AutoRail/src/main/java/com/autorail/pathing/WalkPathfinder.java

package com.autorail.pathing;

import net.minecraft.block.BlockState;
import net.minecraft.block.Blocks;
import net.minecraft.client.world.ClientWorld;
import net.minecraft.registry.tag.FluidTags;
import net.minecraft.util.math.BlockPos;
import net.minecraft.util.math.Direction;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.PriorityQueue;

/**
 * 自动走到远处的起始点：内置 A* 步行寻路（4 向 + 跳 1 格 + 落 3 格 + 可涉水）。
 *
 * 与铺轨寻路解耦：先走过去，再从起点开始铺轨；走不到则明确报错让用户手动接近。
 */
public class WalkPathfinder {

    public enum State { RUNNING, FOUND, FAILED }

    private static final int MARGIN = 32;
    private static final long MAX_EXPANSIONS = 200_000L;
    private static final float COST_JUMP = 1.7f;
    private static final float COST_WATER = 0.8f;

    private final ClientWorld world;
    private final BlockPos start;
    private final BlockPos goal;

    private final PriorityQueue<Node> open = new PriorityQueue<>(Comparator.comparingDouble(n -> n.f));
    private final Map<Long, Node> best = new HashMap<>();
    private final List<BlockPos> result = new ArrayList<>();

    private State state = State.RUNNING;
    private String failKey;
    private long expansions;
    private int opened;

    private int minX, minY, minZ, maxX, maxY, maxZ;

    public WalkPathfinder(ClientWorld world, BlockPos start, BlockPos goal) {
        this.world = world;
        this.start = start.toImmutable();
        this.goal = goal.toImmutable();
        computeRegion();
        Node s = new Node(this.start, null, 0f, h(this.start));
        best.put(this.start.asLong(), s);
        open.add(s);
        opened++;
    }

    public State getState() {
        return state;
    }

    public String getFailKey() {
        return failKey;
    }

    public List<BlockPos> getResult() {
        return result;
    }

    public int getOpened() {
        return opened;
    }

    /** 每帧分片执行，最多展开 budget 个节点，不卡客户端线程。 */
    public void step(int budget) {
        if (state != State.RUNNING) {
            return;
        }
        for (int b = 0; b < budget; b++) {
            Node cur = poll();
            if (cur == null) {
                fail("autorail.err.walkFail");
                return;
            }
            if (cur.pos.equals(goal)) {
                rebuild(cur);
                state = State.FOUND;
                return;
            }
            expand(cur);
            if (++expansions > MAX_EXPANSIONS) {
                fail("autorail.err.walkFail");
                return;
            }
        }
    }

    private Node poll() {
        while (true) {
            Node n = open.poll();
            if (n == null) {
                return null;
            }
            if (best.get(n.pos.asLong()) == n) {
                return n;
            }
        }
    }

    private void expand(Node cur) {
        for (Direction d : Direction.Type.HORIZONTAL) {
            BlockPos base = cur.pos.offset(d);
            // 跳上 1 格
            if (passHead(cur.pos.up(2))) {
                tryMove(cur, base.up(), COST_JUMP, true);
            }
            // 平走 / 涉水
            tryMove(cur, base, 1.0f, false);
            // 落下 1-3 格
            for (int f = 1; f <= 3; f++) {
                tryMove(cur, base.down(f), 1.0f + 0.25f * f, false);
            }
        }
    }

    private void tryMove(Node cur, BlockPos target, float baseCost, boolean jump) {
        if (!inRegion(target) || !walkable(target) || !passHead(target.up())) {
            return;
        }
        BlockState sup = world.getBlockState(target.down());
        boolean solid = sup.isSolidBlock(world, target.down());
        boolean water = sup.getFluidState().isIn(FluidTags.WATER);
        if (!solid && !water) {
            return;
        }
        if (jump && !solid) {
            return; // 跳跃必须落在实心方块上
        }
        float cost = baseCost;
        if (world.getBlockState(target).getFluidState().isIn(FluidTags.WATER)) {
            cost += COST_WATER;
        }
        float g = cur.g + cost;
        long key = target.asLong();
        Node old = best.get(key);
        if (old == null || g < old.g - 1.0e-4f) {
            Node n = new Node(target, cur, g, g + h(target));
            best.put(key, n);
            open.add(n);
            opened++;
        }
    }

    private void rebuild(Node goalNode) {
        result.clear();
        for (Node n = goalNode; n != null; n = n.parent) {
            result.add(n.pos);
        }
        java.util.Collections.reverse(result);
    }

    private void computeRegion() {
        minX = Math.min(start.getX(), goal.getX()) - MARGIN;
        maxX = Math.max(start.getX(), goal.getX()) + MARGIN;
        minZ = Math.min(start.getZ(), goal.getZ()) - MARGIN;
        maxZ = Math.max(start.getZ(), goal.getZ()) + MARGIN;
        minY = Math.max(world.getBottomY() + 1, Math.min(start.getY(), goal.getY()) - MARGIN);
        maxY = Math.min(world.getTopYInclusive() - 2, Math.max(start.getY(), goal.getY()) + MARGIN);
    }

    private boolean inRegion(BlockPos p) {
        return p.getX() >= minX && p.getX() <= maxX
                && p.getY() >= minY && p.getY() <= maxY
                && p.getZ() >= minZ && p.getZ() <= maxZ;
    }

    private float h(BlockPos p) {
        return 1.0f * (Math.abs(p.getX() - goal.getX())
                + Math.abs(p.getY() - goal.getY())
                + Math.abs(p.getZ() - goal.getZ()));
    }

    private void fail(String key) {
        failKey = key;
        state = State.FAILED;
    }

    /** 站立格：空气 / 可替换 / 铁轨（无碰撞）/ 水。岩浆不可走。 */
    private boolean walkable(BlockPos pos) {
        BlockState s = world.getBlockState(pos);
        return s.isAir()
                || (s.getFluidState().isEmpty() && s.isReplaceable())
                || s.isOf(Blocks.RAIL)
                || s.isOf(Blocks.POWERED_RAIL)
                || s.isOf(Blocks.DETECTOR_RAIL)
                || s.getFluidState().isIn(FluidTags.WATER);
    }

    private boolean passHead(BlockPos pos) {
        return walkable(pos);
    }

    private static final class Node {
        final BlockPos pos;
        final Node parent;
        final float g;
        final float f;

        Node(BlockPos pos, Node parent, float g, float f) {
            this.pos = pos;
            this.parent = parent;
            this.g = g;
            this.f = f;
        }
    }
}