Add a guide on the upcoming data maps

docs/datamaps/_category_.json

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+{
+    "label": "Data Maps"
+}

docs/datamaps/index.md

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+# Data Maps
+
+A registry data map contains data-driven, reloadable objects that can be attached to a registry object.  
+This system allows more easily data-driving game behaviour, as they provide functionality such as syncing or conflict resolution, leading to a better and more configurable user experience.  
+
+You can think of tags as entry->boolean maps, while data maps are more flexible entry->object maps.
+
+A data map can be attached to both static, built-in, registries and dynamic data-driven datapack registries.
+
+## Registration
+A data map type should be statically created and then registered to the `RegisterDataMapTypesEvent` (which is fired on the mod event bus). The `DataMapType` can be created using a `DataMapType$Builder`, through `DataMapType#builder`.  
+
+A simple `DataMapType` has two generic arguments: `T` (the values that are being attached) and `R` (the type of the registry the data map is for). A data map of `SomeObject`s that are attached to `Item`s can, as such, be represented as `DataMapType<SomeObject, Item>`.  
+
+Data maps are serialized and deserialized using [Codecs](../datastorage/codecs.md).
+
+Let's take the following record representing the data map value as an example:
+```java
+public record DropHealing(
+        float amount, float chance
+) {
+    public static final Codec<DropHealing> CODEC = RecordCodecBuilder.create(in -> in.group(
+            Codec.FLOAT.fieldOf("amount").forGetter(DropHealing::amount),
+            Codec.floatRange(0, 1).fieldOf("chance").forGetter(DropHealing::chance)
+    ).apply(in, DropHealing::new));
+}
+```
+
+:::warning
+The value should be an *immutable* object, as otherwise weird behaviour can be caused if the object is attached to all values within a tag (since no copy is created).
+:::
+
+For the purposes of this example, we will use this data map to heal players when they drop an item.  
+The `DataMapType` can be created as such:
+```java
+public static final DataMapType<DropHealing, Item> DROP_HEALING = DataMapType.builder(
+        new ResourceLocation("mymod:drop_healing"), Registries.ITEM, DropHealing.CODEC
+).build();
+```
+and then registered to the `RegisterDataMapTypesEvent` using `RegisterDataMapTypesEvent#register`.
+
+## Syncing
+A synced data map will have its values synced to clients. A data map can be marked as synced using `DataMapType$Builder#synced(Codec<T> networkCodec, boolean mandatory)`.   
+The values of the data map will then be synced using the `networkCodec`.  
+If the `mandatory` flag is set to `true`, clients that do not support the data map (including Vanilla clients) will not be able to connect to the server, nor vice-versa. A non-mandatory data map is, on the other side, optional, so it will not prevent any clients from joining.
+
+## JSON Structure and location
+Data maps are loaded from a JSON file located at `:mapNamespace/data_maps/:registryNamespace/:registryPath/:mapPath.json`.  
+For more information, please [check out the dedicated page](./structure.md).
+
+## Usage
+As data maps can be used on any registry, they can be queried through `Holder`s, and not through the actual registry objects.  
+You can query a data map value using `Holder#getData(DataMapType)`. If that object doesn't have a value attached, the method will return `null`.
+
+:::note
+Only reference holders will return a value in that method. `Named` holders will **not**.  Generally, you will only encounter reference holders (which are returned by methods such as `Registry#wrapAsHolder`, `Registry#getHolder` or the different `builtInRegistryHolder` methods).
+:::
+
+To continue the example above, we can implement our intended behaviour as follows:
+```java
+public static void onItemDrop(final ItemTossEvent event) {
+    final ItemStack stack = event.getEntity().getItem();
+    // ItemStack has a getItemHolder method that will return a Holder<Item> which points to the item the stack is of
+    //highlight-next-line
+    final DropHealing value = stack.getItemHolder().getData(DROP_HEALING);
+    // Since getData returns null if the item will not have a drop healing value attached, we guard against it being null
+    if (value != null) {
+        // And here we simply use the values
+        if (event.getPlayer().level().getRandom().nextFloat() > value.chance()) {
+            event.getPlayer().heal(value.amount());
+        }
+    }
+}
+```
+
+## Advanced data maps
+Advanced data maps are data maps which have added functionality. Namely, the ability of merging values and selectively removing them, through a remover. Implementing some form of merging and removers is highly recommended for data maps whose values are collection-likes (like `Map`s or `List`s).
+
+`AdvancedDataMapType` have one more generic besides `T` and `R`: `VR extends DataMapValueRemover<T, R>`. This additional generic allows you to datagen remove objects with increased type safety, but it can otherwise be ignored and treated as wilcard (`?`) for most use cases.
+
+### Creation
+You create an `AdvancedDataMapType` using `AdvancedDataMapType#builder`. Unlike the normal builder, the builder returned by that method will have two more methods (`merger` and `remover`), and it will return an `AdvancedDataMapType`.  
+
+Registration methods remain the same.
+
+### Mergers
+An advanced data map can provide a `DataMapValueMerger` through `AdvancedDataMapType#merger`. This merger will be used to handle conflicts between data packs that attempt to attach a value to the same object.  
+The merger will be given the two conflicting values, and their sources (as an `Either<TagKey<R>, ResourceKey<R>>` since values can be attached to all entries within a tag, not just individual entries), and is expected to return the value that will actually be attached.  
+Generally, mergers should simply merge the values, and should not perform "hard" overwrites unless necessary (i.e. if merging isn't possible). A value can be overwritten by specifying the object-level `replace` field, which will bypass the merger.
+
+We provide some default mergers for merging lists, sets and maps in `DataMapValueMerger`.  
+
+The default merger (`DataMapValueMerger#defaultMerger`) has the typical first come last serverd priority-based overwriting behaviour that you'd expect from normal data packs, where the newest value always wins.
+
+### Removers
+An advanced data map can provide a `DataMapValueRemover` through `AdvancedDataMapType#remover`. The remover will allow selective removals of data map values, effectively decomposition.  
+While by default a datapack can only remove the whole object attached to a registry entry, with a remover it can remove just speciffic values from the attached object (i.e. just the entry with a given key in the case of a map, or the entry with a specific property in the case of a list).  
+
+The codec that is passed to the builder will decode removers that will then be given the value currently attached and its source, and is expected to create a new object that will have the properties requested by the `remove` object removed. Alternatively, an empty `Optional` will lead to the value being completely removed.  
+
+An example of a remover that will remove a value with a specific key from a `Map`-based data map:
+```java
+public record MapRemover(String key) implements DataMapValueRemover<Map<String, String>, Item> {
+    public static final Codec<MapRemover> CODEC = Codec.STRING.xmap(MapRemover::new, MapRemover::key);
+
+    @Override
+    public Optional<Map<String, String>> remove(Map<String, String> value, Registry<Item> registry, Either<TagKey<Item>, ResourceKey<Item>> source, Item object) {
+        final Map<String, String> newMap = new HashMap<>(value);
+        newMap.remove(key);
+        return Optional.of(newMap);
+    }
+}
+```
+
+With the remover above in mind, we're attaching maps of string to string to items. Take the following data map JSON file:
+```js
+{
+    "values": {
+        //highlight-start
+        "minecraft:carrot": {
+            "somekey1": "value1",
+            "somekey2": "value2"
+        }
+        //highlight-end
+    }
+}
+```
+That file will attach the map `[somekey1=value1, somekey2=value2]` to the `minecraft:carrot` item. Now, another datapack can come on top of it and remove just the value with the `somekey1` key, as such:
+```js
+{
+    "remove": {
+        // As the remover is decoded as a string, we can use a string as the value here. If it were decoded as an object, we would have needed to use an object.
+        //highlight-next-line
+        "minecraft:carrot": "somekey1"
+    }
+}
+```
+After the second datapack is read and applied, the new value attached to the `minecraft:carrot` item will be `[somekey2=value2]`.
+
+## Datagen
+Data maps can be generated through `DataMapProvider`.  
+You should extend that class, and then override the `generate` method to create your entries, similar to tag generation.
+
+Considering the drop healing example from the start, we could generate some values as follows:
+```java
+public class DropHealingGen extends DataMapProvider {
+
+    public DropHealingGen(PackOutput packOutput, CompletableFuture<HolderLookup.Provider> lookupProvider) {
+        super(packOutput, lookupProvider);
+    }
+
+    @Override
+    protected void gather() {
+        // In the examples below, we do not need to forcibly replace any value as that's the default behaviour since a merger isn't provided, so the third parameter can be false.
+
+        // If you were to provide a merger for your data map, then the third parameter will cause the old value to be overwritten if set to true, without invoking the merger
+        builder(DROP_HEALING)
+                // Always give entities that drop any item in the minecraft:fox_food tag 12 hearts
+                .add(ItemTags.FOX_FOOD, new DropHealing(12, 1f), false)
+                // Have a 10% chance of healing entities that drop an acacia boat by one point
+                .add(Items.ACACIA_BOAT.builtInRegistryHolder(), new DropHealing(1, 0.1f), false);
+    }
+}
+```

docs/datamaps/neo_maps.md

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+# Built-in Data Maps
+NeoForge provides a few data maps that mostly replace hardcoded in-code vanilla maps.  
+These dats maps can be found in `NeoForgeDataMaps`.
+
+## `neoforge:compostables`
+NeoForge provides a data map that allows configuring composter values, as a replacement for `ComposterBlock#COMPOSTABLES` (which is now ignored).  
+This data map is located at `neoforged/data_maps/item/compostables.json` and its objects have the following structure:
+```js
+{
+    // A 0 to 1 (inclusive) float representing the chance that the item will update the level of the composter
+    "chance": 1,
+
+    // A 1 to 7 integer that indicates how many levels should be added to the composter when the item is successfully composted. Defaults to 1 level.
+    "amount": 1 // int
+}
+```
+
+Example:
+```js
+{
+    "values": {
+        // Give acacia logs a 50% chance that they will fill a composter with 2 levels
+        "minecraft:acacia_log": {
+            "chance": 0.5,
+            "amount": 2
+        }
+    }
+}
+```

docs/datamaps/structure.md

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+# JSON Structure
+For the purposes of this page, we will use a data map which is an object with two float keys: `amount` and `chance` as an example. The codec for that object can be found [here](./index.md#registration).
+
+## Location
+Data maps are loaded from a JSON file located at `:mapNamespace/data_maps/:registryNamespace/:registryPath/:mapPath.json`, where:
+- `mapNamespace` is the namespace of the ID of the data map
+- `mapPath` is the path of the ID of the data map
+- `registryNamespace` is the namespace of the ID of the registry
+- `registryPath` is the path of the ID of the registry
+
+:::note
+The registry namespace is ommited if it is `minecraft`.
+:::
+
+Examples:
+- For a data map named `mymod:drop_healing` for the `minecraft:item` registry (as in the example), the path will be `mymod/data_maps/item/drop_healing.json`.
+- For a data map named `somemod:somemap` for the `minecraft:block` registry, the path will be `somemod/data_maps/block/somemap.json`.
+- For a data map named `example:stuff` for the `somemod:custom` registry, the path will be `example/data_maps/somemod/custom/stuff.json`.
+
+## Global `replace` field
+The JSON file has an optional, global `replace` field, which is similar to tags, and when `true` will remove all previously attached values of that data map. This is useful for datapacks that want to completely change the entire data map.
+
+## Adding values
+Values can be attached to objects using the `values` map. Each key will represent either the ID of an individual registry entry to attach the value to, or a tag key, preceeded by `#`. If it is a tag, the same value will be attached to all entries in that tag.  
+The key will be the object to attach.
+
+```js
+{
+    "values": {
+        // Attach a value to the carrot item
+        "minecraft:carrot": {
+            "amount": 12,
+            "chance": 1
+        },
+        // Attach a value to all items in the logs tag
+        "#minecraft:logs": {
+            "amount": 1,
+            "chance": 0.1
+        }
+    }
+}
+```
+
+:::info
+The above structure will invoke mergers in the case of [advanced data maps](./index.md#advanced-data-maps). If you do not want to invoke the merger for a specific object, then you will have to use a structure similar to this one:
+```js
+{
+    "values": {
+        // Overwrite the value of the carrot item
+        "minecraft:carrot": {
+            "replace": true,
+            // The new value will be under a value sub-object
+            "value": {
+                "amount": 12,
+                "chance": 1
+            }
+        }
+    }
+}
+```
+:::
+
+## Removing values
+
+A JSON file can also remove values previously attached to objects, through the use of the `remove` array:
+```js
+{
+    // Remove the value attached to apples and potatoes
+    "remove": ["minecraft:apple", "minecraft:potato"]
+}
+```
+The array contains a list of registry entry IDs or tags to remove the value from.
+
+:::warning
+Removals happen after the values in the current JSON file have been attached, so you can use the removal feature to remove a value attached to an object through a tag:
+```js
+{
+    "values": {
+        "#minecraft:logs": 12
+    },
+    // Remove the value from the acacia log, so that all logs but acacia have the value 12 attached to them
+    "remove": ["minecraft:acacia_log"]
+}
+```
+:::
+
+:::info
+In the case of [advanced data maps](./index.md#advanced-data-maps) that provide a custom remover, the arguments of the remover can be provided by transforming the `remove` array into a map.  
+Let's assume that the remover object is serialized as a string and removes the value with a given key for a `Map`-based data map:
+```js
+{
+    "remove": {
+        // The remover will be deserialized from the value (`somekey1` in this case)
+        // and applied to the value attached to the carrot item
+        "minecraft:carrot": "somekey1"
+    }
+}
+```
+:::