bitmap >>> 1;
+      }
+      return {
+        type: ARRAY_NODE,
+        size: n + 1,
+        array: nodes,
+      };
+    } else {
+      // else there is still space in this index node
+      // simply insert a new entry at the hash index
+      const newArray = spliceIn(root.array, idx, {
+        type: ENTRY,
+        k: key,
+        v: val,
+      });
+      addedLeaf.val = true;
+      return {
+        type: INDEX_NODE,
+        bitmap: root.bitmap | bit,
+        array: newArray,
+      };
+    }
+  }
+}
+/**
+ * @template T,K,V
+ * @type {AssocFunction<CollisionNode<K,V>,K,V>}
+ */
+function assocCollision(root, shift, hash, key, val, addedLeaf) {
+  // if there is a hash collision
+  if (hash === root.hash) {
+    const idx = collisionIndexOf(root, key);
+    // if this key already exists replace the entry with the new value
+    if (idx !== -1) {
+      const entry = root.array[idx];
+      if (entry.v === val) {
+        return root;
+      }
+      return {
+        type: COLLISION_NODE,
+        hash: hash,
+        array: cloneAndSet(root.array, idx, { type: ENTRY, k: key, v: val }),
+      };
+    }
+    // otherwise insert the entry at the end of the array
+    const size = root.array.length;
+    addedLeaf.val = true;
+    return {
+      type: COLLISION_NODE,
+      hash: hash,
+      array: cloneAndSet(root.array, size, { type: ENTRY, k: key, v: val }),
+    };
+  }
+  // if there is no hash collision, upgrade to an index node
+  return assoc(
+    {
+      type: INDEX_NODE,
+      bitmap: bitpos(root.hash, shift),
+      array: [root],
+    },
+    shift,
+    hash,
+    key,
+    val,
+    addedLeaf
+  );
+}
+/**
+ * Find the index of a key in the collision node's array
+ * @template K,V
+ * @param {CollisionNode<K,V>} root
+ * @param {K} key
+ * @returns {number}
+ */
+function collisionIndexOf(root, key) {
+  const size = root.array.length;
+  for (let i = 0; i < size; i++) {
+    if (isEqual(key, root.array[i].k)) {
+      return i;
+    }
+  }
+  return -1;
+}
+/**
+ * @template T,K,V
+ * @callback FindFunction
+ * @param {T} root
+ * @param {number} shift
+ * @param {number} hash
+ * @param {K} key
+ * @returns {undefined | Entry<K,V>}
+ */
+/**
+ * Return the found entry or undefined if not present in the root
+ * @template K,V
+ * @type {FindFunction<Node<K,V>,K,V>}
+ */
+function find(root, shift, hash, key) {
+  switch (root.type) {
+    case ARRAY_NODE:
+      return findArray(root, shift, hash, key);
+    case INDEX_NODE:
+      return findIndex(root, shift, hash, key);
+    case COLLISION_NODE:
+      return findCollision(root, key);
+  }
+}
+/**
+ * @template K,V
+ * @type {FindFunction<ArrayNode<K,V>,K,V>}
+ */
+function findArray(root, shift, hash, key) {
+  const idx = mask(hash, shift);
+  const node = root.array[idx];
+  if (node === undefined) {
+    return undefined;
+  }
+  if (node.type === ENTRY) {
+    return node;
+  }
+  return find(node, shift + SHIFT, hash, key);
+}
+/**
+ * @template K,V
+ * @type {FindFunction<IndexNode<K,V>,K,V>}
+ */
+function findIndex(root, shift, hash, key) {
+  const bit = bitpos(hash, shift);
+  if ((root.bitmap & bit) === 0) {
+    return undefined;
+  }
+  const idx = index(root.bitmap, bit);
+  const node = root.array[idx];
+  if (node.type !== ENTRY) {
+    return find(node, shift + SHIFT, hash, key);
+  }
+  if (isEqual(key, node.k)) {
+    return node;
+  }
+  return undefined;
+}
+/**
+ * @template K,V
+ * @param {CollisionNode<K,V>} root
+ * @param {K} key
+ * @returns {undefined | Entry<K,V>}
+ */
+function findCollision(root, key) {
+  const idx = collisionIndexOf(root, key);
+  if (idx < 0) {
+    return undefined;
+  }
+  return root.array[idx];
+}
+/**
+ * @template T,K,V
+ * @callback WithoutFunction
+ * @param {T} root
+ * @param {number} shift
+ * @param {number} hash
+ * @param {K} key
+ * @returns {undefined | Node<K,V>}
+ */
+/**
+ * Remove an entry from the root, returning the updated root.
+ * Returns undefined if the node should be removed from the parent.
+ * @template K,V
+ * @type {WithoutFunction<Node<K,V>,K,V>}
+ * */
+function without(root, shift, hash, key) {
+  switch (root.type) {
+    case ARRAY_NODE:
+      return withoutArray(root, shift, hash, key);
+    case INDEX_NODE:
+      return withoutIndex(root, shift, hash, key);
+    case COLLISION_NODE:
+      return withoutCollision(root, key);
+  }
+}
+/**
+ * @template K,V
+ * @type {WithoutFunction<ArrayNode<K,V>,K,V>}
+ */
+function withoutArray(root, shift, hash, key) {
+  const idx = mask(hash, shift);
+  const node = root.array[idx];
+  if (node === undefined) {
+    return root; // already empty
+  }
+  let n = undefined;
+  // if node is an entry and the keys are not equal there is nothing to remove
+  // if node is not an entry do a recursive call
+  if (node.type === ENTRY) {
+    if (!isEqual(node.k, key)) {
+      return root; // no changes
+    }
+  } else {
+    n = without(node, shift + SHIFT, hash, key);
+    if (n === node) {
+      return root; // no changes
+    }
+  }
+  // if the recursive call returned undefined the node should be removed
+  if (n === undefined) {
+    // if the number of child nodes is at the minimum, pack into an index node
+    if (root.size <= MIN_ARRAY_NODE) {
+      const arr = root.array;
+      const out = new Array(root.size - 1);
+      let i = 0;
+      let j = 0;
+      let bitmap = 0;
+      while (i < idx) {
+        const nv = arr[i];
+        if (nv !== undefined) {
+          out[j] = nv;
+          bitmap |= 1 << i;
+          ++j;
+        }
+        ++i;
+      }
+      ++i; // skip copying the removed node
+      while (i < arr.length) {
+        const nv = arr[i];
+        if (nv !== undefined) {
+          out[j] = nv;
+          bitmap |= 1 << i;
+          ++j;
+        }
+        ++i;
+      }
+      return {
+        type: INDEX_NODE,
+        bitmap: bitmap,
+        array: out,
+      };
+    }
+    return {
+      type: ARRAY_NODE,
+      size: root.size - 1,
+      array: cloneAndSet(root.array, idx, n),
+    };
+  }
+  return {
+    type: ARRAY_NODE,
+    size: root.size,
+    array: cloneAndSet(root.array, idx, n),
+  };
+}
+/**
+ * @template K,V
+ * @type {WithoutFunction<IndexNode<K,V>,K,V>}
+ */
+function withoutIndex(root, shift, hash, key) {
+  const bit = bitpos(hash, shift);
+  if ((root.bitmap & bit) === 0) {
+    return root; // already empty
+  }
+  const idx = index(root.bitmap, bit);
+  const node = root.array[idx];
+  // if the item is not an entry
+  if (node.type !== ENTRY) {
+    const n = without(node, shift + SHIFT, hash, key);
+    if (n === node) {
+      return root; // no changes
+    }
+    // if not undefined, the child node still has items, so update it
+    if (n !== undefined) {
+      return {
+        type: INDEX_NODE,
+        bitmap: root.bitmap,
+        array: cloneAndSet(root.array, idx, n),
+      };
+    }
+    // otherwise the child node should be removed
+    // if it was the only child node, remove this node from the parent
+    if (root.bitmap === bit) {
+      return undefined;
+    }
+    // otherwise just remove the child node
+    return {
+      type: INDEX_NODE,
+      bitmap: root.bitmap ^ bit,
+      array: spliceOut(root.array, idx),
+    };
+  }
+  // otherwise the item is an entry, remove it if the key matches
+  if (isEqual(key, node.k)) {
+    if (root.bitmap === bit) {
+      return undefined;
+    }
+    return {
+      type: INDEX_NODE,
+      bitmap: root.bitmap ^ bit,
+      array: spliceOut(root.array, idx),
+    };
+  }
+  return root;
+}
+/**
+ * @template K,V
+ * @param {CollisionNode<K,V>} root
+ * @param {K} key
+ * @returns {undefined | Node<K,V>}
+ */
+function withoutCollision(root, key) {
+  const idx = collisionIndexOf(root, key);
+  // if the key not found, no changes
+  if (idx < 0) {
+    return root;
+  }
+  // otherwise the entry was found, remove it
+  // if it was the only entry in this node, remove the whole node
+  if (root.array.length === 1) {
+    return undefined;
+  }
+  // otherwise just remove the entry
+  return {
+    type: COLLISION_NODE,
+    hash: root.hash,
+    array: spliceOut(root.array, idx),
+  };
+}
+/**
+ * @template K,V
+ * @param {undefined | Node<K,V>} root
+ * @param {(value:V,key:K)=>void} fn
+ * @returns {void}
+ */
+function forEach(root, fn) {
+  if (root === undefined) {
+    return;
+  }
+  const items = root.array;
+  const size = items.length;
+  for (let i = 0; i < size; i++) {
+    const item = items[i];
+    if (item === undefined) {
+      continue;
+    }
+    if (item.type === ENTRY) {
+      fn(item.v, item.k);
+      continue;
+    }
+    forEach(item, fn);
+  }
+}
+/**
+ * Extra wrapper to keep track of map size and clean up the API
+ * @template K,V
+ */
+export default class PMap {
+  /**
+   * @template V
+   * @param {Record<string,V>} o
+   * @returns {PMap<string,V>}
+   */
+  static fromObject(o) {
+    const keys = Object.keys(o);
+    /** @type PMap<string,V> */
+    let m = PMap.new();
+    for (let i = 0; i < keys.length; i++) {
+      const k = keys[i];
+      m = m.set(k, o[k]);
+    }
+    return m;
+  }
+  /**
+   * @template K,V
+   * @param {Map<K,V>} o
+   * @returns {PMap<K,V>}
+   */
+  static fromMap(o) {
+    /** @type PMap<K,V> */
+    let m = PMap.new();
+    o.forEach((v, k) => {
+      m = m.set(k, v);
+    });
+    return m;
+  }
+  static new() {
+    return new PMap(undefined, 0);
+  }
+  /**
+   * @param {undefined | Node<K,V>} root
+   * @param {number} size
+   */
+  constructor(root, size) {
+    this.root = root;
+    this.size = size;
+  }
+  /**
+   * @template NotFound
+   * @param {K} key
+   * @param {NotFound} notFound
+   * @returns {NotFound | V}
+   */
+  get(key, notFound) {
+    if (this.root === undefined) {
+      return notFound;
+    }
+    const found = find(this.root, 0, getHash(key), key);
+    if (found === undefined) {
+      return notFound;
+    }
+    return found.v;
+  }
+  /**
+   * @param {K} key
+   * @param {V} val
+   * @returns {PMap<K,V>}
+   */
+  set(key, val) {
+    const addedLeaf = { val: false };
+    const root = this.root === undefined ? EMPTY : this.root;
+    const newRoot = assoc(root, 0, getHash(key), key, val, addedLeaf);
+    if (newRoot === this.root) {
+      return this;
+    }
+    return new PMap(newRoot, addedLeaf.val ? this.size + 1 : this.size);
+  }
+  /**
+   * @param {K} key
+   * @returns {PMap<K,V>}
+   */
+  delete(key) {
+    if (this.root === undefined) {
+      return this;
+    }
+    const newRoot = without(this.root, 0, getHash(key), key);
+    if (newRoot === this.root) {
+      return this;
+    }
+    if (newRoot === undefined) {
+      return PMap.new();
+    }
+    return new PMap(newRoot, this.size - 1);
+  }
+  /**
+   * @param {K} key
+   * @returns {boolean}
+   */
+  has(key) {
+    if (this.root === undefined) {
+      return false;
+    }
+    return find(this.root, 0, getHash(key), key) !== undefined;
+  }
+  /**
+   * @returns {[K,V][]}
+   */
+  entries() {
+    if (this.root === undefined) {
+      return [];
+    }
+    /** @type [K,V][] */
+    const result = [];
+    this.forEach((v, k) => result.push([k, v]));
+    return result;
+  }
+  /**
+   *
+   * @param {(val:V,key:K)=>void} fn
+   */
+  forEach(fn) {
+    forEach(this.root, fn);
+  }
+  hashCode() {
+    let h = 0;
+    this.forEach((v, k) => {
+      h = (h + hashMerge(getHash(v), getHash(k))) | 0;
+    });
+    return h;
+  }
+  /**
+   * @param {unknown} o
+   * @returns {boolean}
+   */
+  equals(o) {
+    if (!(o instanceof PMap)) {
+      return false;
+    }
+    let equal = true;
+    this.forEach((v, k) => {
+      equal = equal && isEqual(o.get(k, !v), v);
+    });
+    return equal;
+  }
+}
+
diff --git a/test/gleam/bit_string_test.gleam b/test/gleam/bit_string_test.gleam
index c52dd854..581cc1c6 100644
--- a/test/gleam/bit_string_test.gleam
+++ b/test/gleam/bit_string_test.gleam
@@ -9,6 +9,11 @@ pub fn byte_size_test() {
   |> should.equal(0)
 }
 
+pub fn not_equal_test() {
+  bit_string.from_string("test")
+  |> should.not_equal(bit_string.from_string("asdf"))
+}
+
 pub fn append_test() {
   bit_string.from_string("Test")
   |> bit_string.append(bit_string.from_string(" Me"))
diff --git a/test/gleam/map_test.gleam b/test/gleam/map_test.gleam
index 2666579f..43e17a01 100644
--- a/test/gleam/map_test.gleam
+++ b/test/gleam/map_test.gleam
@@ -2,6 +2,8 @@ import gleam/map
 import gleam/option.{None, Some}
 import gleam/should
 import gleam/string
+import gleam/list
+import gleam/int
 
 pub fn from_list_test() {
   [#(4, 0), #(1, 0)]
@@ -82,6 +84,21 @@ pub fn get_test() {
   m
   |> map.get(C)
   |> should.equal(Error(Nil))
+
+  let proplist = [#(<<1, 2, 3>>, 0), #(<<3, 2, 1>>, 1)]
+  let m = map.from_list(proplist)
+
+  m
+  |> map.get(<<1, 2, 3>>)
+  |> should.equal(Ok(0))
+
+  m
+  |> map.get(<<3, 2, 1>>)
+  |> should.equal(Ok(1))
+
+  m
+  |> map.get(<<1, 3, 2>>)
+  |> should.equal(Error(Nil))
 }
 
 pub fn insert_test() {
@@ -103,6 +120,7 @@ pub fn keys_test() {
   [#("a", 0), #("b", 1), #("c", 2)]
   |> map.from_list
   |> map.keys
+  |> list.sort(string.compare)
   |> should.equal(["a", "b", "c"])
 }
 
@@ -110,6 +128,7 @@ pub fn values_test() {
   [#("a", 0), #("b", 1), #("c", 2)]
   |> map.from_list
   |> map.values
+  |> list.sort(int.compare)
   |> should.equal([0, 1, 2])
 }
 
@@ -189,13 +208,131 @@ pub fn fold_test() {
   |> map.fold(0, add)
   |> should.equal(6)
 
-  let concat = fn(acc, k, _) { string.append(acc, k) }
+  let prepend = fn(acc, k, _) { list.prepend(acc, k) }
 
   dict
-  |> map.fold("", concat)
-  |> should.equal("abcd")
+  |> map.fold([], prepend)
+  |> list.sort(string.compare)
+  |> should.equal(["a", "b", "c", "d"])
 
   map.from_list([])
   |> map.fold(0, add)
   |> should.equal(0)
 }
+
+fn range(start, end, a) {
+  case end - start {
+    n if n < 1 -> a
+    _ -> range(start, end - 1, [end - 1, ..a])
+  }
+}
+
+fn list_to_map(list) {
+  list
+  |> list.map(fn(n) { #(n, n) })
+  |> map.from_list
+}
+
+fn grow_and_shrink_map(initial_size, final_size) {
+  range(0, initial_size, [])
+  |> list_to_map
+  |> list.fold(
+    range(final_size, initial_size, []),
+    _,
+    fn(map, item) { map.delete(map, item) },
+  )
+}
+
+// maps should be equal even if the insert/removal order was different
+pub fn insert_order_equality_test() {
+  grow_and_shrink_map(8, 2)
+  |> should.equal(grow_and_shrink_map(4, 2))
+  grow_and_shrink_map(17, 10)
+  |> should.equal(grow_and_shrink_map(12, 10))
+  grow_and_shrink_map(2000, 1000)
+  |> should.equal(grow_and_shrink_map(1000, 1000))
+}
+
+// ensure operations on a map don't mutate it
+pub fn persistence_test() {
+  let a = list_to_map([0])
+  map.insert(a, 0, 5)
+  map.insert(a, 1, 6)
+  map.delete(a, 0)
+  map.get(a, 0)
+  |> should.equal(Ok(0))
+}
+
+// using maps as keys should work (tests hash function)
+pub fn map_as_key_test() {
+  let l = range(0, 1000, [])
+  let a = list_to_map(l)
+  let a2 = list_to_map(list.reverse(l))
+  let a3 = grow_and_shrink_map(2000, 1000)
+  let b = grow_and_shrink_map(60, 50)
+  let c = grow_and_shrink_map(50, 20)
+  let d = grow_and_shrink_map(2, 2)
+
+  let map1 =
+    map.new()
+    |> map.insert(a, "a")
+    |> map.insert(b, "b")
+    |> map.insert(c, "c")
+    |> map.insert(d, "d")
+
+  map.get(map1, a)
+  |> should.equal(Ok("a"))
+  map.get(map1, a2)
+  |> should.equal(Ok("a"))
+  map.get(map1, a3)
+  |> should.equal(Ok("a"))
+  map.get(map1, b)
+  |> should.equal(Ok("b"))
+  map.get(map1, c)
+  |> should.equal(Ok("c"))
+  map.get(map1, d)
+  |> should.equal(Ok("d"))
+  map.insert(map1, a2, "a2")
+  |> map.get(a)
+  |> should.equal(Ok("a2"))
+  map.insert(map1, a3, "a3")
+  |> map.get(a)
+  |> should.equal(Ok("a3"))
+}
+
+pub fn large_n_test() {
+  let n = 10_000
+  let l = range(0, n, [])
+
+  let m = list_to_map(l)
+  list.map(l, fn(i) { should.equal(map.get(m, i), Ok(i)) })
+
+  let m = grow_and_shrink_map(n, 0)
+  list.map(l, fn(i) { should.equal(map.get(m, i), Error(Nil)) })
+}
+
+pub fn size_test() {
+  let n = 1000
+  let m = list_to_map(range(0, n, []))
+  map.size(m)
+  |> should.equal(n)
+
+  let m = grow_and_shrink_map(n, n / 2)
+  map.size(m)
+  |> should.equal(n / 2)
+
+  let m =
+    grow_and_shrink_map(n, 0)
+    |> map.delete(0)
+  map.size(m)
+  |> should.equal(0)
+
+  let m = list_to_map(range(0, 18, []))
+
+  map.insert(m, 1, 99)
+  |> map.size()
+  |> should.equal(18)
+  map.insert(m, 2, 99)
+  |> map.size()
+  |> should.equal(18)
+}
diff --git a/test/gleam/set_test.gleam b/test/gleam/set_test.gleam
index aae5256d..123dc43d 100644
--- a/test/gleam/set_test.gleam
+++ b/test/gleam/set_test.gleam
@@ -70,6 +70,7 @@ pub fn filter_test() {
   |> set.from_list()
   |> set.filter(for: int.is_even)
   |> set.to_list
+  |> list.sort(int.compare)
   |> should.equal([4, 6, 44])
 }