/*
 * mono.c
 *
 * This file contains functions that deal with reading in monomials
 * (components of polynomials, like -2x^14) from standard input and
 * returning them in 'mono' structs.
 */
#include <stdio.h>
#include "mono.h"

/* get a character from stdin, excluding tab and space */

char get_nonwhite_char (void) {
	char	ch;

	ch = getchar ();
	while (strchr (" \t", ch) && !feof (stdin)) ch = getchar ();
	return ch;
}

/* read a single monomial into the reference parameter t.
 * return 0 on success, 1 on end of file or end of line
 */
int read_mono (mono *t) {
	static char lookahead = ' ';	/* always contains the next char */
	static int init = 1;		/* true if we need to initialize */
	int	i, 			/* loop index */
		mul;			/* 1 or -1, for negative numbers */
	char	s[100];			/* build up a number in this string */

	/* if this is our first time, initialize lookahead from stdin */

	if (init) {
		lookahead = get_nonwhite_char ();
		init = 0;
	}

	/* end of file?  return with 1 */

	if (feof (stdin)) return 1;

	/* end of line?  return with 1 */

	if (lookahead == '\n') return 1;

	/* assume coefficient is positive */

	mul = 1;


	if (lookahead == '+') {

		/* if we find a +, we'll just ignore it as a separator */

		lookahead = get_nonwhite_char ();
	} else if (lookahead == '-') {

		/* if we find a -, negate mul so later we'll know it
		 * was a negative number
		 */
		lookahead = get_nonwhite_char ();
		mul = -mul;
	}

	/* get the coefficient.  we'll get it by building it up in
	 * s.  there might not be any digits, but that will be OK.
	 */
	i = 0;

	/* while the next character is a digit or period ... */

	while (strchr ("0123456789.", lookahead)) {

		/* put it in the string */

		s[i++] = lookahead;

		/* get the next char */

		lookahead = getchar ();
	}

	/* null terminator for string */

	s[i] = 0;


	if (!i) 
		/* if we didn't get any digits, the coefficient 
		 * is assumed to be 1 	
		 */

		t->coeff = 1.0; 
	else
		/* otherwise we'll find the value */

		sscanf (s, "%lf", &t->coeff);

	/* if we found a minus sign before, multiply by -1 */

	t->coeff *= mul;

	/* if there is whitespace here, get a nonwhite char */

	if (strchr (" \t", lookahead)) lookahead = get_nonwhite_char ();

	/* if we didn't find x... */

	if (lookahead != 'x') {

		/* but we found something that couldn't be the
		 * next character in a correct polynomial, then
		 * complain and exit.
		 */

		if (!strchr (" \t+-\n", lookahead)) {
			printf ("unexpected input!\n");
			exit (1);
		}

		/* otherwise, it's just that there's no x and
		 * the power is 0 (coeff * x^0) and we can return
		 */
		t->power = 0.0;
		return 0;
	}

	/* if we did find an x, then maybe there's an exponent */

	lookahead = get_nonwhite_char ();
	if (lookahead != '^') {

		/* or maybe not.  then it's just 1. (coeff * x^1) */

		t->power = 1.0;
		return 0;
	}

	/* there's an exponent.  we'll start reading in digits
	 * and see what it is.
	 */
	lookahead = get_nonwhite_char ();
	i = 0;
	while (strchr ("-0123456789.", lookahead)) {
		s[i++] = lookahead;
		lookahead = getchar ();
	}
	s[i] = 0;

	/* this time, the user told us '^' so there should be something
	 * there.  if not, we don't know what to do so we won't handle it :-)
	 */
	sscanf (s, "%lf", &t->power);

	/* prime lookahead for next time */

	if (strchr (" \t", lookahead)) lookahead = get_nonwhite_char ();
	return 0;
}