point_cloud_t convex_hull_monotone_chain(point_cloud_t cloud) {
point_cloud_t hull = init_point_cloud(cloud.size);
int j = 0;
for (int k = 0; k < cloud.size; k++) {
while (j >= 2 && turn_direction(hull.pc[j-2], hull.pc[j-1], cloud.pc[k]) <= 0) --j;
hull.pc[j++] = cloud.pc[k];
}
for (int k = cloud.size-2, t = j+1; k >= 0; --k) {
while (j >= t && turn_direction(hull.pc[j-2], hull.pc[j-1], cloud.pc[k]) <= 0) --j;
hull.pc[j++] = cloud.pc[k];
}
hull.size = j;
return hull;
}
int chull_2d(const double *original_points,
int npoints,
double *vertices,
int *nvertices)
{
int rval = 0;
int pivot_idx;
double pivot[2];
int *stack = 0;
int stack_length = 0;
double *points = 0;
stack = (int*) malloc(npoints * sizeof(int));
points = (double*) malloc(2 * npoints * sizeof(double));
abort_if(!stack, "could not allocate stack");
abort_if(!points, "could not allocate points");
memcpy(points, original_points, 2 * npoints * sizeof(double));
// find point with lowest y-coordinate
pivot_idx = 0;
memcpy(pivot, points, 2 * sizeof(double));
for (int i = 0; i < npoints; i++)
{
double *point = &points[2*i];
if(point[1] > pivot[1]) continue;
if(point[1] == pivot[1] && point[0] > pivot[0]) continue;
memcpy(pivot, point, 2 * sizeof(double));
pivot_idx = i;
}
// remove pivot
swap(points[2*(npoints-1)], points[2*pivot_idx], double);
swap(points[2*(npoints-1)+1], points[2*pivot_idx+1], double);
npoints--;
// sort points counterclockwise
qsort_r(points, npoints, 2 * sizeof(double), _qsort_cmp_angle_distance,
pivot);
// adds pivot and first point to the stack
stack[stack_length++] = npoints;
stack[stack_length++] = 0;
int k = 1;
while(k < npoints)
{
double *p0 = &points[2*stack[stack_length-2]];
double *p1 = &points[2*stack[stack_length-1]];
double *p2 = &points[2*k];
double t = turn_direction(p0, p1, p2);
if(DOUBLE_leq(t, 0))
stack_length--;
if(DOUBLE_geq(t, 0))
{
stack[stack_length++] = k;
k++;
}
}
for(int i = 0; i < stack_length; i++)
{
int j = stack_length - i - 1;
vertices[2*j] = points[2*stack[i]];
vertices[2*j+1] = points[2*stack[i]+1];
}
*nvertices = stack_length;
CLEANUP:
if(points) free(points);
if(stack) free(stack);
return rval;
}
