/* Infix notation calculator--calc */

%{
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#define YYSTYPE int

static int power (int base, int exponent);
static int read_signed_integer (FILE *stream);
static void yyerror (const char *s);
extern void perror (const char *s);
%}

/* BISON Declarations */
%token NUM
%left '-' '+'
%left '*' '/'
%left NEG     /* negation--unary minus */
%right '^'    /* exponentiation        */

/* Grammar follows */
%%
input:    /* empty string */
        | input line
;

line:     '\n'
        | exp '\n'  { printf ("%d", $1);     }
;

exp:      NUM                { $$ = $1;             }
        | exp '+' exp        { $$ = $1 + $3;        }
        | exp '-' exp        { $$ = $1 - $3;        }
        | exp '*' exp        { $$ = $1 * $3;        }
        | exp '/' exp        { $$ = $1 / $3;        }
        | '-' exp  %prec NEG { $$ = -$2;            }
        | exp '^' exp        { $$ = power ($1, $3); }
        | '(' exp ')'        { $$ = $2;             }
;
%%
/* The input. */
FILE *yyin;

static void
yyerror (const char *s)
{
  fprintf (stderr, "%s\n", s);
}

static int
read_signed_integer (FILE *stream)
{
  int c = getc (stream);
  int sign = 1;
  int n = 0;

  if (c == '-')
    {
      c = getc (stream);
      sign = -1;
    }

  while (isdigit (c))
    {
      n = 10 * n + (c - '0');
      c = getc (stream);
    }

  ungetc (c, stream);

  return sign * n;
}

/*---------------------------------------------------------------.
| Lexical analyzer returns an integer on the stack and the token |
| NUM, or the ASCII character read if not a number.  Skips all   |
| blanks and tabs, returns 0 for EOF.                            |
`---------------------------------------------------------------*/

int
yylex ()
{
  int c;

  /* Skip white space.  */
  while ((c = getc (yyin)) == ' ' || c == '\t')
    ;
  /* process numbers   */
  if (c == '.' || isdigit (c))
    {
      ungetc (c, yyin);
      yylval = read_signed_integer (yyin);
      return NUM;
    }
  /* Return end-of-file.  */
  if (c == EOF)
    return 0;
  /* Return single chars. */
  return c;
}

static int
power (int base, int exponent)
{
  int res = 1;
  if (exponent < 0)
    exit (1);
  for (/* Niente */; exponent; --exponent)
    res *= base;
  return res;
}

int
main (int argn, const char **argv)
{
  if (argn == 2)
    yyin = fopen (argv[1], "r");
  else
     yyin = stdin;

  if (!stdin)
    {
      perror (argv[1]);
      exit (1);
    }

#if YYDEBUG
  yydebug = 1;
#endif
  yyparse ();
  return 0;
}