/*
 * File: scanner.h
 * ---------------
 * This file is the interface to a package that divides
 * a line into individual "tokens".  A token is defined
 * to be either
 *
 * 1. a string of consecutive letters and digits representing
 *    a word, or
 *
 * 2. a one-character string representing a separator
 *    character, such as a space or a punctuation mark.
 *
 * To use this package, you must first call
 *
 *        InitScanner(line);
 *
 * where line is the string (typically a line returned by
 * GetLine) that is to be divided into tokens.  To retrieve
 * each token in turn, you call
 *
 *        token = GetNextToken();
 *
 * When the last token has been read, the predicate function
 * AtEndOfLine returns TRUE, so that the loop structure
 *
 *        while (!AtEndOfLine()) {
 *            token = GetNextToken();
 *            . . . process the token . . .
 *        }
 *
 * serves as an idiom for processing each token on the line.
 *
 * Further details for each function are given in the
 * individual descriptions below.
 */

#ifndef _scanner_h
#define _scanner_h

#include <stdio.h>
#include <ctype.h>

#include "genlib.h"
#include "strlib.h"
#include "scanner.h"

void InitScanner(string line);
string GetNextToken(void);
bool AtEndOfLine(void);

/*
 * Private variables
 * -----------------
 * buffer  -- Private copy of the string passed to InitScanner
 * buflen  -- Length of the buffer, saved for efficiency
 * cpos    -- Current character position in the buffer
 */

static string buffer;
static int buflen;
static int cpos;

/*
 * Function: InitScanner
 * ---------------------
 * All this function has to do is initialize the private
 * variables used in the package.
 */

void InitScanner(string line)
{
    buffer = line;
    buflen = StringLength(buffer);
    cpos = 0;
}

/*
 * Function: GetNextToken
 * ----------------------
 * The implementation of GetNextToken follows its behavioral
 * description as given in the interface: if the next character
 * is alphanumeric (i.e., a letter or digit), the function
 * searches to find an unbroken string of such characters and
 * returns the entire string.  If the current character is not
 * a letter or digit, a one-character string containing that
 * character is returned.
 */

string GetNextToken(void)
{
    char ch;
    int start;

    if (cpos >= buflen) Error("No more tokens");
    ch = IthChar(buffer, cpos);
    if (isalnum(ch)) {
        start = cpos;
        while (cpos < buflen && isalnum(IthChar(buffer, cpos))) {
            cpos++;
        }
        return (SubString(buffer, start, cpos - 1));
    } else {
        cpos++;
        return (CharToString(ch));
    }
}

/*
 * Function: AtEndOfLine
 * ---------------------
 * This implementation compares the current buffer position
 * against the saved length.
 */

bool AtEndOfLine(void)
{
    return (cpos >= buflen);
}


#endif