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FastMapCodec project artwork

CurseForge · Minecraft mod

FastMapCodec

Provides more efficient BaseMapCodec.decode implementation

Choose a version Pick your version below, then grab the matching file.

Quick answer

Which FastMapCodec release matches 1.21.6 Fabric?

Updated last month
Best match for your filters FastMapCodec 1.0.4
Game version 1.21, 1.21.8, 1.21.5
Loader Fabric

FastMapCodec 1.0.4 targets 1.21, 1.21.8, 1.21.5 with Fabric. The project page does not say whether this file belongs on the client, dedicated server, or both. No extra mods listed for this file.

Where it goes

Is FastMapCodec required on the client, server, or both?

The project page does not say whether this file belongs on the client, dedicated server, or both.

Client Source doesn’t say
Dedicated server Source doesn’t say
Loader for this release Fabric
Required install it here Optional supported, not mandatory Not supported do not install here Source doesn’t say do not assume

The source does not explicitly classify this release as client-only or server-only.

Understand client and server requirements Read the Minecraft installation guide

What else does FastMapCodec 1.0.4 need?

FastMapCodec 1.0.4 on 1.21.6 Fabric. Every mod below is checked against that same setup.

No extra mods listed for this file

This file does not list any required mods. Do not add a library just because a different file uses it.

This file does not list any required or optional mods.

Before you install it

Add FastMapCodec without breaking your instance.

Built for FastMapCodec 1.0.4 on 1.21.6 Fabric. Pick another file and the loader, install side or required mods may change.

  1. 01

    Stick to this file

    Use FastMapCodec 1.0.4. It targets 1.21, 1.21.8, 1.21.5 with Fabric; another release may have different loader, side or dependency requirements.

  2. 02

    Bring the mods it needs

    This file does not list any required mods. Do not add a library just because a different file uses it.

  3. 03

    Put it on the correct side

    The project page does not say whether this file belongs on the client, dedicated server, or both.

  4. 04

    Pick the file you checked

    Use the “Get this file” button beside FastMapCodec 1.0.4. It opens that exact file at the source.

About this project

What does FastMapCodec add?

Mainly fixes TPS freeze on player join caused by large advancement files (and the same class of freeze anywhere else in the game, or in other mods, that arises from decoding a large map through DFU).

It is okay to use this mod on server only or client only.

Fixes O(N²) behavior in Mojang's BaseMapCodec (part of com.mojang:datafixerupper) when decoding large map-based Codecs. Since the fix lives in BaseMapCodec itself, it applies to every codec built on top of it - UnboundedMapCodec, SimpleMapCodec, and anything else sharing the same decode/encode logic - not just player advancements.

This bug was introduced in 1.21 and is present up to modern versions.

Benchmarks comparing this mod's fix to vanilla behaviour (with some context) are available here:

nekotxs/BaseMapCodec-bench

The fix has also been submitted upstream to Mojang's DataFixerUpper:

Mojang/DataFixerUpper#110

If Mojang accepts the PR, this mod won't be needed on newer versions.

NeoForge disclaimer

On NeoForge, this mod uses some cursed techniques to modify DFU behaviour, which may cause incompatibility with other mods. If you notice any issues, please open an issue on GitHub.

It is known to be incompatible with Sodium Extra.

This only applies to NeoForge — the Fabric version is not affected.

Project description from CurseForge.

Pick your setup

FastMapCodec by Minecraft version and loader

Choose the version and loader you play, then open the matching release.

34 available setups

Showing the newest 12 of 17 game versions. Older files are in the list below.

Check the dependencies, then try the file in a copied instance before changing a world you care about.

Recent files

FastMapCodec versions and loaders

5 of 10 releases match
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Looking for an older file? The official CurseForge project page is in Resources.