70 lines
2.5 KiB
Typst
70 lines
2.5 KiB
Typst
#import "/lib.typ": note, tip, todo
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= Core Mechanics
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== Entity IDs
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Entities are the dynamic movable objects in the game. Minecraft tracks every entity with a unique
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ID number. Whenever an entity is added to the game world, a global counter is incremented
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and the new entity is assigned the current value.
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A large number of game objects are tracked in this way, and so creating any one of them will increment that global
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counter. To reiterate, *this is a global counter, shared for all entities in the game*.
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== Stationary Item Optimization
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Typically, item entities fall to the floor and sit stationary soon after being created. Processing
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movement for these stationary entities would be wasteful, so an optimization was added in 1.14: if
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an item is sitting stationary on a block, then only check for gravity every 4 ticks.
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Here is the relevant code path for the optimization.
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#note[ `onGround` is only ever updated from `move()`, and this branch is the *only* place that
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`move()` is called. ]
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```java
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public class ItemEntity extends Entity {
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public void tick() {
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...
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if (this.onGround() && !(this.getDeltaMovement().horizontalDistanceSqr() > (double)1.0E-5F) && (this.tickCount + this.getId()) % 4 != 0) {
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...
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} else {
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this.move(MoverType.SELF, this.getDeltaMovement());
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...
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}
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...
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}
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}
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public class Entity {
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public void move(final MoverType moverType, Vec3 delta) {
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...
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this.setOnGroundWithMovement(this.verticalCollisionBelow, this.horizontalCollision, movement);
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...
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}
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public void setOnGroundWithMovement(final boolean onGround, final boolean horizontalCollision, final Vec3 movement) {
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this.onGround = onGround;
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...
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}
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}
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```
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== Observable Drop Delay
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The effect is that if the item is on a block and has negligible horizontal momentum, it *cannot
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fall* until the condition is satisfied, even if it's no longer on the ground. It can fall early if
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it gains horizontal momentum or is pushed.
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#tip[ You can rewrite the condition as `(age % 4 == (-id) % 4)` ]
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#todo[ An animation. Spawn a few items, note their ID and age. Let them settle on a block,
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then remove that supporting block. There will be a few ticks where the item hovers in place before
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it falls. ]
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For simplicity, suppose we spawn all our items on the same tick. Then `this.tickCount` will be the
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same for all of them, and we only need to consider the ID.
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The result of this is that if you spawn several item entities, allow them to settle on a block, then
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remove that block
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