* ------------------------------------------------
* SIMPLE INTRODUCTORY VERSION,
* WITH ASSIGNMENT, ADDITION, AND MULTIPLICATION,
* ON INTEGER TYPES ONLY
* ------------------------------------------------
*
* PascalInterpreter is a parse-tree interpreter for a subset of the Pascal * programming language. Its primary public method is eval, which takes a * parse tree and symbol table, and evaluates the tree if it is an executable * portion of a program. *
* The interpreter works with an Object-valued program memory array. Prior to * calling eval, the initInterpreter method is used to allocate array storage * and perform other initialization. The constructor calls initInterpreter, * and it may be called subsequently for reinitialization. *
* Calls to eval use the memory array as the working store for the program * being evaluated. The memory retains its state between calls to eval, unless * initInterpreter is called again, or an already-allocated memory is * re-initialized to null using the clearMemory method. *
* All of the public methods called by eval are publicly accessible. * However, they are likely of lesser value to outside callers, given that eval * dispatches to the appropriate submethods based on the type of tree it * receives. *
* See the method comments for further details. * */ public class PascalInterpreter { /** * Construct this with the given static pool and stack size. Call * initInterpter to do the work. */ public PascalInterpreter (int staticSize, int stackSize) { initInterpreter(staticSize, stackSize); } /** * Evaluate the given tree node, in the context of the given symbol table. * The tree must be that of an executable Pascal construct, which is one of * the following: a list of statements, a single statement, or an * expression. *
* For statement evaluation, the output of eval is a modified program
* memory. For expression evaluation, output is the returned Value, and a
* modified memory if the expression has side effects. The evaluation of
* statements and statement lists always produces a null return value.
*
* CSC 330 NOTE: The statement structure of EJay is a bit different than
* Pascal. In particular, data decls are intermingled with stmts in a
* block. This is not the case in a Pascal compound statement. So, you'll
* have to do things a bit differently at the top level of the EJay eval
* method than is done here for Pascal. One thing for sure, you should not
* print out an error if you see a data decl here. You should either pick
* out the stmts in advance, or ignore data decls if they come up.
*
* ANOTHER 330 NOTE: To simplify Assignment 4, no nested blocks will be
* part of the test data, so you don't have to deal with them here.
*/
public Value eval(TreeNode node, SymbolTable symtab) {
if (isStmtList(node)) {
evalStmtList((TreeNodeList) node, symtab);
return null;
}
else if (isStmt(node)) {
evalStmt(node, symtab);
return null;
}
else if (isExpr(node)) {
return evalExpr(node, symtab);
}
else {
Utilities.error(node,
"Node type " + symNames.map[node.id] + " is unevaluatable.");
return null;
}
}
/**
* Evaluate a stmtlist by looping through each and call eval on it.
*/
public void evalStmtList(TreeNodeList nodes, SymbolTable symtab) {
TreeNode node;
TreeNodeList rest;
boolean done = false;
for (node = nodes.node, rest = nodes.siblings; !done; ) {
eval(node, symtab);
if (rest == null) {
done = true;
}
else {
node = rest.node;
rest = rest.siblings;
}
}
}
/**
* Evaluate an individual Pascal statement, which is one of null,
* assignment, if, procedure call, or compound.
*/
public void evalStmt(TreeNode stmt, SymbolTable symtab) {
switch (stmt.id) {
case 0:
break;
case sym.ASSMNT:
evalAssmnt((TreeNode2) stmt, symtab);
break;
case sym.IF:
evalIf((TreeNode3) stmt, symtab);
break;
case sym.PROCEDURE:
evalProcCall((TreeNode2) stmt, symtab);
break;
case sym.BEGIN:
eval(((TreeNode1) stmt).child, symtab);
}
}
/**
* Evaluate an assignment statement by evaluating the LHS as an l-value and
* the RHS as an r-value expression. If the type of the LHS and RHS are
* the same, then assign the r-value to the memory location designated by
* the l-value. If the LHS does not designate a memory location, output an
* error message. Also output an error if the LHS and RHS types differ.
*/
public void evalAssmnt(TreeNode2 assmntStmt, SymbolTable symtab) {
LValue lval = evalIdentLValue((LeafNode) assmntStmt.child1, symtab);
Value rval = evalExpr(assmntStmt.child2, symtab);
/*
* Outta here if the LHS does not designate a memory address.
*/
if (lval == null) {
Utilities.error(assmntStmt.child1,
"The LHS of an assignment must be an assignable designator.");
return;
}
/*
* Also outta here if RHS is null, which means there was an error
* during its evaluation. The error message(s) for this will have
* already occurred, so don't do any more.
*/
if (rval == null) {
return;
}
/*
* Do the assignment if the LHS and RHS are the same type. This is the
* BOTTOM LINE when it comes to the evaluation of assignment.
*
* Have a close look here -- we're defining assignment in EJay using
* assignment in Java. It's taken a bit of work to get here, but
* ultimately this is what interpretation is about. I.e., the
* fundamental constructs of the language being interpreted are defined
* in terms of the fundamental constructs of the language in which the
* interpreter is implemented. Pretty curious, actually.
*/
if (Types.equiv(lval.type, rval.type)) {
memory[lval.getVal()] = rval.val;
}
else {
Utilities.error(assmntStmt,
"The type of the LHS and RHS of an assignment must be the same.");
}
}
/**
* Eval an if by first evaluating its test expr. Then, eval the then part
* of the statement if the test expr is true, else eval the else part if
* present, or return if the if part is false. If the value is not
* boolean, or its value is null, then output and error and return without
* evaluating either part of the statement.
*/
public void evalIf(TreeNode3 ifStmt, SymbolTable symtab) {
Value ifTest = evalExpr(ifStmt.child1, symtab);
if (Types.isBool(ifTest.type)) {
/* evaluate the then or else, as appropriate */
}
else {
Utilities.error(ifStmt.child1,
"The type of the expression in an if statement must be boolean");
}
}
/**
* Eval a proc call by performing these steps.
*
* (1) looking up the procedure name in the symtab
* (2) pushing an activation record onto the stack
* (3) binding actuals to formals, in the callING environment
* (4) eval'ing the proc body, in the callED environment
* (5) popping the activation record and returning the value, if any
*
* Output an error message under the following circumstances:
*
* (1) the proc name lookup fails or is not that of a procedure
* (2) the number or types of actuals and formals do not agree
*/
public Value evalProcCall(TreeNode2 procCall, SymbolTable symtab) {
//
// CSC 330 NOTE: The implementation details of this are left for your
// work in assignment 4. We'll discuss conceptual details in class.
//
return null;
}
/**
* Eval an expr by switching on its node id. This will the id of a literal
* value, an identifier, or an operator. All cases are handled in the
* appropriate methods.
*/
public Value evalExpr(TreeNode expr, SymbolTable symtab) {
switch (expr.id) {
case sym.INT:
return evalInt((LeafNode) expr);
//
// Other literals go here.
//
case sym.IDENT:
return evalIdent((LeafNode) expr, symtab);
case sym.PLUS:
return evalPlus((TreeNode2) expr, symtab);
case sym.TIMES:
return evalTimes((TreeNode2) expr, symtab);
//
// Other binary and unary exprs go here.
//
}
return null; // to shut the compiler up
}
/**
* Eval an addition expression by evaluating both operands, adding them
* together, and returning a Value for the result. Output an error if both
* operands are not the same numeric type, and return null. Also, return
* null if either or both operands evaluates to null.
*/
public Value evalPlus(TreeNode2 expr, SymbolTable symtab) {
Value v1 = eval(expr.child1, symtab);
Value v2 = eval(expr.child2, symtab);
/*
* Propagate a null return from either or both args. No new error
* message is necessary here.
*/
if ((v1 == null) || (v2 == null)) {
return null;
}
/*
* Output an error if both operands aren't the same numeric type.
*/
if (! ((v1.type.id == v2.type.id) && Types.equiv(v1.type, v2.type))) {
Utilities.error(expr,
"Operands of '+' must be the same numeric type.");
}
/*
* Add the value of v2 to v1, and return v1. In order to cast out in
* Java, we have to separate the int and float cases. Note that in
* Java 1.5, these casts are not necessary.
*
* This is another BOTTOM LINE place, where some real work is getting
* done. Here we are implementing EJay addition directly in terms of
* Java's addition.
*/
if (Types.isInt(v1.type)) {
v1.val = new Integer(((Integer)v1.val).intValue() +
((Integer) v2.val).intValue());
}
else {
v1.val = new Float(((Float)v1.val).floatValue() +
((Float) v2.val).floatValue());
}
return v1;
}
/**
* A la evalPlus.
*/
public Value evalTimes(TreeNode2 expr, SymbolTable symtab) {
Value v1 = eval(expr.child1, symtab);
Value v2 = eval(expr.child2, symtab);
if ((v1 == null) || (v2 == null)) {
return null;
}
if (! ((v1.type.id == v2.type.id) && Types.equiv(v1.type, v2.type))) {
Utilities.error(expr,
"Operands of '*' must be the same numeric type.");
}
if (Types.isInt(v1.type)) {
v1.val = new Integer(((Integer)v1.val).intValue() *
((Integer) v2.val).intValue());
}
else {
v1.val = new Float(((Float)v1.val).floatValue() *
((Float) v2.val).floatValue());
}
return v1;
}
/**
* Evaluate a designator expression as an r-value. In Pascal, a designator
* is either an identifier for an array reference. In the case of
* identifier, this method just passes the buck to evalIdent. The case of
* an array ref will be dealt with in the next version of the example (Week
* 6).
*/
public Value evalDesig(TreeNode desig, SymbolTable symtab) {
if (isIdent(desig)) {
return evalIdent((LeafNode) desig, symtab);
}
//
// CSC 330 NOTE: Compute the LValue for an array or struct selector.
//
return null;
}
/**
* Eval the given ident as an r-value. First, look it up in the given
* symtab. If an entry is found, get the address of the ident via getAddr.
* Then return a new Value that contains the memory value at the computed
* address, and the type of the ident. If the ident lookup fails, or if
* the entry is not a variable, then output an error message and return
* null.
*/
public Value evalIdent(LeafNode ident, SymbolTable symtab) {
SymbolTableEntry entry = symtab.lookup((String) ident.value);
if (entry == null) {
Utilities.error(ident,
"Identifier " + ident.value + " is not defined.");
return null;
}
if (! entry.getClass().getName().equals("VariableEntry")) {
Utilities.error(ident,
"Identifier " + ident.value + " is not a variable.");
return null;
}
int addr = getAddr((VariableEntry) entry);
return new Value(memory[addr], entry.type);
}
/**
* Eval the given designator as an l-value. This follows the same
* procedure as evalDesig, except here a new LValue is returned with the
* address value, instead of the contents of memory at that address.
*/
public LValue evalDesigLValue(TreeNode desig, SymbolTable symtab) {
if (isIdent(desig)) {
return evalIdentLValue((LeafNode) desig, symtab);
}
//
// CSC 330 NOTE: Compute the LValue for an array or struct selector.
//
return null;
}
/**
* Eval the given ident as an l-value. This follows the same procedure as
* evalIdent, except here a new LValue is returned with the address value,
* instead of the contents of memory at that address.
*/
public LValue evalIdentLValue(LeafNode ident, SymbolTable symtab) {
SymbolTableEntry entry = symtab.lookup((String) ident.value);
if (entry == null) {
Utilities.error(ident,
"Identifier " + ident.value + " is not defined.");
return null;
}
if (! entry.getClass().getName().equals("VariableEntry")) {
Utilities.error(ident,
"Identifier " + ident.value + " is not a variable.");
return null;
}
int addr = getAddr((VariableEntry) entry);
return new LValue(new Integer(addr), entry.type);
}
/**
* Eval an integer literal by making a new value for it.
*/
public Value evalInt(LeafNode integer) {
return new Value(integer.value, Types.IntType);
}
/**
* Return true if the given node is an identifier.
*/
public boolean isIdent(TreeNode node) {
return node.id == sym.IDENT;
}
/**
* Get the memory address of the given variable entry. If the entry is at
* level 0, then the address is relative to the 0th element of the static
* pool. If the lexical level of the entry is > 0, then the address is
* relative to the current tos.
*/
public int getAddr(VariableEntry entry) {
if (entry.level == 0) {
return entry.memoryLocation;
}
else {
return entry.memoryLocation + tos;
}
}
/**
* Return true if the given tree node is a statement list.
*/
public boolean isStmtList(TreeNode node) {
return node.getClass().getName().equals("TreeNodeList");
}
/**
* Return true if the given tree node is a statement.
*/
public boolean isStmt(TreeNode node) {
return
(node.id == sym.ASSMNT)
||
(node.id == sym.IF)
||
(node.id == sym.PROCEDURE)
||
(node.id == sym.BEGIN)
||
(node.id == 0); // a null stmt
}
/**
* Return true if the given tree node is an expr.
*/
public boolean isExpr(TreeNode node) {
return
(node.id == sym.IDENT) ||
(node.id == sym.REAL) ||
(node.id == sym.INT) ||
(node.id == sym.CHAR) ||
(node.id == sym.LEFT_BRKT) ||
(node.id == sym.LESS) ||
(node.id == sym.GTR) ||
(node.id == sym.EQ) ||
(node.id == sym.LESS_EQ) ||
(node.id == sym.GTR_EQ) ||
(node.id == sym.NOT_EQ) ||
(node.id == sym.PLUS) ||
(node.id == sym.MINUS) ||
(node.id == sym.OR) ||
(node.id == sym.TIMES) ||
(node.id == sym.DIVIDE) ||
(node.id == sym.AND) ||
(node.id == sym.NOT);
}
/**
* Initialize the interpreter by allocating the memory and initializing the
* top-of-stack pointer (tos). tos is initialized to the value of the
* memory index after the end of the static pool, which is the value of the
* given staticSize parameter. The total size of memory is staticSize +
* stackSize. The stackSize and stackSize data fields are also
* initialized, for subsequent runtime reference.
*/
public void initInterpreter(int staticSize, int stackSize) {
memory = Memory.allocate(staticSize + stackSize);
tos = staticSize;
curActRecSize = 0;
this.staticSize = staticSize;
this.stackSize = stackSize;
}
/**
* Clear an already-allocated memory by setting all of its elements to
* null. Initialize the tos to staticSize, i.e., the stack is empty.
*/
public void clearMemory() {
Memory.clear(memory);
tos = staticSize;
curActRecSize = 0;
}
/**
* Dump all of currently used memory to stdout, both static pool and
* stack.
*/
public void dumpMemory() {
System.out.println();
dumpStaticPool();
dumpStack();
}
/**
* Dump the static pool to stdout.
*/
public void dumpStaticPool() {
System.out.println("Static Pool:");
Memory.dump(memory, staticSize);
System.out.println();
}
/**
* Dump the stack to stdout, starting with tos as the relative 0 address in
* the dump.
*/
public void dumpStack() {
System.out.print("Stack:");
if (tos == staticSize) {
System.out.println(" empty");
}
else {
System.out.println();
Memory.dumpRelative(memory, staticSize + 1, tos + curActRecSize);
}
}
/** Program memory, organized logically with a static pool on the top, and
* a growable stack starting after the last allocated element of static
* memory. Organizing static and stack storage into a single common memory
* pool allows uniform handling of static and stack addresses. Viz., both
* types of address are an integer index into the common memory. Static
* addresses are relative to offset 0. Stack addresses are relative to the
* current value of the tos index. */
public Object[] memory;
/** Current top-of-stack index. */
public int tos;
/** The fixed size of the static memory pool, as initialized by
* initInterpreter. */
public int staticSize;
/** The maximum size of the stack, as initialized by initInterpreter. */
public int stackSize;
/** Size of the most recently pushed activation. This is only used as a
* convenience for full stack dumps.
*/
public int curActRecSize;
}