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====== Jigsaws ======
Jigsaws are good for advanced structures such as dungeons & villages, and allow you to spend more time on actually building content vs. messing with procedural generation code. 

A repository with the finished code from this tutorial can be found [[https://github.com/Draylar/jigsaw-example-mod|here for 1.14 - 1.16]].
A full example with structure files and more can be found [[https://github.com/TelepathicGrunt/StructureTutorialMod/tree/1.18.x-Fabric-Jigsaw|here for 1.15 - 1.18]].

===== Creating a StructureFeature =====

A ''StructureFeature'' is an advanced ''Feature'': it keeps track of its location and bounds, and also has the ability to generate itself from a structure file((While you can generate your StructureFeature from a ''.nbt'' file, most vanilla StructureFeatures simply override the ''generate'' method inside their given Piece class.)). If it helps, you can think of it as a ''Structure'' + ''Feature''. We'll need to create one for our jigsaw generated structure. To start, create a class that extends ''StructureFeature<DefaultFeatureConfig>''((AbstractTempleFeature is another option. It automatically spaces out the structures similar to the existing temples-- this logic is also used by villages.)). Feature naming convention is "structure name" + "Feature"; a few vanilla examples are ''EndCityFeature'', ''OceanRuinFeature'', and ''VillageFeature''.

//Note: while Feature is the proper name for something generated in the world, we'll refer to our addition as a Structure. This is to distinguish between a StructureFeature and a standard Feature.//

We'll keep the constructor as-is. The ''StructurePoolFeatureConfig'' parameter is the structure config:

<code java [enable_line_numbers="false"]>
public ExampleFeature(Codec<StructurePoolFeatureConfig> codec) {
    super(codec);
}
</code>

//shouldStartAt// answers the question "should I start generating at the given Chunk?" ''AbstractTempleFeature'' is a class offered by vanilla that offers an answer to this question: it guarantees each structure is spaced out from others of the same type. The standard village feature uses the same logic. By returning true, every chunk will have your feature.

<code java [enable_line_numbers="false"]>
@Override
protected boolean shouldStartAt(ChunkGenerator chunkGenerator, BiomeSource biomeSource, long l, ChunkRandom chunkRandom, int i, int j, Biome biome, ChunkPos chunkPos, StructurePoolFeatureConfig featureConfig) {
    return chunkRandom.nextInt(150) == 0; // 1 in 150
}
</code>

Finally, //getStructureStartFactory// is the generation portion of your StructureFeature. You'll have to return a factory method to create new StructureStarts-- we can simply method reference the constructor. Our implementation will look like this, with ''ExampleStructure.Start'' being the next step in this tutorial:
<code java [enable_line_numbers="false"]>
@Override
public StructureStartFactory<StructurePoolFeatureConfig> getStructureStartFactory() {
    return Start::new;
}
</code>

==== Creating a ExampleFeature.Start class ====

''Start'' is like the initialization stage of generating our structure in the world. For now, create a simple child class with a constructor that also overrides initialize:
<code java [enable_line_numbers="true"]>
public static class Start extends StructureStart<StructurePoolFeatureConfig> {

    Start(StructureFeature<StructurePoolFeatureConfig> feature, int x, int z, BlockBox box, int int_3, long seed) {
        super(feature, x, z, box, int_3, seed);
    }

    @Override
    public void init(ChunkGenerator chunkGenerator, StructureManager structureManager, int x, int z, Biome biome,
                     StructurePoolFeatureConfig config) {
 
    }
}
</code>

Now all we have to do is add our starting piece in our ''init'' method:
<code java [enable_line_numbers="false"]>
@Override
public void init(ChunkGenerator chunkGenerator, StructureManager structureManager, int x, int z, Biome biome,
                 StructurePoolFeatureConfig config) {
    BlockPos pos = new BlockPos(x * 16, 80, z * 16);

    boolean randomYPos = false;
    boolean calculateMaxYFromPiecePositions = false;

    ExamplePiece.init();

    StructurePoolBasedGenerator.method_30419(config.getStartPool(), config.getSize(), ExamplePiece::new, chunkGenerator, structureManager,
            pos, children, random, calculateMaxYFromPiecePositions, randomYPos);

    setBoundingBoxFromChildren();
}
</code>

The Identifier is the starting pool to select from, the int is the size of the entire structure (with 7 being "7 squares out"), and the 3rd argument is a factory for the piece we'll register in a second.

Our finalized ''ExampleFeature'' class:
<code java [enable_line_numbers="true"]>
public class ExampleFeature extends StructureFeature<StructurePoolFeatureConfig> {

    public ExampleFeature(Codec<StructurePoolFeatureConfig> codec) {
        super(codec);
    }

    @Override
    public StructureStartFactory<StructurePoolFeatureConfig> getStructureStartFactory() {
        return Start::new;
    }

    @Override
    protected boolean shouldStartAt(ChunkGenerator chunkGenerator, BiomeSource biomeSource, long l, ChunkRandom chunkRandom, int i, int j, Biome biome, ChunkPos chunkPos, StructurePoolFeatureConfig featureConfig) {
        return chunkRandom.nextInt(150) == 0; // 1 in 150
    }

    public static class Start extends StructureStart<StructurePoolFeatureConfig> {

        Start(StructureFeature<StructurePoolFeatureConfig> feature, int x, int z, BlockBox box, int int_3, long seed) {
            super(feature, x, z, box, int_3, seed);
        }

        @Override
        public void init(ChunkGenerator chunkGenerator, StructureManager structureManager, int x, int z, Biome biome,
                         StructurePoolFeatureConfig config) {
            BlockPos pos = new BlockPos(x * 16, 80, z * 16);

            boolean randomYPos = false;
            boolean calculateMaxYFromPiecePositions = false;

            ExamplePiece.init();

            StructurePoolBasedGenerator.addPieces(config.startPool, config.size, ExamplePiece::new, chunkGenerator, structureManager,
                    pos, children, random, calculateMaxYFromPiecePositions, randomYPos);

            setBoundingBoxFromChildren();
        }
    }
}
</code>

===== Creating a Piece =====
This portion is very simple. A piece represents one section or element in your full structure. You'll need to create a basic piece class, and we'll register it later:
<code java [enable_line_numbers="false"]>
public class ExamplePiece extends PoolStructurePiece {

    ExamplePiece(StructureManager structureManager_1, StructurePoolElement structurePoolElement_1, BlockPos blockPos_1, int int_1, BlockRotation blockRotation_1, BlockBox mutableIntBoundingBox_1) {
        super(TutorialJigsaws.EXAMPLE_PIECE, structureManager_1, structurePoolElement_1, blockPos_1, int_1, blockRotation_1, mutableIntBoundingBox_1);
    }

    public ExamplePiece(StructureManager manager, CompoundTag tag) {
        super(manager, tag, TutorialJigsaws.EXAMPLE_PIECE);
    }
}
</code>

Where ''ExampleMod.EXAMPLE_PIECE'' is a reference to our registered piece.


 In a static block at the top of our class, we're going to register our structure pools using ''StructurePoolBasedGenerator.REGISTRY'':
<code java [enable_line_numbers="true"]>
public static void init() { }

static {
    StructurePoolBasedGenerator.REGISTRY.add(
            new StructurePool(
                    TutorialJigsaws.BASE_POOL,
                    new Identifier("empty"),
                    ImmutableList.of(
                            Pair.of(new LegacySinglePoolElement("tutorial:black_square"), 1),
                            Pair.of(new LegacySinglePoolElement("tutorial:white_square"), 1)
                    ),
                    StructurePool.Projection.RIGID
            )
    );

    StructurePoolBasedGenerator.REGISTRY.add(
            new StructurePool(
                    TutorialJigsaws.COLOR_POOL,
                    new Identifier("empty"),
                    ImmutableList.of(
                            Pair.of(new LegacySinglePoolElement("tutorial:lime_square"), 1),
                            Pair.of(new LegacySinglePoolElement("tutorial:magenta_square"), 1),
                            Pair.of(new LegacySinglePoolElement("tutorial:orange_square"), 1),
                            Pair.of(new LegacySinglePoolElement("tutorial:light_blue_square"), 1)
                    ),
                    StructurePool.Projection.RIGID
            )
    );
}
</code>

Here, we're registering 2 pools (base & color) and then adding their respective children to them. The StructurePool constructor is as follows:
  * registry name of the pool, same as target pool at top of a jigsaw
  * @Draylar if you know what this one does
  * a list of pool elements
  * the projection type of the pool

For the list of elements, we add Pairs((com.mojang.datafixers.util)) of pool elements and integers. The string passed into the element is the location of the structure in the data directory, and the int is the weight of the element within the entire target pool. Using 1 for each element ensures each one will be picked evenly.

The projection is how the pool is placed in the world. Rigid means it will be placed directly as is, and terrain matching means it will be bent to sit on top of the terrain. The latter may be good for a wheat field structure that moves with the terrain shape, whereas the first would be better for houses with solid floors.


==== Jigsaws and pieces ====

To understand what happens here, we'll have to dive into jigsaws (https://minecraft.gamepedia.com/Jigsaw_Block) and structure blocks (https://minecraft.gamepedia.com/Structure_Block).

Structure Blocks are a simple way of saving a structure to a .nbt file for future use. Jigsaws are a component of structure blocks that assemble multiple structures into a single one; similar to normal jigsaws, each piece of the structure connects at a jigsaw block, which is like a connection wedge in a puzzle piece. We'll assume you're familiar with saving structures-- if you aren't, read up on the structure block page before going any further.

The jigsaw menu consists of 3 fields:

  * target pool
  * attachment type
  * turns into

{{ https://i.imgur.com/owaJ0k2.png?nolink&600 |Blank Jigsaw}}

When thinking about this as a puzzle, the target pool is the group of puzzle pieces you can search through. If you have a total of 10 pieces, one target pool may have 7 of the total pieces. This field is how a jigsaw specifies, "Hi, I'd like a piece from group B to connect to me!" In the case of a village, this may be a road saying, "Give me a house!" The target pools of 2 jigsaws do not have to match: the requestor gets to decide who they select from. It is **not** defining what type //the given// jigsaw block is, but rather what type should be on the //other side//.

The attachment type can be seen as a more specific filter within target pools-- a jigsaw can only connect to other jigsaws with the same attachment type. This is like the type of connector on a puzzle piece. The usages for this are a little bit more specific.

Finally, the "turns into" field is simply what the jigsaw is replaced with after it finds a match. If the jigsaw is inside your cobblestone floor, it should probably turn into cobblestone.

Here's an example implementation: the given jigsaw will draw from the //tutorial:my_pool// structure pool, looks for any jigsaws with the //tutorial:any// type, and turns into cobblestone when it's done.

{{ https://i.imgur.com/f9tP2sv.png?nolink&600 |Example Finished Jigsaw}}

Our finalized structure will consist of multiple colored squares connecting to each other. It will have a white or a black square in the center, and orange, magenta, light blue, and lime squares branching off on the sides randomly. Here is the setup of our 2 initial squares:

{{ https://i.imgur.com/dVFADy8.png?nolink&400 |Initial Squares}}

This jigsaw will ask for any other jigsaw that:
  * is in the //tutorial:color_pool// target pool
  * has an attachment type of //tutorial:square_edge//
It then turns into white concrete to match the rest of the platform.

For demo purposes, we've made 2 starting platforms: one is white, and one is black. The only difference is what they turn into. We'll save these as structure files using structure blocks:

{{ https://i.imgur.com/31LAORw.png?nolink&400 |Finalized Initial Squares}}

For our randomized edge platforms, we've made 4 extra squares of different colors. Again, despite being used for a different purpose, the jigsaw construction is //the same// aside from the "turns into" field.

{{ https://i.imgur.com/OngxweJ.png?nolink&400 |Colored Squares}}

We now have 6 saved ''.nbt'' files. These can be found in our world save folder under ''generated'':

{{ https://i.imgur.com/ZKIoZT9.png?nolink&400 |Saved NBT files}}

For usage, we'll move these to ''resources/data/tutorial/structures'', where "tutorial" is your modid:

{{ https://i.imgur.com/kaiy84U.png?nolink&400 |Moved NBT files}}

The setup is complete! We now have 6 total squares. Let's briefly recap the goal:
  * have a white or black square selected as the center for our structure
  * have a pool of the 4 other colors
  * branch off from the center square with our 4 extra colors

Let's head back to our ''TutorialJigsaws'' class. We'll need 2 Identifiers to label our 2 pools (black&white, 4 colors):
<code java [enable_line_numbers="false"]>
private static final Identifier BASE_POOL = new Identifier("tutorial:base_pool");
private static final Identifier COLOR_POOL = new Identifier("tutorial:color_pool");
</code>

Remember: every jigsaw ends up searching through the color pool, but we still have a base pool! This is to keep our black & white squares out of the outside generated squares. It's also going to be our origin pool, where we randomly select 1 structure from to begin our generation.

===== Registering Everything =====
We'll need to register our structure as a structure feature, and also register our piece.
<code java [enable_line_numbers="true"]>
public static final StructureFeature<StructurePoolFeatureConfig> FEATURE = StructureFeature.register(
        "tutorial:example_feature",
        new ExampleFeature(StructurePoolFeatureConfig.CODEC),
        GenerationStep.Feature.SURFACE_STRUCTURES
);

public static final StructurePieceType EXAMPLE_PIECE = StructurePieceType.register(
        ExamplePiece::new,
        "tutorial:example_piece"
);
</code>

===== Spawning Our Structure =====
Finally, we'll have to spawn our structure. A basic example which adds it to every biome is:
<code java [enable_line_numbers="false"]>
public static final ConfiguredStructureFeature<StructurePoolFeatureConfig, ? extends StructureFeature<StructurePoolFeatureConfig>> FEATURE_CONFIGURED
        = FEATURE.configure(new StructurePoolFeatureConfig(BASE_POOL, 7));

static {
    StructuresConfigAccessor.setDefaultStructures(
            new ImmutableMap.Builder<StructureFeature<?>, StructureConfig>()
                    .putAll(StructuresConfig.DEFAULT_STRUCTURES)
                    .put(TutorialJigsaws.FEATURE, new StructureConfig(32, 8, 10387312))
                    .build()
    );
}

@Override
public void onInitialize() {
    Registry.BIOME.forEach(biome -> {
        biome.addStructureFeature(FEATURE_CONFIGURED);
    });
}
</code>
===== Finished! =====
As you can see, we have a single white square in the center, with boxes going off the edges. Note that the radius in this screenshot was increased to 14 instead of the 7 used in the tutorial.

{{ https://i.imgur.com/qndZzZu.png?nolink&600 |Finalized}}
===== Jigsaw Tips =====
Ideally, you do not want structure pieces to be bigger than 32x32x32, so breaking them into chunk-sized pieces is the best option. 
You cannot generate other structure pieces of the same pool through jigsaws. So, if you have a piece in pool A and you try to generate another piece, you will have to have another pool.