using Microsoft.Xna.Framework;
using Xunit;

namespace MrGameEng.Pathfinding.Tests;

public class FlowFieldTests
{
    [Fact]
    public void Build_DistancesGrowFromGoal_DirectionsDescend()
    {
        var grid = new TestGrid(".....", ".###.", ".....");
        var builder = new FlowFieldBuilder(grid);
        var field = new FlowField();

        builder.Build([new Point(0, 0)], field);

        Assert.Equal(0f, field.DistanceAt(0, 0));
        Assert.True(field.DistanceAt(4, 2) > field.DistanceAt(1, 0));

        // Из любой достижимой не-целевой клетки направление ведёт к клетке с меньшей дистанцией.
        for (var y = 0; y < grid.Height; y++)
        {
            for (var x = 0; x < grid.Width; x++)
            {
                if (
                    !grid.IsPassable(x, y)
                    || !field.IsReachable(x, y)
                    || field.DistanceAt(x, y) == 0f
                )
                {
                    continue;
                }

                var direction = field.DirectionAt(x, y);
                Assert.NotEqual(Vector2.Zero, direction);
                var nx = x + Math.Sign(MathF.Round(direction.X * 10f));
                var ny = y + Math.Sign(MathF.Round(direction.Y * 10f));
                Assert.True(
                    field.DistanceAt(nx, ny) < field.DistanceAt(x, y),
                    $"direction at ({x},{y}) does not descend"
                );
            }
        }
    }

    [Fact]
    public void Build_UnreachablePocket_IsFlagged()
    {
        var grid = new TestGrid("..#..", "..#..", "..#..");
        var builder = new FlowFieldBuilder(grid);
        var field = new FlowField();

        builder.Build([new Point(0, 1)], field);

        Assert.False(field.IsReachable(4, 1));
        Assert.Equal(Vector2.Zero, field.DirectionAt(4, 1));
        Assert.True(field.IsReachable(1, 2));
    }

    [Fact]
    public void Build_MultipleGoals_EachCellFlowsToNearest()
    {
        var grid = new TestGrid("..........");
        var builder = new FlowFieldBuilder(grid);
        var field = new FlowField();

        builder.Build([new Point(0, 0), new Point(9, 0)], field);

        Assert.True(field.DirectionAt(2, 0).X < 0f); // ближе к левой цели
        Assert.True(field.DirectionAt(7, 0).X > 0f); // ближе к правой
        Assert.Equal(0f, field.DistanceAt(9, 0));
    }

    [Fact]
    public void Build_Rebuild_OverwritesPreviousField()
    {
        var grid = new TestGrid(".....");
        var builder = new FlowFieldBuilder(grid);
        var field = new FlowField();

        builder.Build([new Point(0, 0)], field);
        Assert.True(field.DirectionAt(4, 0).X < 0f);

        builder.Build([new Point(4, 0)], field);
        Assert.True(field.DirectionAt(0, 0).X > 0f);
        Assert.Equal(0f, field.DistanceAt(4, 0));
    }

    [Fact]
    public void Build_NoValidGoals_EverythingUnreachable()
    {
        var grid = new TestGrid(".#.");
        var builder = new FlowFieldBuilder(grid);
        var field = new FlowField();

        builder.Build([new Point(1, 0)], field); // цель — стена

        Assert.False(field.IsReachable(0, 0));
        Assert.False(field.IsReachable(2, 0));
    }
}