+use crate::lexer::{Lexer, Token};
+
+#[derive(Debug, PartialEq)]
+pub enum ExprAST {
+    /// Number - Expression class for numeric literals like "1.0".
+    Number(f64),
+
+    /// Variable - Expression class for referencing a variable, like "a".
+    Variable(String),
+
+    /// Binary - Expression class for a binary operator.
+    Binary(char, Box<ExprAST>, Box<ExprAST>),
+
+    /// Call - Expression class for function calls.
+    Call(String, Vec<ExprAST>),
+}
+
+/// PrototypeAST - This class represents the "prototype" for a function,
+/// which captures its name, and its argument names (thus implicitly the number
+/// of arguments the function takes).
+#[derive(Debug, PartialEq)]
+pub struct PrototypeAST(pub String, pub Vec<String>);
+
+/// FunctionAST - This class represents a function definition itself.
+#[derive(Debug, PartialEq)]
+pub struct FunctionAST(pub PrototypeAST, pub ExprAST);
+
+/// Parse result with String as Error type (to be compliant with tutorial).
+type ParseResult<T> = Result<T, String>;
+
+/// Parser for the `kaleidoscope` language.
+pub struct Parser<I>
+where
+    I: Iterator<Item = char>,
+{
+    lexer: Lexer<I>,
+    cur_tok: Option<Token>,
+}
+
+impl<I> Parser<I>
+where
+    I: Iterator<Item = char>,
+{
+    pub fn new(lexer: Lexer<I>) -> Self {
+        Parser {
+            lexer,
+            cur_tok: None,
+        }
+    }
+
+    // -----------------------
+    //   Simple Token Buffer
+    // -----------------------
+
+    /// Implement the global variable `int CurTok;` from the tutorial.
+    ///
+    /// # Panics
+    /// Panics if the parser doesn't have a current token.
+    pub fn cur_tok(&self) -> &Token {
+        self.cur_tok.as_ref().expect("Parser: Expected cur_token!")
+    }
+
+    /// Advance the `cur_tok` by getting the next token from the lexer.
+    ///
+    /// Implement the fucntion `int getNextToken();` from the tutorial.
+    pub fn get_next_token(&mut self) {
+        self.cur_tok = Some(self.lexer.gettok());
+    }
+
+    // ----------------------------
+    //   Basic Expression Parsing
+    // ----------------------------
+
+    /// numberexpr ::= number
+    ///
+    /// Implement `std::unique_ptr<ExprAST> ParseNumberExpr();` from the tutorial.
+    fn parse_num_expr(&mut self) -> ParseResult<ExprAST> {
+        match *self.cur_tok() {
+            Token::Number(num) => {
+                // Consume the number token.
+                self.get_next_token();
+                Ok(ExprAST::Number(num))
+            }
+            _ => unreachable!(),
+        }
+    }
+
+    /// parenexpr ::= '(' expression ')'
+    ///
+    /// Implement `std::unique_ptr<ExprAST> ParseParenExpr();` from the tutorial.
+    fn parse_paren_expr(&mut self) -> ParseResult<ExprAST> {
+        // Eat '(' token.
+        assert_eq!(*self.cur_tok(), Token::Char('('));
+        self.get_next_token();
+
+        let v = self.parse_expression()?;
+
+        if *self.cur_tok() == Token::Char(')') {
+            // Eat ')' token.
+            self.get_next_token();
+            Ok(v)
+        } else {
+            Err("expected ')'".into())
+        }
+    }
+
+    /// identifierexpr
+    ///   ::= identifier
+    ///   ::= identifier '(' expression* ')'
+    ///
+    /// Implement `std::unique_ptr<ExprAST> ParseIdentifierExpr();` from the tutorial.
+    fn parse_identifier_expr(&mut self) -> ParseResult<ExprAST> {
+        let id_name = match self.cur_tok.take() {
+            Some(Token::Identifier(id)) => {
+                // Consume identifier.
+                self.get_next_token();
+                id
+            }
+            _ => unreachable!(),
+        };
+
+        if *self.cur_tok() != Token::Char('(') {
+            // Simple variable reference.
+            Ok(ExprAST::Variable(id_name))
+        } else {
+            // Call.
+
+            // Eat '(' token.
+            self.get_next_token();
+
+            let mut args: Vec<ExprAST> = Vec::new();
+
+            // If there are arguments collect them.
+            if *self.cur_tok() != Token::Char(')') {
+                loop {
+                    let arg = self.parse_expression()?;
+                    args.push(arg);
+
+                    if *self.cur_tok() == Token::Char(')') {
+                        // Eat ')' token.
+                        self.get_next_token();
+                        break;
+                    }
+
+                    if *self.cur_tok() != Token::Char(',') {
+                        return Err("Expected ')' or ',' in argument list".into());
+                    }
+
+                    self.get_next_token();
+                }
+            }
+
+            Ok(ExprAST::Call(id_name, args))
+        }
+    }
+
+    /// primary
+    ///   ::= identifierexpr
+    ///   ::= numberexpr
+    ///   ::= parenexpr
+    ///
+    /// Implement `std::unique_ptr<ExprAST> ParsePrimary();` from the tutorial.
+    fn parse_primary(&mut self) -> ParseResult<ExprAST> {
+        match *self.cur_tok() {
+            Token::Identifier(_) => self.parse_identifier_expr(),
+            Token::Number(_) => self.parse_num_expr(),
+            Token::Char('(') => self.parse_paren_expr(),
+            _ => Err("unknown token when expecting an expression".into()),
+        }
+    }
+
+    // -----------------------------
+    //   Binary Expression Parsing
+    // -----------------------------
+
+    /// /// expression
+    ///   ::= primary binoprhs
+    ///
+    /// Implement `std::unique_ptr<ExprAST> ParseExpression();` from the tutorial.
+    fn parse_expression(&mut self) -> ParseResult<ExprAST> {
+        let lhs = self.parse_primary()?;
+        self.parse_bin_op_rhs(0, lhs)
+    }
+
+    /// binoprhs
+    ///   ::= ('+' primary)*
+    ///
+    /// Implement `std::unique_ptr<ExprAST> ParseBinOpRHS(int ExprPrec, std::unique_ptr<ExprAST> LHS);` from the tutorial.
+    fn parse_bin_op_rhs(&mut self, expr_prec: isize, mut lhs: ExprAST) -> ParseResult<ExprAST> {
+        loop {
+            let tok_prec = get_tok_precedence(self.cur_tok());
+
+            // Not a binary operator or precedence is too small.
+            if tok_prec < expr_prec {
+                return Ok(lhs);
+            }
+
+            let binop = match self.cur_tok.take() {
+                Some(Token::Char(c)) => {
+                    // Eat binary operator.
+                    self.get_next_token();
+                    c
+                }
+                _ => unreachable!(),
+            };
+
+            // lhs BINOP1 rhs BINOP2 remrhs
+            //     ^^^^^^     ^^^^^^
+            //     tok_prec   next_prec
+            //
+            // In case BINOP1 has higher precedence, we are done here and can build a 'Binary' AST
+            // node between 'lhs' and 'rhs'.
+            //
+            // In case BINOP2 has higher precedence, we take 'rhs' as 'lhs' and recurse into the
+            // 'remrhs' expression first.
+
+            // Parse primary expression after binary operator.
+            let mut rhs = self.parse_primary()?;
+
+            let next_prec = get_tok_precedence(self.cur_tok());
+            if tok_prec < next_prec {
+                // BINOP2 has higher precedence thatn BINOP1, recurse into 'remhs'.
+                rhs = self.parse_bin_op_rhs(tok_prec + 1, rhs)?
+            }
+
+            lhs = ExprAST::Binary(binop, Box::new(lhs), Box::new(rhs));
+        }
+    }
+
+    // --------------------
+    //   Parsing the Rest
+    // --------------------
+
+    /// prototype
+    ///   ::= id '(' id* ')'
+    ///
+    /// Implement `std::unique_ptr<PrototypeAST> ParsePrototype();` from the tutorial.
+    fn parse_prototype(&mut self) -> ParseResult<PrototypeAST> {
+        let id_name = match self.cur_tok.take() {
+            Some(Token::Identifier(id)) => {
+                // Consume the identifier.
+                self.get_next_token();
+                id
+            }
+            other => {
+                // Plug back current token.
+                self.cur_tok = other;
+                return Err("Expected function name in prototype".into());
+            }
+        };
+
+        if *self.cur_tok() != Token::Char('(') {
+            return Err("Expected '(' in prototype".into());
+        }
+
+        let mut args: Vec<String> = Vec::new();
+        loop {
+            self.get_next_token();
+
+            match self.cur_tok.take() {
+                Some(Token::Identifier(arg)) => args.push(arg),
+                Some(Token::Char(',')) => {}
+                other => {
+                    self.cur_tok = other;
+                    break;
+                }
+            }
+        }
+
+        if *self.cur_tok() != Token::Char(')') {
+            return Err("Expected ')' in prototype".into());
+        }
+
+        // Consume ')'.
+        self.get_next_token();
+
+        Ok(PrototypeAST(id_name, args))
+    }
+
+    /// definition ::= 'def' prototype expression
+    ///
+    /// Implement `std::unique_ptr<FunctionAST> ParseDefinition();` from the tutorial.
+    pub fn parse_definition(&mut self) -> ParseResult<FunctionAST> {
+        // Consume 'def' token.
+        assert_eq!(*self.cur_tok(), Token::Def);
+        self.get_next_token();
+
+        let proto = self.parse_prototype()?;
+        let expr = self.parse_expression()?;
+
+        Ok(FunctionAST(proto, expr))
+    }
+
+    /// external ::= 'extern' prototype
+    ///
+    /// Implement `std::unique_ptr<PrototypeAST> ParseExtern();` from the tutorial.
+    pub fn parse_extern(&mut self) -> ParseResult<PrototypeAST> {
+        // Consume 'extern' token.
+        assert_eq!(*self.cur_tok(), Token::Extern);
+        self.get_next_token();
+
+        self.parse_prototype()
+    }
+
+    /// toplevelexpr ::= expression
+    ///
+    /// Implement `std::unique_ptr<FunctionAST> ParseTopLevelExpr();` from the tutorial.
+    pub fn parse_top_level_expr(&mut self) -> ParseResult<FunctionAST> {
+        let e = self.parse_expression()?;
+        let proto = PrototypeAST("".into(), Vec::new());
+        Ok(FunctionAST(proto, e))
+    }
+}
+
+/// Get the binary operator precedence.
+///
+/// Implement `int GetTokPrecedence();` from the tutorial.
+fn get_tok_precedence(tok: &Token) -> isize {
+    match tok {
+        Token::Char('<') => 10,
+        Token::Char('+') => 20,
+        Token::Char('-') => 20,
+        Token::Char('*') => 40,
+        _ => -1,
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::{ExprAST, FunctionAST, Parser, PrototypeAST};
+    use crate::lexer::Lexer;
+
+    fn parser(input: &str) -> Parser<std::str::Chars> {
+        let l = Lexer::new(input.chars());
+        let mut p = Parser::new(l);
+
+        // Drop initial coin, initialize cur_tok.
+        p.get_next_token();
+
+        p
+    }
+
+    #[test]
+    fn parse_number() {
+        let mut p = parser("13.37");
+
+        assert_eq!(p.parse_num_expr(), Ok(ExprAST::Number(13.37f64)));
+    }
+
+    #[test]
+    fn parse_variable() {
+        let mut p = parser("foop");
+
+        assert_eq!(
+            p.parse_identifier_expr(),
+            Ok(ExprAST::Variable("foop".into()))
+        );
+    }
+
+    #[test]
+    fn parse_primary() {
+        let mut p = parser("1337 foop \n bla(123)");
+
+        assert_eq!(p.parse_primary(), Ok(ExprAST::Number(1337f64)));
+
+        assert_eq!(p.parse_primary(), Ok(ExprAST::Variable("foop".into())));
+
+        assert_eq!(
+            p.parse_primary(),
+            Ok(ExprAST::Call("bla".into(), vec![ExprAST::Number(123f64)]))
+        );
+    }
+
+    #[test]
+    fn parse_binary_op() {
+        // Operator before RHS has higher precedence, expected AST
+        //
+        //       -
+        //      / \
+        //     +     c
+        //    / \
+        //   a   b
+        let mut p = parser("a + b - c");
+
+        let binexpr_ab = ExprAST::Binary(
+            '+',
+            Box::new(ExprAST::Variable("a".into())),
+            Box::new(ExprAST::Variable("b".into())),
+        );
+
+        let binexpr_abc = ExprAST::Binary(
+            '-',
+            Box::new(binexpr_ab),
+            Box::new(ExprAST::Variable("c".into())),
+        );
+
+        assert_eq!(p.parse_expression(), Ok(binexpr_abc));
+    }
+
+    #[test]
+    fn parse_binary_op2() {
+        // Operator after RHS has higher precedence, expected AST
+        //
+        //       +
+        //      / \
+        //     a   *
+        //        / \
+        //       b   c
+        let mut p = parser("a + b * c");
+
+        let binexpr_bc = ExprAST::Binary(
+            '*',
+            Box::new(ExprAST::Variable("b".into())),
+            Box::new(ExprAST::Variable("c".into())),
+        );
+
+        let binexpr_abc = ExprAST::Binary(
+            '+',
+            Box::new(ExprAST::Variable("a".into())),
+            Box::new(binexpr_bc),
+        );
+
+        assert_eq!(p.parse_expression(), Ok(binexpr_abc));
+    }
+
+    #[test]
+    fn parse_prototype() {
+        let mut p = parser("foo(a,b)");
+
+        let proto = PrototypeAST("foo".into(), vec!["a".into(), "b".into()]);
+
+        assert_eq!(p.parse_prototype(), Ok(proto));
+    }
+
+    #[test]
+    fn parse_definition() {
+        let mut p = parser("def bar( arg0 , arg1 ) arg0 + arg1");
+
+        let proto = PrototypeAST("bar".into(), vec!["arg0".into(), "arg1".into()]);
+
+        let body = ExprAST::Binary(
+            '+',
+            Box::new(ExprAST::Variable("arg0".into())),
+            Box::new(ExprAST::Variable("arg1".into())),
+        );
+
+        let func = FunctionAST(proto, body);
+
+        assert_eq!(p.parse_definition(), Ok(func));
+    }
+
+    #[test]
+    fn parse_extern() {
+        let mut p = parser("extern baz()");
+
+        let proto = PrototypeAST("baz".into(), vec![]);
+
+        assert_eq!(p.parse_extern(), Ok(proto));
+    }
+}
+

+