Map v8.0.0, it's finally here

Hi everyone!

It’s been a long time since v8.0.0-alpha.1 came out and v8.0.0 release version is finally here
Here is the description of changes v8.0.0 provides

Completely new algorithm

Since its first version, Map has been using the deterministic hash map algorithm proposed by Tyler Close. It uses a data array to store entries in an order of insertion. Another array (hash table) is used to map a hash (key) to an index in a data array (value). Although having some modifications, the main paradigms of a deterministic hash map agree with the Map behavior, meaning it will need to rehash (extend its data and hash arrays) when they fill up. Deleted items leave an empty space both in a data array and in a bucket. Despite having great performance characteristics, the above traits can be considered algorithm weaknesses in some scenarios.

V8 Map completely overhauls this behavior and moves to an in-house built hash map algorithm that eliminates these weaknesses, the main advantage being that it completely eliminates the need of rehashing by using a tree-like structure to store its entries. As a supplement to the new hash map algorithm, as well as to preserve the feature of saving insertion order, V8 Map also supports DEQ links to the next/prev inserted item. Together these new features open up a wide variety of possibilities. Lets go through all improvements it provides.

V7 Map algorithm leaves an empty space in a bucket on entry deletion. This leads to a problem where an entry deletion slows operations with other entries in the same bucket. More so, in the worst case scenario, subsequent delete/insert operations on a single key could lead to a half-linear complexity of operations on entries in the same bucket.

V8 Map uses a tree-like structure to store entries using 4 links that bond an entry with 4 branches on an upper level. Each such layer uses 2 hash bits to determine a branch, meaning all branches forking from an ancestor will have their lower n-hash-bits equal to the ancestor’s. This leads us to a simple way of deleting entries - we just replace them with any descendant leaf. This prevents any possible holes in a map, eliminates the need to rebalance the structure (opposed to conventional tree algorithms) while the tree structure itself eliminates the need to rehash (opposed to most hash map algorithms).

V7 Map algorithm leaves an empty space in a data array on entry deletion. This leads us to 2 inconvenient consequences. Successive delete/insert operations will inevitably lead to rehashes even without size overflow (as deleted space still adds up). We can not easily implement DEQ methods (like pop and peek) as they could have up to a half-linear complexity in the worst case scenario.

V8 Map algorithm uses DEQ links to preserve insertion order instead of an array. This opens up a lot of possibilities, such as full DEQ methods support, and the ability to insert entries mid-map, while eliminating the need to leave holes in a structure.

Conventional hash map algorithms can only make use of all hash bits when they are max size. Although upper hash bits are not needed for such algorithms in small maps, it is in fact possible to choose keys that will have conflicting hashes and appear in one bucket. V8 Map eliminates this possibility as it will use up to all hash bits with any-size maps if an operation leads to a collision.

To determine key equality, V8 Map now compares hash equality first to discard false-cases earlier, as hash comparison will be a lot faster for most key types.

New methods

As the new algorithm supports DEQ links, there are many possibilities for new methods:

  • A lot of methods received their Front/Desc counterparts

  • Most DEQ-originating methods were added to V8 Map

  • V8 Map extends the default iterator logic and adds many new ways to traverse maps

  • Ability to insert entries mid-map (mid-deq), move entries, update values without a prior get call

  • Support for map mutation during iteration, maintaining iteration order predictable

  • Ability to easily convert Map to an Array, new ways to create Map from iterator

Set methods

V7 Map version had 2 methods to add new entries: put and set where the only difference was that the latter ignores the result.

V8 Map adds putFront and setFront counterparts to prepend new entries to a map (deq) instead of putting them at the end.

New replace method was added to skip insertion and make changes only when the specified key already exists. Similarly, add and addFront methods allow you to insert a new entry and skip modification when the specified key already exists.

Move methods (putMove, putMoveFront, replaceMove, replaceMoveFront) were added as a convenience for cases where you want to move a value to the back/front if it already exists.

New methods (putBefore, putAfter) were added allowing you to put a new key/value pair before/after a specified key. These methods do not have their Move twins as they move existing entries after/before a specified key by default.

Update methods

A set of update methods was added (update, updateFront, updateMove, updateMoveFront). They are similar to the put method but instead of a newValue they expect you to pass a function that receives an optional old value and returns an optional new value.

As in the case of put, these support Front/Move variants which allow you to prepend new entries and move existing values to the back/front.

DEQ methods

DEQ links existence allows you to use full DEQ functionality alongside map methods.

New pop and popFront methods remove the last/first entry and return it.

To get the last/first entry without removing it, you can use peek and peekFront.

There are also cycle and cycleFront methods which take the last/first entry, move it to the front/back and return it.


Iterators were completely overhauled. Map extends the default iterator object that has a single next method and uses the next structure for its iterators (prev, next, peekPrev, peekNext, current, started, finished, reset, movePrev, moveNext) where prev iterates backward, current returns the current value without moving an iterator, reset moves an iterator to its initial position, started and finished return a boolean indicating an iterator state, peekPrev and peekNext return prev/next value without moving an iterator, movePrev and moveNext work similar to prev and next but return the iterator itself, instead of a value.

Of course, a for…in loop will not be able to use these methods automatically, but manual calls will be very helpful in some situations.

For the convenience of iterating backward with a for…in loop, there are new methods keysDesc, valsDesc, entriesDesc. These will iterate forward if you use prev, peekPrev and movePrev methods on them.

Sometimes you don’t want to iterate your map from the beginning and have a specific entry to start iterating from. Instead of skipping a number of iterations you can now use a new group of From iterators (keysFrom, keysFromDesc, valsFrom, valsFromDesc, entriesFrom, entriesFromDesc) which expect you to pass a key to start iterating from. By default this key will not be included in iteration (meaning you will start iterating from the next/prev item) but you can easily override this behavior by chaining movePrev or moveNext methods.

V8 Map runs additional checks to make sure iteration order remains predictable when you remove/add an entry either before/after a currently iterating one. This behavior also extends to other iterating methods (like map, filter, find, toArrayMap and others).

On top of that, all Map iterators are reusable, that is, after returning null once, the next next or prev call will start a new iteration cycle.


Before, you had fromIter method to create new maps from iterators which expected you to pass an iterator that returns a key/value tuple on every iteration. V8 Map adds a fromIterMap method, which allows you to map and filter iterator items. This is helpful if your iterator does not have the expected key/value structure by default. The corresponding fromIterDesc and fromIterMapDesc were also added to prepend iterator items to a map instead of appending them.

For the convenience of converting your maps to arrays there are 4 new methods toArray, toArrayDesc, toArrayMap, toArrayMapDesc. If you just want an array with a full set of your map’s key/value pairs, you can use toArray. If you want to map/filter entries before putting them to an array, use toArrayMap. To traverse your map backward while creating a resulting array, use toArrayDesc and toArrayMapDesc.

New hash functions

V7 Map supported numeric hash functions for Nat (nhash) and Int (ihash) keys natively. With V8 Map support for all combinations on numeric keys were added (nhash, n8hash, n16hash, n32hash, n64hash, ihash, i8hash, i16hash, i32hash, i64hash). Also, nhash and ihash now use 64 lower bits for hash calculation instead of 32.

Misc methods

Continuing the trend with Desc twins, forEach, every and some are also receiving the ability to move through a map backward using corresponding forEachDesc, everyDesc and someDesc methods. To keep the naming scheme aligned, the findLast method was renamed to findDesc.

The new empty method is a shorthand for size == 0 check.

For the ease of cloning a map, a clone method was added, which removes the need to use filter together with a function that returns true, or similar methods.

Optimized files

Some code changes lead to worse readability while improving performance by a small margin. It is better to avoid such changes. That is what I am doing for the Map. But the perfectionist part inside me still eagerly wants to leave these optimizations on. That’s why I’m creating a script to automatically parse the source files and generate fully optimized code (down to removing return keywords which scratches off 1 cycle). The parser is a very simple regexp search, which is only suitable for my cases and does not has full syntax understanding whatsoever.

Performance metrics

As a consequence of using a tree-like structure, which on average needs more iterations to find a key (has worse algorithm complexity than a common hash map algorithm), V8 Map has a decrease in performance when comparing best case scenarios for both maps. Due to high code optimizations it was possible to decrease this performance deviation to 30% on average. V8 Map will be faster in a general case scenario as it’s algorithm eliminates a lot of (if not all) performance flaws of the previous one and has low-to-none performance deviation in a best vs worst case.

Here is a performance comparison between some of the data structures using Nat/Nat key/value types. The number of entries is 100_000.

Delete Desc is a cycle cost of deleting all items starting from the last. It has deviations from a regular Delete for some algorithms.

Heap Space is an rts_heap_size call at the end of the first Delete stage. It includes garbage for Insert, Get, Update and Delete operations.

Space is an rts_heap_size call in the next message (when all garbage collection is done), which is the amount of space needed to store 100_000 entries.

Structure Insert Get Update Delete Delete Desc Heap Space Space
V8 Map 91_496_534 59_378_554 83_895_094 124_591_501 105_893_982 8_000_000 5_600_000
V7 Map 91_238_554 30_270_177 41_784_888 80_128_837 77_557_928 20_665_288 3_448_660
Stable Hash Map 2_707_678_519 1_144_832_218 1_200_854_954 1_171_944_898 1_175_349_479 398_126_032 3_724_452
Stable RB Tree 2_374_238_947 264_585_170 1_582_756_699 613_056_039 612_856_037 487_921_556 7_600_028
Trie 1_274_569_564 432_541_994 1_172_783_140 1_015_169_296 1_115_608_817 351_920_140 6_894_652

Percentage table, comparing all structures to V8 Map

Structure Insert Get Update Delete Delete Desc Heap Space Space
V8 Map 1.000 1.000 1.000 1.000 1.000 1.000 1.000
V7 Map 0.997 0.510 0.498 0.643 0.732 2.583 0.616
Stable Hash Map 29.593 19.280 14.314 9.406 11.099 49.766 0.655
Stable RB Tree 25.949 4.456 18.866 4.921 5.787 60.990 1.357
Trie 13.930 7.284 13.979 8.148 10.535 43.990 1.231

Upgrade strategy

  • Rename all findLast methods to findDesc

  • Add HashUtils as a second parameter to map, filter and mapFilter calls

  • The new method now expects you to pass HashUtils as a first param

  • Migrate all your stable maps using let newMap = Map_8.fromIter(Map_7.entries(oldMap), Map_8.x_hash)

Custom hash upgrade strategy

  • IMPORTANT! Your custom hash function must exclude 0xffffffff hash value as this one is reserved

  • Nat32 should be used as a hash type instead of Nat

  • New HashUtils has a third compound getNullKey (a function that should return any key with corresponding type, the value does not matter as it will never be compared to any other key or be passed to a hash function)


@ZhenyaUsenko do you have plan to add this to the base library?

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I am good with it being in the base repo, interesting to know what Motoko team thinks about it.
Among my concerns is the introduction of breaking changes in the future versions… being more constrained with the improvements I can provide. It is always better to avoid breaking changes in such libraries with multiple data structure as it could hold the upgrade for those willing to apply only non-breaking changes.

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Is the code that computes the performance comparison also available publicly?

That’s what I use to track performance changes after every code change. This code was a bit modified for the tests above (e.g. uses Nat/Nat key/value pairs to be comparable to other structures as they do not natively support Nat32 hashing).

let { ihash; nhash; n32hash; n64hash; thash; phash; bhash; lhash } = Map;

type PerfStats = {
  cost: [{ #setCost: Nat64; #getCost: Nat64; #updateCost: Nat64; #deleteCost: Nat64; #deleteDescCost: Nat64 }];
  space: [{ #setSpace: Nat; #getSpace: Nat; #updateSpace: Nat; #deleteSpace: Nat }];


public shared func testPerf(): async PerfStats {
  let map =<Nat32, Nat32>(n32hash);

  let startSpace = Prim.rts_heap_size();

  let setCost = IC.countInstructions(func() {
    var i = 0:Nat32;

    while (i != 100000) { Map.set(map, n32hash, i, i); i +%= 1 };

  let setSpace = Prim.rts_heap_size() - startSpace:Nat;

  let getCost = IC.countInstructions(func() {
    var i = 0:Nat32;

    while (i != 100000) { ignore Map.get(map, n32hash, i); i +%= 1 };

  let getSpace = Prim.rts_heap_size() - startSpace:Nat - setSpace:Nat;

  let updateCost = IC.countInstructions(func() {
    var i = 0:Nat32;

    while (i != 100000) { Map.set(map, n32hash, i, i); i +%= 1 };

  let updateSpace = Prim.rts_heap_size() - startSpace:Nat - setSpace:Nat - getSpace:Nat;

  let deleteCost = IC.countInstructions(func() {
    var i = 0:Nat32;

    while (i != 100000) { Map.delete(map, n32hash, i); i +%= 1 };

  let deleteSpace = Prim.rts_heap_size() - startSpace:Nat - setSpace:Nat - getSpace:Nat - updateSpace:Nat;

  var i = 0:Nat32;

  while (i != 100000) { Map.set(map, n32hash, i, i); i +%= 1 };

  let deleteDescCost = IC.countInstructions(func() {
    var i = 100000:Nat32;

    while (i != 0) { i -%= 1; Map.delete(map, n32hash, i) };

  return {
    cost = [#setCost(setCost), #getCost(getCost), #updateCost(updateCost), #deleteCost(deleteCost), #deleteDescCost(deleteDescCost)];
    space = [#setSpace(setSpace), #getSpace(getSpace), #updateSpace(updateSpace), #deleteSpace(deleteSpace)];
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Map v8.1.0. Small update this time

Added contains method. Functions similarly to has (checks key existence) but returns null if a map is empty

Could be useful, for example, in some situations where an empty map is used to mark disabled state of some functionality. In such cases a combined check of either disabled functionalty or key existence can be made with Map.contains(map, xhash, key) != ?false

Plans for the next minor version

  1. Set operations (union, intersection, difference, symmetricDifference as well as their Desc twins)… maybe symmetricDifference needs a different name as this one is a bit too long
  2. Documentation update (method signatures with brief descriptions). Current one doesn’t even mention the Set data structure existence
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Have you considered uploading this on mops? IIRC there is already a version but it’s a fork and now outdated.


You are not the first one asking for this… will do it shortly. I am using vessel myself

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This is added to mops?

Map V9 is out!

It is 2.25 times more space efficient than V7

It is 4.00 times more space efficient than V8

The primary goal of this release is to fix the issue some people started experiencing some time ago which can prevent canister upgrades when your map grows too big.

V9 reverts its internal structure to the one that is more similar to V7 (as linked nature on V8 was the subject of upgrade issues) but keeps all the improvements described in the original post (like DEQ methods, improved iterators, bidirectionality). The only things that were taken out (impossible to support with the current internal structure) are:

  • putBefore and putAfter are no longer present
  • mutation during iteration is no longer supported
  • (not a direct functionality downgrade) a shift in internal structure makes rehashes necessary again, decreasing the worst case complexity of some methods to linear

V8 map is moving to another repo (Motoko Hash Tree) and will be supported in the future. Its algorithm still has some advantages that can not be achieved with more common hash map algorithms. Though, its usage will be discouraged until we get a motoko compiler that fully fixes serialization/deserialization on upgrades for linked data structures.

On top of fixing the upgrade issue V9 provides huge improvements in terms of consumed space. On average it will consume 1.7 times less space than V7 and 2.7 times less space than V8. In the best case scenario these differences jump up to 2 and 3.5 times respectively. V9 also does not experience an increase in space consumption with Incremental GC enabled (unlike V7 and V8) which will further increase the multipliers to 2.25 and 4. Here I need to note that although V8 seems terrible in terms of space consumption looking at those multipliers, its worst case scenario was always better than V7 (more testing below).

Here is a performance comparison between recent Map versions using 100_000 (Nat32, Nat32) entries.

Structure Insert Get Update Delete Delete Desc Space + Garbage Space
V9 Map 100_565_493 31_135_243 32_997_376 93_732_517 91_153_853 6_292_948 2_097_264
V8 Map 78_535_383 54_324_546 73_834_132 109_237_393 93_839_974 8_000_016 5_600_016
V7 Map 78_533_461 27_591_504 37_182_651 66_453_451 64_171_242 18_706_608 3_448_676

Percentage table, comparing other versions to V9.

Structure Insert Get Update Delete Delete Desc Space + Garbage Space
V9 Map 1.000 1.000 1.000 1.000 1.000 1.000 1.000
V8 Map 0.781 1.745 2.238 1.165 1.029 1.271 2.670
V7 Map 0.781 0.886 1.127 0.709 0.704 2.973 1.644

The next table shows the amount of bytes each map version occupies per single (Nat32, Nat32) entry. Both the default GC and incremental GC were tested.

Structure Min Max Incremental Min Incremental Max
V9 Map 16.00 42.66 16.00 42.66
V8 Map 56.00 56.00 64.00 64.00
V7 Map 32.00 63.50 36.00 67.50

Upgrade strategy

  • Motoko compiler v0.9.8 (or higher) is required for Map V9
  • Add HashUtils as a second parameter to pop and cycle methods
  • The new method no longer requires HashUtils as a first param
  • Migrate all your stable maps using let newMap = Map_9.fromIter(Map_8.entries(oldMap), Map_9.x_hash)
  • Custom HashUtils no longer needs its third compound getNullKey which was a point of misunderstanding for some people (it also no longer requires to exclude 0xffffffff hash value)

@ZhenyaUsenko Could you please publish V9 on mops as well?

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Done. Sorry for the delay

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Nice, thank you! (min 20 chars)

Recently I got some reports about “Map V9 not working”

Therefore, it is important to note that Motoko compiler v0.9.8 is required for Map V9

I’ll update Upgrade strategy accordingly

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Map README update

Since its first version, the Map library had just a bunch of code example in its README. It was not the easiest way to recognize different aspects of various methods, and even know what’s actually in there as the full list of methods was only briefly mentioned in some of the release notes.

Today I am fixing this and providing a comprehensive description of each and every method present in the library, including Map interface, Set interface, Iterator interface and Hash Utils.


9v Seems to work since dfx 0.15.

Question: Can or should I pass Map as a prop to other modules. Having issues passing the phash. Maybe I am thinking about it the wrong way and I should just pass what I need as an Array. Thanks :+1:

Can you post the sample code? If another module needs an array you can usually dump them with Map.entries, Map.keys, or Map.vals depending on what you need.

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Thanks! that helps. I want to split up my test CRUD app into different modules. Just testing things out. What I can/ can’t do. Basically I am just trying to access the map from different modules and just not sure the best way to go about doing that. What you said makes sense and I will try that. Thanks again!