postfix_evaluator.h

// -*- mode: C++ -*-

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// postfix_evaluator.h: Postfix (reverse Polish) notation expression evaluator.
//
// PostfixEvaluator evaluates an expression, using the expression itself
// in postfix (reverse Polish) notation and a dictionary mapping constants
// and variables to their values.  The evaluator supports standard
// arithmetic operations, assignment into variables, and when an optional
// MemoryRange is provided, dereferencing.  (Any unary key-to-value operation
// may be used with a MemoryRange implementation that returns the appropriate
// values, but PostfixEvaluator was written with dereferencing in mind.)
//
// The expression language is simple.  Expressions are supplied as strings,
// with operands and operators delimited by whitespace.  Operands may be
// either literal values suitable for ValueType, or constants or variables,
// which reference the dictionary.  The supported binary operators are +
// (addition), - (subtraction), * (multiplication), / (quotient of division),
// % (modulus of division), and @ (data alignment). The alignment operator (@)
// accepts a value and an alignment size, and produces a result that is a
// multiple of the alignment size by truncating the input value.
// The unary ^ (dereference) operator is also provided.  These operators
// allow any operand to be either a literal value, constant, or variable.
// Assignment (=) of any type of operand into a variable is also supported.
//
// The dictionary is provided as a map with string keys.  Keys beginning
// with the '$' character are treated as variables.  All other keys are
// treated as constants.  Any results must be assigned into variables in the
// dictionary.  These variables do not need to exist prior to calling
// Evaluate, unless used in an expression prior to being assigned to.  The
// internal stack state is not made available after evaluation, and any
// values remaining on the stack are treated as evidence of incomplete
// execution and cause the evaluator to indicate failure.
//
// PostfixEvaluator is intended to support evaluation of "program strings"
// obtained from MSVC frame data debugging information in pdb files as
// returned by the DIA APIs.
//
// Author: Mark Mentovai

#ifndef PROCESSOR_POSTFIX_EVALUATOR_H__
#define PROCESSOR_POSTFIX_EVALUATOR_H__


#include <map>
#include <string>
#include <vector>

#include "common/using_std_string.h"

namespace google_breakpad {

using std::map;
using std::vector;

class MemoryRegion;

template<typename ValueType>
class PostfixEvaluator {
 public:
  typedef map<string, ValueType> DictionaryType;
  typedef map<string, bool> DictionaryValidityType;

  // Create a PostfixEvaluator object that may be used (with Evaluate) on
  // one or more expressions.  PostfixEvaluator does not take ownership of
  // either argument.  |memory| may be NULL, in which case dereferencing
  // (^) will not be supported.  |dictionary| may be NULL, but evaluation
  // will fail in that case unless set_dictionary is used before calling
  // Evaluate.
  PostfixEvaluator(DictionaryType* dictionary, const MemoryRegion* memory)
      : dictionary_(dictionary), memory_(memory), stack_() {}

  // Evaluate the expression, starting with an empty stack. The results of
  // execution will be stored in one (or more) variables in the dictionary.
  // Returns false if any failures occur during execution, leaving
  // variables in the dictionary in an indeterminate state. If assigned is
  // non-NULL, any keys set in the dictionary as a result of evaluation
  // will also be set to true in assigned, providing a way to determine if
  // an expression modifies any of its input variables.
  bool Evaluate(const string& expression, DictionaryValidityType* assigned);

  // Like Evaluate, but provides the value left on the stack to the
  // caller. If evaluation succeeds and leaves exactly one value on
  // the stack, pop that value, store it in *result, and return true.
  // Otherwise, return false.
  bool EvaluateForValue(const string& expression, ValueType* result);

  DictionaryType* dictionary() const { return dictionary_; }

  // Reset the dictionary.  PostfixEvaluator does not take ownership.
  void set_dictionary(DictionaryType* dictionary) {dictionary_ = dictionary; }

 private:
  // Return values for PopValueOrIdentifier
  enum PopResult {
    POP_RESULT_FAIL = 0,
    POP_RESULT_VALUE,
    POP_RESULT_IDENTIFIER
  };

  // Retrieves the topmost literal value, constant, or variable from the
  // stack.  Returns POP_RESULT_VALUE if the topmost entry is a literal
  // value, and sets |value| accordingly.  Returns POP_RESULT_IDENTIFIER
  // if the topmost entry is a constant or variable identifier, and sets
  // |identifier| accordingly.  Returns POP_RESULT_FAIL on failure, such
  // as when the stack is empty.
  PopResult PopValueOrIdentifier(ValueType* value, string* identifier);

  // Retrieves the topmost value on the stack.  If the topmost entry is
  // an identifier, the dictionary is queried for the identifier's value.
  // Returns false on failure, such as when the stack is empty or when
  // a nonexistent identifier is named.
  bool PopValue(ValueType* value);

  // Retrieves the top two values on the stack, in the style of PopValue.
  // value2 is popped before value1, so that value1 corresponds to the
  // entry that was pushed prior to value2.  Returns false on failure.
  bool PopValues(ValueType* value1, ValueType* value2);

  // Pushes a new value onto the stack.
  void PushValue(const ValueType& value);

  // Evaluate expression, updating *assigned if it is non-zero. Return
  // true if evaluation completes successfully. Do not clear the stack
  // upon successful evaluation.
  bool EvaluateInternal(const string& expression,
                        DictionaryValidityType* assigned);

  bool EvaluateToken(const string& token,
                     const string& expression,
                     DictionaryValidityType* assigned);

  // The dictionary mapping constant and variable identifiers (strings) to
  // values.  Keys beginning with '$' are treated as variable names, and
  // PostfixEvaluator is free to create and modify these keys.  Weak pointer.
  DictionaryType* dictionary_;

  // If non-NULL, the MemoryRegion used for dereference (^) operations.
  // If NULL, dereferencing is unsupported and will fail.  Weak pointer.
  const MemoryRegion* memory_;

  // The stack contains state information as execution progresses.  Values
  // are pushed on to it as the expression string is read and as operations
  // yield values; values are popped when used as operands to operators.
  vector<string> stack_;
};

}  // namespace google_breakpad


#endif  // PROCESSOR_POSTFIX_EVALUATOR_H__