// Copyright 2001, softSurfer (www.softsurfer.com)
// This code may be freely used and modified for any purpose
// providing that this copyright notice is included with it.
// SoftSurfer makes no warranty for this code, and cannot be held
// liable for any real or imagined damage resulting from its use.
// Users of this code must verify correctness for their application.

// Assume that a class is already given for the object:
//    Point with coordinates {float x, y;}
//===================================================================

// isLeft(): tests if a point is Left|On|Right of an infinite line.
//    Input:  three points P0, P1, and P2
//    Return: >0 for P2 left of the line through P0 and P1
//            =0 for P2 on the line
//            <0 for P2 right of the line
//    See: the January 2001 Algorithm on Area of Triangles

#include "hull.h"


inline float
isLeft( Point P0, Point P1, Point P2 )
{
    return (P1.x - P0.x)*(P2.y - P0.y) - (P2.x - P0.x)*(P1.y - P0.y);
}
//===================================================================
 

// chainHull_2D(): Andrew's monotone chain 2D convex hull algorithm
//     Input:  P[] = an array of 2D points
//                   presorted by increasing x- and y-coordinates
//             n = the number of points in P[]
//     Output: H[] = an array of the convex hull vertices (max is n)
//     Return: the number of points in H[]
int
chainHull_2D( Point* P, int n, Point* H )
{
    // the output array H[] will be used as the stack
    int    bot=0, top=(-1);  // indices for bottom and top of the stack
    int    i;                // array scan index

    // Get the indices of points with min x-coord and min|max y-coord
    int minmin = 0, minmax;
    float xmin = P[0].x;
    for (i=1; i<n; i++)
        if (P[i].x != xmin) break;
    minmax = i-1;
    if (minmax == n-1) {       // degenerate case: all x-coords == xmin
        H[++top] = P[minmin];
        if (P[minmax].y != P[minmin].y) // a nontrivial segment
            H[++top] = P[minmax];
        H[++top] = P[minmin];           // add polygon endpoint
        return top+1;
    }

    // Get the indices of points with max x-coord and min|max y-coord
    int maxmin, maxmax = n-1;
    float xmax = P[n-1].x;
    for (i=n-2; i>=0; i--)
        if (P[i].x != xmax) break;
    maxmin = i+1;

    // Compute the lower hull on the stack H
    H[++top] = P[minmin];      // push minmin point onto stack
    i = minmax;
    while (++i <= maxmin)
    {
        // the lower line joins P[minmin] with P[maxmin]
        if (isLeft( P[minmin], P[maxmin], P[i]) >= 0 && i < maxmin)
            continue;          // ignore P[i] above or on the lower line

        while (top > 0)        // there are at least 2 points on the stack
        {
            // test if P[i] is left of the line at the stack top
            if (isLeft( H[top-1], H[top], P[i]) > 0)
                break;         // P[i] is a new hull vertex
            else
                top--;         // pop top point off stack
        }
        H[++top] = P[i];       // push P[i] onto stack
    }

    // Next, compute the upper hull on the stack H above the bottom hull
    if (maxmax != maxmin)      // if distinct xmax points
        H[++top] = P[maxmax];  // push maxmax point onto stack
    bot = top;                 // the bottom point of the upper hull stack
    i = maxmin;
    while (--i >= minmax)
    {
        // the upper line joins P[maxmax] with P[minmax]
        if (isLeft( P[maxmax], P[minmax], P[i]) >= 0 && i > minmax)
            continue;          // ignore P[i] below or on the upper line

        while (top > bot)    // at least 2 points on the upper stack
        {
            // test if P[i] is left of the line at the stack top
            if (isLeft( H[top-1], H[top], P[i]) > 0)
                break;         // P[i] is a new hull vertex
            else
                top--;         // pop top point off stack
        }
        H[++top] = P[i];       // push P[i] onto stack
    }
    if (minmax != minmin)
        H[++top] = P[minmin];  // push joining endpoint onto stack

    return top+1;
}