#define _POSIX_C_SOURCE 200112L
#define _DEFAULT_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <assert.h>
#include <sys/select.h>
#include <sys/types.h>
#include <sys/prctl.h>
#include <pty.h>
#include <termios.h>

#define writeout(buff, size) if(write(STDOUT_FILENO, buff, size) < 0) goto fail
#define put(str) writeout(str, strlen(str))
#define MAX(a, b) ((a) > (b) ? (a) : (b))

#define BUFF_SIZE 1024

int main(int argc, char *argv[])
{
	if(argc < 2)
	{
		put("usage: ./this program");
		exit(EXIT_FAILURE);
	}
	struct termios parentTermios;
	if(tcgetattr(STDIN_FILENO, &parentTermios) < 0)
		goto fail;;
	cfmakeraw(&parentTermios); //TODO: should restore terminal state
	if(tcsetattr(STDOUT_FILENO, TCSANOW, &parentTermios) < 0)
		goto fail;
	int fdChild;
	pid_t pid = forkpty(&fdChild, NULL, NULL, NULL); //FIXME: non-standard
	//TODO: read all input and terminate on SIGCHILD?
	if(pid < 0)
		goto fail;
	else if(pid == 0) /* child */
	{
		//if(setsid() < 0)
			//goto fail;
		//if(ioctl(STDIN_FILENO, TIOCSCTTY, 1) < 0)
			//goto fail;
		//TODO: set raw mode
		//TODO: set echo off
		//TODO: set session leader. forkpty(2) does that
		//TODO: set controlling terminal. forkpty(2) does that
		//raw mode => echo off
		//this should be done to the parent terminal
		put("I'm child! ");
		for(char i='a'; i<='z'; i++)
			writeout(&i, 1);
		put("\n");
		execvp(argv[1], argv + 1);
		goto fail;
	}
	else /* parent */
	{
		prctl(PR_SET_NAME, "parent");
		char downBuff[BUFF_SIZE], upBuff[BUFF_SIZE];
		size_t downRead = 0, downWrite = 0;
		size_t upRead = 0, upWrite = 0;

		int nfds = 0;
		nfds = MAX(nfds, STDOUT_FILENO);
		nfds = MAX(nfds, STDIN_FILENO);
		nfds = MAX(nfds, fdChild);
		fd_set readSet, writeSet;
loop:
		FD_ZERO(&readSet);
		FD_ZERO(&writeSet);

		if(downRead < BUFF_SIZE)
			FD_SET(STDIN_FILENO, &readSet);
		if(downWrite < downRead)
			FD_SET(fdChild, &writeSet);

		if(upRead < BUFF_SIZE)
			FD_SET(fdChild, &readSet);
		if(upWrite < upRead)
			FD_SET(STDOUT_FILENO, &writeSet);

		if(select(nfds + 1, &readSet, &writeSet, NULL, NULL) < 0)
			goto fail;
		ssize_t nbytes;
		if(FD_ISSET(STDIN_FILENO, &readSet))
		{
			assert(BUFF_SIZE - downRead > 0);
			nbytes = read(STDIN_FILENO, downBuff + downRead, BUFF_SIZE - downRead);
			if(nbytes < 0)
				goto fail;
			else if(nbytes == 0)
				goto eof;
			downRead += nbytes;
			assert(downRead <= BUFF_SIZE);
		}
		/* mangle downBuff as you wish */
		if(FD_ISSET(fdChild, &writeSet))
		{
			assert(downRead - downWrite > 0);
			nbytes = write(fdChild, downBuff + downWrite, downRead - downWrite);
			if(nbytes < 0)
				goto fail;
			else if(nbytes == 0)
				goto eof;
			downWrite += nbytes;
			assert(downWrite <= BUFF_SIZE);
		}
		if(downWrite == downRead)
			downWrite = downRead = 0;
		if(FD_ISSET(fdChild, &readSet))
		{
			assert(BUFF_SIZE - upRead > 0);
			nbytes = read(fdChild, upBuff + upRead, BUFF_SIZE - upRead);
			if(nbytes < 0)
				goto fail;
			else if(nbytes == 0)
				goto eof;
			upRead += nbytes;
			assert(upRead <= BUFF_SIZE);
		}
		/* mangle upBuff as you wish */
		for(size_t i=0; i<upRead; i++)
			if(upBuff[i] == 'a')
				upBuff[i] = '!';
		if(FD_ISSET(STDOUT_FILENO, &writeSet))
		{
			assert(upRead - upWrite > 0);
			nbytes = write(STDOUT_FILENO, upBuff + upWrite, upRead - upWrite);
			if(nbytes < 0)
				goto fail;
			else if(nbytes == 0)
				goto eof;
			upWrite += nbytes;
			assert(upWrite <= BUFF_SIZE);
		}
		if(upWrite == upRead)
			upWrite = upRead = 0;
		goto loop;
	}
	return 0;
eof:
	fprintf(stderr, "end of file\n");
	exit(EXIT_SUCCESS);
fail:
	perror(NULL);
	exit(EXIT_FAILURE);
}