/*
Program: DROP.C
Author : Kim Moser
Date   : 02/01/89
System : IBM PC, Borland Turbo-C 2.0
Descrip: "Drops" text on screen.

Usage  :

	DROP

Hardware required:
	IBM PC or compatible with color or monochrome card.

Compiler: Borland's Turbo C 2.0

	Compiler settings:

	Linker settings:
		o Graphics library: ON
*/

/**************************************************************************
 Screen I/O identifiers
**************************************************************************/

#define MAXROW 25
  /* Width and height of screen [in characters]; may be changed if
     video card supports more rows and/or columns */

/* Types for screen I/O: */
typedef unsigned char scrnbyte;

typedef struct scrnchar {
  scrnbyte ch, at;  /* Character and its attribute [on screen] */
};

typedef unsigned int scrncoord;
typedef unsigned short int color;
typedef struct scrnchar far *vid_addr;
  /* Must be FAR because video memory is in a different data segment */

/* @@@ NOT USED
static scrncoord curx=1, cury=1;  /* Cursor coordinates for write() procedures */
*/

#define COLOR 0xB8000000L
#define MONO  0xB0000000L

unsigned char far *CRT_MODE = (unsigned char far *) 0x00400049L;
  /* BIOS video mode number */
unsigned int far *CRT_COL = (unsigned int far *) 0x0040004AL;
  /* Pointer to number of columns */
vid_addr SCREEN = (vid_addr) COLOR;  /* Default */

#define xytoadr(x,y) ((scrncoord) ( (y-1)* *CRT_COL + (x-1) ) )

/*
scrncoord xytoadr( x, y )
scrncoord x, y;
/* Returns offset from (0,0), of location (x,y) on screen, according
   to screen width and height #defined */
{
	return (scrncoord) ( (y-1)* *CRT_COL + (x-1) );
}
*/

struct scrnchar *peeksc( x, y, c )
/* Sets c-> to char and color at (x,y) [offset from 1,1] */
scrncoord x,y;
struct scrnchar *c;
{
	register vid_addr scrn = (vid_addr) (SCREEN + xytoadr(x,y));

	*c = *scrn;  /* Copy from video to mem */
	return c;  /* Return pointer */
}

void writeat( x, y, ch )
scrncoord x,y;
char ch;
{
	register vid_addr scrn = (vid_addr) (SCREEN + xytoadr(x,y));

	scrn->ch = ch;
}

void writeatsc( x, y, c )
/* Writes scrnchar pointed to by 'c' at (x,y) [offset from 1,1] */
scrncoord x,y;
struct scrnchar *c;
{
	register vid_addr scrn = (vid_addr) (SCREEN + xytoadr(x,y));

	*scrn = *c;  /* Copy from mem to vid */
}

void main(void)
{
	register scrncoord x,y;
	struct scrnchar c;
	register char some;

	/* Initialization: */
	SCREEN = ((*CRT_MODE==7) ? (vid_addr) MONO : (vid_addr) COLOR );

	do {
		some = 0;
		for (x=1; x<=*CRT_COL; x++) {
			for (y=MAXROW; y>1; y--)
				if ( (peeksc(x,y,&c)->ch) == ' ' )
					if ( (peeksc(x,y-1,&c)->ch) != ' ' ) {
						writeatsc( x, y, &c );
						writeat( x, y-1, ' ' );
						some = 1;
					}
		}
	} while (some);
}