//-------------------------------------------------------------------
//	persists.cpp
//
//	This program causes the soundcard to play a continuous tone 
//	till a signal is received indicating that <ESCAPE> was hit. 
//
//	We thank Thomas Cheong for diagnosing the 'early exit' bug.
//
//	NOTE: We added variable-names that clarify the mathematical
//	basis for our computations of pulse-codes for a given tone. 
//
//	programmer: ALLAN CRUSE
//	written on: 22 SEP 2005
//	bug repair: 23 SEP 2005 -- use 'atexit()' in case of exit()  
//	revised on: 24 SEP 2005 -- supply clarifying variable-names 
//-------------------------------------------------------------------

#include <stdio.h>	// for printf(), perror() 
#include <fcntl.h>	// for open() 
#include <stdlib.h>	// for exit() 
#include <signal.h>	// for signal()
#include <math.h>	// for sin()
#include <termios.h>	// for tcgetattr(), tcsetattr()
#include <unistd.h>	// for read(), write(), close() 
#include <sys/ioctl.h>	// for ioctl()
#include <sys/soundcard.h>

#define SAMPLE_RATE	8000
#define NUM_SAMPLES	(SAMPLE_RATE / 20)
#define TWO_PI		(3.14159 * 2.0)

char		devname[] = "/dev/dsp";
unsigned char	pulsecode[ NUM_SAMPLES ];

struct termios	tty_orig;
void my_atexit( void )
{
	// ignore any further keystrokes	
	signal( SIGIO, SIG_IGN );

	// restore canonical keyboard-input
	tcsetattr( 0, TCSAFLUSH, &tty_orig );
}

int	done = 0;
void upon_input( int signum ) 
{
	// signal-handler for keyboard-input notifications

	int	inch = 0;

	read( STDIN_FILENO, &inch, sizeof( inch ) );
 	if ( inch == '\e' ) done = 1;
}


int main( int argc, char **argv )
{
	// specify the parameters for our tone's pulse-code data 
	double	frequency = 220.0;	// in cycles-per-second
	int	amplitude = 64;		// midrange volume-gain
	int	silence = 128;		// neutral volume-level
	// compute the sine-argument's rate-of-change in radians-per-sample
	double	radians_per_circle = TWO_PI;
	double	circles_per_second = frequency;
	double	samples_per_second = SAMPLE_RATE;
	double	samples_per_circle = samples_per_second / circles_per_second;
	double	radians_per_sample = radians_per_circle / samples_per_circle;
	// now generate the pulse-code data for our fundamental tone
	double	anglesize = radians_per_sample;
	for (int sample = 0; sample < NUM_SAMPLES; sample++)
		{
		double	x = anglesize * sample;
		double	y = amplitude * sin( x );
		pulsecode[ sample ] = silence + (int)y;
		}

	// enable noncanonical keyboard input
	tcgetattr( STDIN_FILENO, &tty_orig );
	struct termios	tty_work = tty_orig;
	tty_work.c_lflag &= ~( ECHO | ICANON | ISIG );
	tty_work.c_cc[ VTIME ] = 0;
	tty_work.c_cc[ VMIN ] = 1;
	tcsetattr( STDIN_FILENO, TCSAFLUSH, &tty_work );

	// request asynchronous notification of keyboard input
	signal( SIGIO, upon_input );
	fcntl( 0, F_SETOWN, getpid() );
	fcntl( 0, F_SETFL, fcntl( 0, F_GETFL, NULL ) | O_ASYNC );

	// insert our own function into this task's 'exit-list'
	atexit( my_atexit ); 
	
	// open the playback device (and reset its default parameters)
	int	dsp = open( devname, O_WRONLY );
	if ( dsp < 0 ) { perror( devname ); exit(1); }
	if ( ioctl( dsp, SNDCTL_DSP_RESET, NULL ) < 0 )
		{ perror( "ioctl" ); exit(1); }

	// now playback the sound continuously (until 'done' is true)
	while ( !done )
		{
		unsigned char	*data = pulsecode;
		int	morebytes = NUM_SAMPLES;
		while (( !done )&&( morebytes > 0 ))
			{
			int	nbytes = write( dsp, data, morebytes );
			if ( nbytes < 0 ) break; 
			morebytes -= nbytes;
			data += nbytes;
			}
		}

	// reset the playback devive (to stop its sound immediately)
	if ( ioctl( dsp, SNDCTL_DSP_RESET, NULL ) < 0 )
		perror( "soundcard reset failed" );

	// close the audio playback device
	close( dsp );
}