Develop a recursive descent parser and scanner for the EBNF grammar like those described in chapter 8:
EBNF = { Production } EOF .
Production = nonterminal "=" Expression "." .
Expression = Term { "|" Term } .
Term = [ Factor { Factor } ] .
Factor = nonterminal
| terminal
| "[" Expression "]"
| "(" Expression ")"
| "{" Expression "}" .
For your convenience a skeleton of the code appears below
import java.util.*;
import Library.*;
class Token {
public int kind;
public String val;
public Token(int kind, String val) {
this.kind = kind;
this.val = val;
}
}
class EBNFParser {
// +++++++++++++++++++++++++ File Handling and Error handlers ++++++++++++++++++++
static InFile input;
static OutFile output;
static String newFileName(String oldFileName, String ext) {
// as in textbook - no need to expand this
static void reportError(String errorMessage) {
// as in textbook - no need to expand this
static void abort(String errorMessage) {
// Abandons parsing after issuing error message
reportError(errorMessage);
output.close();
System.exit(1);
}
// +++++++++++++++++++++++ token kinds enumeration +++++++++++++++++++++++++
static final int
noSym = 0, // others needed like this
// +++++++++++++++++++++++++++++ Character Handler ++++++++++++++++++++++++++
static final char EOF = '\0';
static boolean atEndOfFile = false;
// Declaring ch as a global variable is done for expediency - global variables
// are not always a good thing
static char ch; // look ahead character for scanner
static void getChar() {
// Obtains next character ch from input, or CHR(0) if EOF reached
// Reflect ch to output
if (atEndOfFile) ch = EOF;
else {
ch = input.readChar();
atEndOfFile = ch == EOF;
if (!atEndOfFile) output.write(ch);
}
}
// +++++++++++++++++++++++++++++++ Scanner ++++++++++++++++++++++++++++++++++
// Declaring sym as a global variable is done for expediency - global variables
// are not always a good thing
static Token sym;
static void getSym() {
// Scans for next sym from input
while (ch > EOF && ch <= ' ') getChar();
// Rest of scanner comes here
sym = new Token(symKind, symLex.toString());
}
// +++++++++++++++++++++++++++++++ Parser +++++++++++++++++++++++++++++++++++
static void accept(int wantedSym, String errorMessage) {
// Checks that lookahead token is wantedSym
if (sym.kind == wantedSym) getSym(); else abort(errorMessage);
}
// Parser Routines come here
// +++++++++++++++++++++ Main driver function +++++++++++++++++++++++++++++++
public static void main(String[] args) {
// Open input and output files from command line arguments
if (args.length == 0) {
System.out.println("Usage: EBNFParser FileName");
System.exit(1);
}
input = new InFile(args[0]);
output = new OutFile(newFileName(args[0], ".out"));
getChar(); // Lookahead character
getSym(); // Lookahead symbol
EBNF(); // Start to parse from the goal symbol
// if we get back here everything must have been satisfactory
System.out.println("Parsed correctly");
output.close();
}
}
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