using System.Collections.Generic;
using UnityEngine;

namespace Atlas.Core
{
    /// <summary>
    /// Used to generate triangles for meshes from a given array of border points.
    /// </summary>
    public class MeshTriangulator
    {
        private readonly List<Vector2> borderPoints = new List<Vector2>();

        public MeshTriangulator(Vector2[] points)
        {
            borderPoints = new List<Vector2>(points);
        }

        public int[] Triangulate()
        {
            var indices = new List<int>();
            var n = borderPoints.Count;

            if (n < 3)
                return indices.ToArray();

            var V = new int[n];

            if (Area() > 0)
            {
                for (var v = 0; v < n; v++)
                    V[v] = v;
            }
            else
            {
                for (var v = 0; v < n; v++)
                    V[v] = (n - 1) - v;
            }

            var nv = n;
            var count = 2 * nv;

            for (int m = 0, v = nv - 1; nv > 2;)
            {
                if ((count--) <= 0)
                    return indices.ToArray();

                var u = v;

                if (nv <= u)
                    u = 0;

                v = u + 1;

                if (nv <= v)
                    v = 0;

                var w = v + 1;

                if (nv <= w)
                    w = 0;

                if (Snip(u, v, w, nv, V))
                {
                    int a, b, c, s, t;
                    a = V[u];
                    b = V[v];
                    c = V[w];
                    indices.Add(a);
                    indices.Add(b);
                    indices.Add(c);
                    m++;

                    for (s = v, t = v + 1; t < nv; s++, t++)
                        V[s] = V[t];

                    nv--;
                    count = 2 * nv;
                }
            }

            indices.Reverse();
            return indices.ToArray();
        }

        private float Area()
        {
            var n = borderPoints.Count;
            var A = 0.0f;

            for (int p = n - 1, q = 0; q < n; p = q++)
            {
                var pval = borderPoints[p];
                var qval = borderPoints[q];
                A += pval.x * qval.y - qval.x * pval.y;
            }

            return (A * 0.5f);
        }

        private bool Snip(int u, int v, int w, int n, int[] V)
        {
            int p;
            var A = borderPoints[V[u]];
            var B = borderPoints[V[v]];
            var C = borderPoints[V[w]];

            if (Mathf.Epsilon > (((B.x - A.x) * (C.y - A.y)) - ((B.y - A.y) * (C.x - A.x))))
                return false;

            for (p = 0; p < n; p++)
            {
                if ((p == u) || (p == v) || (p == w))
                    continue;

                var P = borderPoints[V[p]];

                if (InsideTriangle(A, B, C, P))
                    return false;
            }

            return true;
        }

        private bool InsideTriangle(Vector2 A, Vector2 B, Vector2 C, Vector2 P)
        {
            float ax, ay, bx, by, cx, cy, apx, apy, bpx, bpy, cpx, cpy;
            float cCROSSap, bCROSScp, aCROSSbp;

            ax = C.x - B.x; ay = C.y - B.y;
            bx = A.x - C.x; by = A.y - C.y;
            cx = B.x - A.x; cy = B.y - A.y;
            apx = P.x - A.x; apy = P.y - A.y;
            bpx = P.x - B.x; bpy = P.y - B.y;
            cpx = P.x - C.x; cpy = P.y - C.y;

            aCROSSbp = ax * bpy - ay * bpx;
            cCROSSap = cx * apy - cy * apx;
            bCROSScp = bx * cpy - by * cpx;

            return ((aCROSSbp >= 0.0f) && (bCROSScp >= 0.0f) && (cCROSSap >= 0.0f));
        }
    }
}