Rewrote the parser to have two lookahead tokens

andreabergia · Dec 26, 2014 · 2babdf9 · 2babdf9
1 parent 0153b9a
commit 2babdf9
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Showing 2 changed files with 19 additions and 9 deletions.
diff --git a/sources/parser.cpp b/sources/parser.cpp
@@ -7,18 +7,26 @@
 Parser::Parser(std::istream& istream, std::ostream &ostream)
     :lexer_(istream), ostream_(ostream)
 {
-    advance();
+    for (int i = 0; i < NUM_LOOK_AEAHD_TOKENS; ++i) {
+        if (lexer_.hasNextToken()) {
+            nextTokens_[i] = lexer_.nextToken();
+        }
+    }
 }
 
 void Parser::advance()
 {
-    if (lexer_.hasNextToken()) {
-        nextToken_ = lexer_.nextToken();
-    } else {
-        nextToken_ = Token(TokenType::END_OF_INPUT, "");
+    // Shift tokens one position back
+    for (int i = 0; i < NUM_LOOK_AEAHD_TOKENS - 1; ++i) {
+        nextTokens_[i] = nextTokens_[i + 1];
     }
-}
 
+    // Set last available token
+    nextTokens_[NUM_LOOK_AEAHD_TOKENS - 1] =
+            lexer_.hasNextToken()
+                ? lexer_.nextToken()
+                : Token{TokenType::END_OF_INPUT, ""};
+}
 
 void Parser::parseProgram()
 {

diff --git a/sources/parser.h b/sources/parser.h
@@ -17,12 +17,14 @@ class Parser
     double evalNextExpression();
 
 private:
+    static const int NUM_LOOK_AEAHD_TOKENS = 2;
+
     // A doubleToDoubleFunction is a pointer to a function taking a double and returning a double
     using doubleToDoubleFunction = double(*)(double);
 
     std::ostream &ostream_;
     Lexer lexer_;
-    Token nextToken_;
+    Token nextTokens_[NUM_LOOK_AEAHD_TOKENS];
     std::map<std::string, doubleToDoubleFunction> functions_ {
         {"exp", std::exp},
         {"log", std::log},
@@ -31,8 +33,8 @@ class Parser
         {"tan", std::tan}
     };
 
-    inline const Token &getNextToken() { return nextToken_; }
-    inline bool hasNextToken() const { return nextToken_.getTokenType() != TokenType::END_OF_INPUT; }
+    inline const Token &getNextToken() const { return nextTokens_[0]; }
+    inline bool hasNextToken() const { return getNextToken().getTokenType() != TokenType::END_OF_INPUT; }
     void advance();
 
     double evalNextTerm();