Initial Commit

Corgi/Parser.java

@@ -0,0 +1,202 @@
+/*
+    This class provides a recursive descent parser 
+    for Corgi (a simple calculator language),
+    creating a parse tree which can be interpreted
+    to simulate execution of a Corgi program
+*/
+
+public class Parser {
+
+   private Lexer lex;
+
+   public Parser( Lexer lexer ) {
+      lex = lexer;
+   }
+
+   public Node parseProgram() {
+      return parseStatements();
+   }
+
+   private Node parseStatements() {
+      System.out.println("-----> parsing <statements>:");
+ 
+      Node first = parseStatement();
+ 
+      // look ahead to see if there are more statement's
+      Token token = lex.getNextToken();
+ 
+      if ( token.isKind("eof") ) {
+         return new Node( "stmts", first, null, null );
+      }
+      else {
+         lex.putBackToken( token );
+         Node second = parseStatements();
+         return new Node( "stmts", first, second, null );
+      }
+   }// <statements>
+
+   private Node parseStatement() {
+      System.out.println("-----> parsing <statement>:");
+ 
+      Token token = lex.getNextToken();
+ 
+      // ---------------->>>  print <string>  or   print <expr>
+      if ( token.isKind("print") ) {
+         token = lex.getNextToken();
+ 
+         if ( token.isKind("string") ) {// print <string>
+            return new Node( "prtstr", token.getDetails(),
+                          null, null, null );
+         }
+         else {// must be first token in <expr>
+            // put back the token we looked ahead at
+            lex.putBackToken( token );
+            Node first = parseExpr();
+            return new Node( "prtexp", first, null, null );
+         }
+      // ---------------->>>  newline
+      }
+      else if ( token.isKind("newline") ) {
+         return new Node( "nl", null, null, null );
+      }
+      // --------------->>>   <var> = <expr>
+      else if ( token.isKind("var") ) {
+         String varName = token.getDetails();
+         token = lex.getNextToken();
+         errorCheck( token, "single", "=" );
+         Node first = parseExpr();
+         return new Node( "sto", varName, first, null, null );
+      }
+      else {
+         System.out.println("Token " + token + 
+                             " can't begin a statement");
+         System.exit(1);
+         return null;
+      }
+ 
+   }// <statement>
+
+   private Node parseExpr() {
+      System.out.println("-----> parsing <expr>");
+
+      Node first = parseTerm();
+
+      // look ahead to see if there's an addop
+      Token token = lex.getNextToken();
+ 
+      if ( token.matches("single", "+") ||
+           token.matches("single", "-") 
+         ) {
+         Node second = parseExpr();
+         return new Node( token.getDetails(), first, second, null );
+      }
+      else {// is just one term
+         lex.putBackToken( token );
+         return first;
+      }
+
+   }// <expr>
+
+   private Node parseTerm() {
+      System.out.println("-----> parsing <term>");
+
+      Node first = parseFactor();
+
+      // look ahead to see if there's a multop
+      Token token = lex.getNextToken();
+ 
+      if ( token.matches("single", "*") ||
+           token.matches("single", "/") 
+         ) {
+         Node second = parseTerm();
+         return new Node( token.getDetails(), first, second, null );
+      }
+      else {// is just one factor
+         lex.putBackToken( token );
+         return first;
+      }
+      
+   }// <term>
+
+   private Node parseFactor() {
+      System.out.println("-----> parsing <factor>");
+
+      Token token = lex.getNextToken();
+
+      if ( token.isKind("num") ) {
+         return new Node("num", token.getDetails(), null, null, null );
+      }
+      else if ( token.isKind("var") ) {
+         return new Node("var", token.getDetails(), null, null, null );
+      }
+      else if ( token.matches("single","(") ) {
+         Node first = parseExpr();
+         token = lex.getNextToken();
+         errorCheck( token, "single", ")" );
+         return first;
+      }
+      else if ( token.isKind("bif0") ) {
+         String bifName = token.getDetails();
+         token = lex.getNextToken();
+         errorCheck( token, "single", "(" );
+         token = lex.getNextToken();
+         errorCheck( token, "single", ")" );
+         
+         return new Node( bifName, null, null, null );
+      }
+      else if ( token.isKind("bif1") ) {
+         String bifName = token.getDetails();
+         token = lex.getNextToken();
+         errorCheck( token, "single", "(" );
+         Node first = parseExpr();
+         token = lex.getNextToken();
+         errorCheck( token, "single", ")" );
+         
+         return new Node( bifName, first, null, null );
+      }
+      else if ( token.isKind("bif2") ) {
+         String bifName = token.getDetails();
+         token = lex.getNextToken();
+         errorCheck( token, "single", "(" );
+         Node first = parseExpr();
+         token = lex.getNextToken();
+         errorCheck( token, "single", "," );
+         Node second = parseExpr();
+         token = lex.getNextToken();
+         errorCheck( token, "single", ")" );
+         
+         return new Node( bifName, first, second, null );
+      }
+      else if ( token.matches("single","-") ) {
+         Node first = parseFactor();
+         return new Node("opp", first, null, null );
+      }
+      else {
+         System.out.println("Can't have factor starting with " + token );
+         System.exit(1);
+         return null;
+      }
+      
+   }// <factor>
+
+  // check whether token is correct kind
+  private void errorCheck( Token token, String kind ) {
+    if( ! token.isKind( kind ) ) {
+      System.out.println("Error:  expected " + token + 
+                         " to be of kind " + kind );
+      System.exit(1);
+    }
+  }
+
+  // check whether token is correct kind and details
+  private void errorCheck( Token token, String kind, String details ) {
+    if( ! token.isKind( kind ) || 
+        ! token.getDetails().equals( details ) ) {
+      System.out.println("Error:  expected " + token + 
+                          " to be kind=" + kind + 
+                          " and details=" + details );
+      System.exit(1);
+    }
+  }
+
+}