/* Lex.cpp - Very simple lexical analyzer for an equally simple 
 * 'desk calculator' language.
 *
 * Essentially breaks the input stream up into a series of tokens, 
 * returning the token code
 */
#include <iostream>
#include <cmath>
#include <string>

using namespace std;

#include "Lex.h"

namespace Lex 
{
  // Code for end of file
  const int ENDOFFILE = -1;

  // Code for a numeric constant (cur_val() returns the value)
  const int NUMBER = 128;

  // Code for a symbol (cur_sym() returns the name)
  const int SYMBOL = 129;

  // Keyword token codes
  const int SET_KW = 130;
  const int FUN_KW = 131;

  // State information
  static double val = 0.0;
  static string sym;
  static int token = 0;

  const string & cur_sym() {	// Return current symbol, token==SYMBOL
    return sym;
  }

  int cur_tok() {		// Return current token
    return token;
  }

  double cur_val() {		// Return current value, token==NUMBER
    return val;
  }

  int get_next(istream &input)	// Get next token from input
  {
    char ch;
    double intpart=0;
    double exponent=1;
    int after=0;		// # of digits after decimal point
    int before=0;		// # of digits before decimal point

    sym = "";			// Reset the state as needed
    val = 0.0;

    input.get(ch);		// Get first character

    while (ch <= ' ' && ch != '\n') { // Skip whitespace
      if (!input.good()) {
	return (token = ENDOFFILE);
      }
      input.get(ch);
    }

    switch (ch) {
    case '+':			// Characters (not all are supported)
    case '-':
    case '*':
    case '/':
    case '(':
    case ')':
    case '}':
    case '{':
    case '=':
    case '>':
    case '<':
    case '\n':
      return (token = ch);

    default:
      if (ch >= 'a' && ch <= 'z') { // Symbol or keyword
	while (ch >= 'a' && ch <= 'z') {
	  sym += ch;
	  input.get(ch);
	}
	input.putback(ch);
	if (sym == "fun") {
	  return (token = FUN_KW);
	}
	if (sym == "set") {
	  return (token = SET_KW);
	}
	return (token = SYMBOL);
      }
      else if (ch >= '0' && ch <= '9') { // Numeric constant
	intpart = 0;
	exponent = 0;

	while (ch >= '0' && ch <= '9') {
	  intpart = (intpart * 10) + (ch - '0'); // Convert from ASCII
	  before++;
	  input.get(ch);
	}
	if (ch == '.') {	// Is there a fractional part?
	  input.get(ch);
	  while (ch >= '0' && ch <= '9') { // Gather fraction
	    intpart = (intpart * 10) + (ch - '0'); // Convert from ASCII
	    after++;
	    input.get(ch);
	  }
	}
	if (ch == 'e' || ch == 'E') { // Is there an exponent?
	  input.get(ch);
	  while (ch >= '0' && ch <= '9') { // Gather exponent
	    exponent = (exponent * 10) + (ch - '0');
	    input.get(ch);
	  }
	}
	if (after > 0) {
	  exponent -= after;
	}
	if (after + before < 1) {
	  cerr << "Need at least one digit in number\n";
	}
	val = intpart * pow(10, exponent);
	input.putback(ch);
	return (token = NUMBER);
      }
      else {
	cerr << "Illegal input character: " << (int)ch << endl;
	return (token = 0);	// Illegal value
      }
      break;
    }
  }
} /* End namespace */