/****************************************************************/
/*								*/
/*	segment.c	in	~/src/Glos			*/
/*								*/
/*	This function picks up all the points in one segment	*/
/*	and performs los analysis on them.			*/
/*								*/
/****************************************************************/

#include <grass/segment.h>
#include "point.h"

#define	  NEXT_PT		PRESENT_PT->next
#define	  NEXT_PT_BACK_PTR	PRESENT_PT->next->previous
#define	  HEAD_BACK_PTR		head->previous

struct point *segment (
	int segment_no, int xmax, int ymax,
	double slope_1, double slope_2, int flip,
	int sign_on_y, int sign_on_x, int viewpt_elev,
	SEGMENT *seg_in_p, SEGMENT *seg_out_p, SEGMENT *seg_patt_p,
	int row_viewpt, int col_viewpt, int patt_flag)
{
        int lower_limit_y ,upper_limit_y,less,x,y,
		x_actual,y_actual,x_flip,y_flip;
        struct point *head = (struct point *)NULL, *PRESENT_PT;
        int quadrant;

	/*	decide which one of the four quadrants		*/
        quadrant = 1 + (segment_no-1)/4;

        if(slope_1 != 0){ less= ymax/slope_1 + 0.99;
                          xmax= (xmax<=less)?xmax:less;
                        }

	/*	outer loop over x coors for picking up points	*/
        for(x=xmax ; x > 0; x--)
        {

	/*	calculate limits for range of y for a single x	*/
        lower_limit_y = x * slope_1 + 0.9;
        upper_limit_y = x * slope_2 ;
        upper_limit_y = (upper_limit_y<=ymax)?upper_limit_y:ymax;

	/*	loop over y range to pick up correct points	*/
        for(y= upper_limit_y; y>= lower_limit_y ; y--)
        {
	/*  calculate actual x, y that lie in current segment	*/

        if(flip==0) { x_flip = x; y_flip = y;}
        else        { y_flip = x; x_flip = y;}
 
        x_actual = sign_on_x*x_flip ;  
	y_actual = sign_on_y*y_flip;
 
	/*	add chosen point to the point list		*/
        head = make_list(head,y_actual,x_actual,seg_in_p,
		viewpt_elev,quadrant,row_viewpt,col_viewpt);
 
        }
        }  /* end of outer loop */
 
 
	if(head != NULL)
	{
	/*      assign back pointers in linked list             */
        HEAD_BACK_PTR = NULL;
	PRESENT_PT = head;
         
        while(NEXT_PT != NULL)
	{
        NEXT_PT_BACK_PTR = PRESENT_PT;
        PRESENT_PT = NEXT_PT;
        }

	head = hidden_point_elimination(head,viewpt_elev,
		seg_in_p,seg_out_p,seg_patt_p,quadrant,sign_on_y,
		sign_on_x,row_viewpt,col_viewpt,patt_flag);
        }              

        return(head);

} 

/**************** END OF FUNCTION "SEGMENT" *********************/