package com.mythlane.chainlightning.chain;

import java.util.List;

import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertTrue;

final class ParticleTrailTest {

    private static final double EPS = 1e-9;

    @Test
    void zeroDistance() {
        Vec3 p = new Vec3(2, 3, 4);
        List<Vec3> out = ParticleTrail.sample(p, p, 4.0);
        assertTrue(out.isEmpty());
    }

    @Test
    void subBlockDistance() {
        // distance 0.5 along X with density 4 → ceil(2.0) = 2
        Vec3 from = new Vec3(0, 0, 0);
        Vec3 to = new Vec3(0.5, 0, 0);
        List<Vec3> out = ParticleTrail.sample(from, to, 4.0);
        assertEquals(2, out.size());
        // Both points strictly between endpoints
        for (Vec3 v : out) {
            assertTrue(from.distanceSquared(v) > EPS, "sample equals from");
            assertTrue(to.distanceSquared(v) > EPS, "sample equals to");
        }
    }

    @Test
    void integerDistanceCount() {
        Vec3 from = new Vec3(0, 0, 0);
        Vec3 to = new Vec3(5, 0, 0);
        List<Vec3> out = ParticleTrail.sample(from, to, 4.0);
        assertEquals(20, out.size());
    }

    @Test
    void fractionalDistanceCount() {
        Vec3 from = new Vec3(0, 0, 0);
        Vec3 to = new Vec3(3.7, 0, 0);
        List<Vec3> out = ParticleTrail.sample(from, to, 4.0);
        // ceil(3.7 * 4) = ceil(14.8) = 15
        assertEquals(15, out.size());
    }

    @Test
    void endpointsExcluded() {
        Vec3 from = new Vec3(1, 2, 3);
        Vec3 to = new Vec3(7, 8, 9);
        List<Vec3> out = ParticleTrail.sample(from, to, 4.0);
        assertTrue(out.size() > 0);
        for (Vec3 v : out) {
            assertTrue(from.distanceSquared(v) > EPS, "sample equals from endpoint");
            assertTrue(to.distanceSquared(v) > EPS, "sample equals to endpoint");
        }
    }

    @Test
    void uniformSpacing() {
        Vec3 from = new Vec3(0, 0, 0);
        Vec3 to = new Vec3(5, 0, 0);
        List<Vec3> out = ParticleTrail.sample(from, to, 4.0);
        assertEquals(20, out.size());
        double total = from.distance(to);
        double expectedSpacing = total / (out.size() + 1);

        // from -> first
        assertEquals(expectedSpacing, from.distance(out.get(0)), EPS);
        // last -> to
        assertEquals(expectedSpacing, out.get(out.size() - 1).distance(to), EPS);
        // consecutive
        for (int i = 1; i < out.size(); i++) {
            assertEquals(expectedSpacing, out.get(i - 1).distance(out.get(i)), EPS);
        }
    }
}