scan.c

#include "defs.h"
#include "data.h"
#include "decl.h"

// Lexical scanning
// Copyright (c) 2019 Warren Toomey, GPL3

// Return the position of character c
// in string s, or -1 if c not found
static int chrpos(char *s, int c) {
  char *p;

  p = strchr(s, c);
  return (p ? p - s : -1);
}

// Get the next character from the input file.
static int next(void) {
  int c;

  if (Putback) {		// Use the character put
    c = Putback;		// back if there is one
    Putback = 0;
    return (c);
  }

  c = fgetc(Infile);		// Read from input file
  if ('\n' == c)
    Line++;			// Increment line count
  return (c);
}

// Put back an unwanted character
static void putback(int c) {
  Putback = c;
}

// Skip past input that we don't need to deal with, 
// i.e. whitespace, newlines. Return the first
// character we do need to deal with.
static int skip(void) {
  int c;

  c = next();
  while (' ' == c || '\t' == c || '\n' == c || '\r' == c || '\f' == c) {
    c = next();
  }
  return (c);
}

// Scan and return an integer literal
// value from the input file.
static int scanint(int c) {
  int k, val = 0;

  // Convert each character into an int value
  while ((k = chrpos("0123456789", c)) >= 0) {
    val = val * 10 + k;
    c = next();
  }

  // We hit a non-integer character, put it back.
  putback(c);
  return (val);
}

// Scan an identifier from the input file and
// store it in buf[]. Return the identifier's length
static int scanident(int c, char *buf, int lim) {
  int i = 0;

  // Allow digits, alpha and underscores
  while (isalpha(c) || isdigit(c) || '_' == c) {
    // Error if we hit the identifier length limit,
    // else append to buf[] and get next character
    if (lim - 1 == i) {
      fatal("Identifier too long");
    } else if (i < lim - 1) {
      buf[i++] = c;
    }
    c = next();
  }
  // We hit a non-valid character, put it back.
  // NUL-terminate the buf[] and return the length
  putback(c);
  buf[i] = '\0';
  return (i);
}

// Given a word from the input, return the matching
// keyword token number or 0 if it's not a keyword.
// Switch on the first letter so that we don't have
// to waste time strcmp()ing against all the keywords.
static int keyword(char *s) {
  switch (*s) {
    case 'c':
      if (!strcmp(s, "char"))
	return (T_CHAR);
      break;
    case 'e':
      if (!strcmp(s, "else"))
	return (T_ELSE);
      break;
    case 'f':
      if (!strcmp(s, "for"))
	return (T_FOR);
      break;
    case 'i':
      if (!strcmp(s, "if"))
	return (T_IF);
      if (!strcmp(s, "int"))
	return (T_INT);
      break;
    case 'l':
      if (!strcmp(s, "long"))
	return (T_LONG);
      break;
    case 'p':
      if (!strcmp(s, "print"))
	return (T_PRINT);
      break;
    case 'r':
      if (!strcmp(s, "return"))
	return (T_RETURN);
      break;
    case 'w':
      if (!strcmp(s, "while"))
	return (T_WHILE);
      break;
    case 'v':
      if (!strcmp(s, "void"))
	return (T_VOID);
      break;
  }
  return (0);
}

// A pointer to a rejected token
static struct token *Rejtoken = NULL;

// Reject the token that we just scanned
void reject_token(struct token *t) {
  if (Rejtoken != NULL)
    fatal("Can't reject token twice");
  Rejtoken = t;
}

// Scan and return the next token found in the input.
// Return 1 if token valid, 0 if no tokens left.
int scan(struct token *t) {
  int c, tokentype;

  // If we have any rejected token, return it
  if (Rejtoken != NULL) {
    t = Rejtoken;
    Rejtoken = NULL;
    return (1);
  }
  // Skip whitespace
  c = skip();

  // Determine the token based on
  // the input character
  switch (c) {
    case EOF:
      t->token = T_EOF;
      return (0);
    case '+':
      t->token = T_PLUS;
      break;
    case '-':
      t->token = T_MINUS;
      break;
    case '*':
      t->token = T_STAR;
      break;
    case '/':
      t->token = T_SLASH;
      break;
    case ';':
      t->token = T_SEMI;
      break;
    case '{':
      t->token = T_LBRACE;
      break;
    case '}':
      t->token = T_RBRACE;
      break;
    case '(':
      t->token = T_LPAREN;
      break;
    case ')':
      t->token = T_RPAREN;
      break;
    case '=':
      if ((c = next()) == '=') {
	t->token = T_EQ;
      } else {
	putback(c);
	t->token = T_ASSIGN;
      }
      break;
    case '!':
      if ((c = next()) == '=') {
	t->token = T_NE;
      } else {
	fatalc("Unrecognised character", c);
      }
      break;
    case '<':
      if ((c = next()) == '=') {
	t->token = T_LE;
      } else {
	putback(c);
	t->token = T_LT;
      }
      break;
    case '>':
      if ((c = next()) == '=') {
	t->token = T_GE;
      } else {
	putback(c);
	t->token = T_GT;
      }
      break;
    case '&':
      if ((c = next()) == '&') {
	t->token = T_LOGAND;
      } else {
	putback(c);
	t->token = T_AMPER;
      }
      break;
    default:

      // If it's a digit, scan the
      // literal integer value in
      if (isdigit(c)) {
	t->intvalue = scanint(c);
	t->token = T_INTLIT;
	break;
      } else if (isalpha(c) || '_' == c) {
	// Read in a keyword or identifier
	scanident(c, Text, TEXTLEN);

	// If it's a recognised keyword, return that token
	if ((tokentype = keyword(Text)) != 0) {
	  t->token = tokentype;
	  break;
	}
	// Not a recognised keyword, so it must be an identifier
	t->token = T_IDENT;
	break;
      }
      // The character isn't part of any recognised token, error
      fatalc("Unrecognised character", c);
  }

  // We found a token
  return (1);
}