using System;
using System.Diagnostics.CodeAnalysis;

namespace Syntriax.Engine.Core;

/// <summary>
/// Represents a 2D line segment defined by two endpoints.
/// </summary>
/// <param name="from">The starting point of the <see cref="Line2D"/> segment.</param>
/// <param name="to">The ending point of the <see cref="Line2D"/> segment.</param>
/// <remarks>
/// Initializes a new instance of the <see cref="Line2D"/> struct with the specified endpoints.
/// </remarks>
[System.Diagnostics.DebuggerDisplay("From: {From.ToString(),nq}, To: {To.ToString(),nq}, Direction: {Direction.ToString(),nq}, Length: {Length}")]
public readonly struct Line2D(Vector2D from, Vector2D to)
{
    /// <summary>
    /// The starting point of the <see cref="Line2D"/> segment.
    /// </summary>
    public readonly Vector2D From = from;

    /// <summary>
    /// The ending point of the <see cref="Line2D"/> segment.
    /// </summary>
    public readonly Vector2D To = to;

    /// <summary>
    /// The reversed <see cref="Line2D"/> segment.
    /// </summary>
    public readonly Line2D Reversed => new(To, From);

    /// <summary>
    /// The normalized direction <see cref="Vector2D"/> of the <see cref="Line2D"/> segment.
    /// </summary>
    public readonly Vector2D Direction => From.FromTo(To).Normalize();

    /// <summary>
    /// The length of the <see cref="Line2D"/> segment.
    /// </summary>
    public readonly float Length => From.FromTo(To).Length();

    /// <summary>
    /// The squared length of the <see cref="Line2D"/> segment.
    /// </summary>
    public readonly float LengthSquared => From.FromTo(To).LengthSquared();

    /// <summary>
    /// The equation of the <see cref="Line2D"/> defined by this <see cref="Line2D"/> segment.
    /// </summary>
    public static Line2DEquation GetLineEquation(Line2D line)
    {
        Vector2D slopeVector = line.From.FromTo(line.To);
        float slope = slopeVector.Y / slopeVector.X;

        float yOffset = line.From.Y - (slope * line.From.X);

        return new Line2DEquation(slope, yOffset);
    }

    /// <summary>
    /// Determines whether the specified <see cref="Vector2D"/> lies on the <see cref="Line2D"/>.
    /// </summary>
    public static bool Intersects(Line2D line, Vector2D point)
        => Line2DEquation.Resolve(GetLineEquation(line), point.X).ApproximatelyEquals(point.Y);

    /// <summary>
    /// Calculates the parameter 't' representing the point's position on the <see cref="Line2D"/> segment.
    /// </summary>
    public static float GetT(Line2D line, Vector2D point)
    {
        float fromX = MathF.Abs(line.From.X);
        float toX = MathF.Abs(line.To.X);
        float pointX = MathF.Abs(point.X);

        float min = MathF.Min(fromX, toX);
        float max = MathF.Max(fromX, toX) - min;

        pointX -= min;

        float t = pointX / max;

        // FIXME
        // I don't even know, apparently whatever I wrote up there doesn't take into account of the direction of the line
        // Which... I can see how, but I am also not sure how I can make it take into account. Or actually I'm for some reason
        // too unmotivated to find a solution. Future me, find a better way if possible, please.
        if (!Lerp(line, t).ApproximatelyEquals(point))
            return 1f - t;
        return t;
    }

    /// <summary>
    /// Checks if the <see cref="Line2D"/> segment intersects with another <see cref="Line2D"/> segment.
    /// </summary>
    public static bool Intersects(Line2D left, Line2D right)
    {
        int o1 = Vector2D.Orientation(left.From, left.To, right.From);
        int o2 = Vector2D.Orientation(left.From, left.To, right.To);
        int o3 = Vector2D.Orientation(right.From, right.To, left.From);
        int o4 = Vector2D.Orientation(right.From, right.To, left.To);

        if (o1 != o2 && o3 != o4)
            return true;

        if (o1 == 0 && OnSegment(left, right.From)) return true;
        if (o2 == 0 && OnSegment(left, right.To)) return true;
        if (o3 == 0 && OnSegment(right, left.From)) return true;
        if (o4 == 0 && OnSegment(right, left.To)) return true;

        return false;
    }

    /// <summary>
    /// Checks if the point lies within the <see cref="Line2D"/> segment.
    /// </summary>
    public static bool OnSegment(Line2D line, Vector2D point)
    {
        if (point.X <= MathF.Max(line.From.X, line.To.X) && point.X >= MathF.Min(line.From.X, line.To.X) &&
            point.Y <= MathF.Max(line.From.Y, line.To.Y) && point.Y >= MathF.Min(line.From.Y, line.To.Y))
            return true;

        return false;
    }

    /// <summary>
    /// Determines whether two <see cref="Line2D"/> segments intersect.
    /// </summary>
    public static bool Intersects(Line2D left, Line2D right, [NotNullWhen(returnValue: true)] out Vector2D? point)
    {
        point = null;

        bool result = Intersects(left, right);

        if (result)
            point = IntersectionPoint(left, right);

        return result;
    }

    /// <summary>
    /// Finds the point of intersection between two <see cref="Line2D"/> segments.
    /// </summary>
    public static Vector2D IntersectionPoint(Line2D left, Line2D right)
        => Vector2D.Lerp(left.From, left.To, IntersectionParameterT(left, right));

    /// <summary>
    /// Calculates the parameter 't' representing the intersection point's position on the <see cref="Line2D"/> segment.
    /// </summary>
    public static float IntersectionParameterT(Line2D left, Line2D right)
    {
        float numerator = (left.From.X - right.From.X) * (right.From.Y - right.To.Y) - (left.From.Y - right.From.Y) * (right.From.X - right.To.X);
        float denominator = (left.From.X - left.To.X) * (right.From.Y - right.To.Y) - (left.From.Y - left.To.Y) * (right.From.X - right.To.X);

        // Lines are parallel
        if (denominator == 0)
            return float.NaN;

        return numerator / denominator;
    }

    /// <summary>
    /// Linearly interpolates between the two endpoints of the <see cref="Line2D"/> segment using parameter 't'.
    /// </summary>
    public static Vector2D Lerp(Line2D line, float t)
        => Vector2D.Lerp(line.From, line.To, t);

    /// <summary>
    /// Calculates the closest point on the <see cref="Line2D"/> segment to the specified point.
    /// </summary>
    public static Vector2D ClosestPointTo(Line2D line, Vector2D point)
    {
        Vector2D edgeVector = line.From.FromTo(line.To);
        Vector2D pointVector = point - line.From;

        float t = (pointVector.X * edgeVector.X + pointVector.Y * edgeVector.Y) / (edgeVector.X * edgeVector.X + edgeVector.Y * edgeVector.Y);

        t = MathF.Max(0, MathF.Min(1, t));

        float closestX = line.From.X + t * edgeVector.X;
        float closestY = line.From.Y + t * edgeVector.Y;

        return new Vector2D((float)closestX, (float)closestY);
    }

    /// <summary>
    /// Checks if two <see cref="Line2D"/> segments are approximately equal.
    /// </summary>
    /// <param name="left">The first <see cref="Line2D"/>.</param>
    /// <param name="right">The second <see cref="Line2D"/>.</param>
    /// <param name="epsilon">The epsilon range.</param>
    /// <returns><see cref="true"/> if the <see cref="Line2D"/>s are approximately equal; otherwise, <see cref="false"/>.</returns>
    public static bool ApproximatelyEquals(Line2D left, Line2D right, float epsilon = float.Epsilon)
        => left.From.ApproximatelyEquals(right.From, epsilon) && left.To.ApproximatelyEquals(right.To, epsilon);
}

/// <summary>
/// Provides extension methods for the <see cref="Line2D"/> struct.
/// </summary>
public static class Line2DExtensions
{
    /// <inheritdoc cref="Line2D.Lerp(Line2D, float)" />
    public static Vector2D Lerp(this Line2D line, float t) => Line2D.Lerp(line, t);

    /// <inheritdoc cref="Line2D.GetLineEquation(Line2D)" />
    public static Line2DEquation ToLineEquation(this Line2D line) => Line2D.GetLineEquation(line);

    /// <inheritdoc cref="Line2D.Intersects(Line2D, Vector2D)" />
    public static bool Intersects(this Line2D line, Vector2D point) => Line2D.Intersects(line, point);

    /// <inheritdoc cref="Line2D.GetT(Line2D, Vector2D)" />
    public static float GetT(this Line2D line, Vector2D point) => Line2D.GetT(line, point);

    /// <inheritdoc cref="Line2D.Intersects(Line2D, Line2D)" />
    public static bool Intersects(this Line2D left, Line2D right) => Line2D.Intersects(left, right);

    /// <inheritdoc cref="Line2D.Intersects(Line2D, Line2D, out Vector2D?)" />
    public static bool Intersects(this Line2D left, Line2D right, [NotNullWhen(returnValue: true)] out Vector2D? point) => Line2D.Intersects(left, right, out point);

    /// <inheritdoc cref="Line2D.ApproximatelyEquals(Line2D, Line2D, float)" />
    public static bool ApproximatelyEquals(this Line2D left, Line2D right, float epsilon = float.Epsilon) => Line2D.ApproximatelyEquals(left, right, epsilon);
}