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// Copyright (C) 2006 Google Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// 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),
// and % (modulus of division). 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>
namespace google_airbag {
using std::map;
using std::string;
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, MemoryRegion *memory)
: dictionary_(dictionary), memory_(memory), stack_() {}
// Evaluate the expression. The results of execution will be stored
// in one (or more) variables in the dictionary. Returns false if any
// failures occure 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);
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);
// 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.
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_airbag
#endif // PROCESSOR_POSTFIX_EVALUATOR_H__
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