ahem/lib/ahem/parser.rb

class Parser
  def initialize(lexer)
    @lexer = lexer
    @current_token = @lexer.get_token
  end

  def parse
    tree = Array.new

    tree << statement until at_end

    tree
  end

  def statement
    if @current_token.type == TokenKinds::IF
      conditional
    elsif @current_token.type == TokenKinds::LET
      variable_declaration
    elsif @current_token.type == TokenKinds::FUNCTION
      next_token = @lexer.peek
      if next_token && next_token.type == TokenKinds::IDENTIFIER
        function_definition
      else
        expression_statement
      end
    elsif @current_token.type == TokenKinds::CLASS
      class_definition
    elsif @current_token.type == TokenKinds::FOR
      for_loop
    else
      expression_statement
    end
  end

  def expression_statement
    expr = expression
    eat(TokenKinds::SEMICOLON)
    return expr
  end

  def variable_declaration
    eat(TokenKinds::LET)
    name = identifier
    eat(TokenKinds::EQUALS)
    value = expression
    eat(TokenKinds::SEMICOLON)
    AST::VariableDeclaration.new(name, value)
  end

  def function_definition
    eat(TokenKinds::FUNCTION)
    name = identifier
    params = parameters
    body = block
    AST::FunctionDefinition.new(name, params, body)
  end

  def class_definition
    eat(TokenKinds::CLASS)

    class_name = AST::Identifier.new(eat(TokenKinds::CLASS_NAME).value)

    eat(TokenKinds::LBRACE)

    members = Array.new

    while [TokenKinds::PUBLIC, TokenKinds::PRIVATE].include?(
      @current_token.type
    )
      is_public =
        if @current_token.type == TokenKinds::PUBLIC
          eat(TokenKinds::PUBLIC)
          true
        elsif @current_token.type == TokenKinds::PRIVATE
          eat(TokenKinds::PRIVATE)
          false
        end

      while !at_end && true
        name = identifier
        members << AST::Member.new(name, is_public)
        if @current_token.type == TokenKinds::COMMA
          eat(TokenKinds::COMMA)
        else
          break
        end
      end

      eat(TokenKinds::SEMICOLON)
    end

    methods = Array.new
    while @current_token.type == TokenKinds::FUNCTION
      methods << function_definition
    end

    eat(TokenKinds::RBRACE)

    AST::ClassDefinition.new(class_name, members, methods)
  end

  def conditional
    branches = Array.new
    eat(TokenKinds::IF)
    condition = expression
    block = self.block
    branches << AST::Branch.new(condition, block)

    while @current_token.type == TokenKinds::ELSEIF
      eat(TokenKinds::ELSEIF)
      condition = expression
      block = self.block
      branches << AST::Branch.new(condition, block)
    end

    if @current_token.type == TokenKinds::ELSE
      eat(TokenKinds::ELSE)
      block = self.block
      branches << AST::Branch.new(AST::Boolean.new(true), block)
    end

    AST::Conditional.new(branches)
  end

  def for_loop
    eat(TokenKinds::FOR)
    iterator = identifier
    eat(TokenKinds::IN)
    iterable = expression
    body = block
    AST::ForLoop.new(iterator, iterable, body)
  end

  def parameters
    params = Array.new
    eat(TokenKinds::LPAREN)

    while @current_token.type == TokenKinds::IDENTIFIER
      params << identifier

      if @current_token.type == TokenKinds::COMMA
        eat(TokenKinds::COMMA)
      else
        break
      end
    end

    eat(TokenKinds::RPAREN)

    params
  end

  def block
    statements = Array.new

    eat(TokenKinds::LBRACE)
    statements << statement until @current_token.type == TokenKinds::RBRACE
    eat(TokenKinds::RBRACE)

    AST::Block.new(statements)
  end

  def expression
    assignment
  end

  private

  def assignment
    expr = binary

    if @current_token.type == TokenKinds::EQUALS
      unless expr.is_a?(AST::Identifier)
        raise 'Invalid left hand side of assignment'
      end
      eat(TokenKinds::EQUALS)
      value = expression
      AST::Assignment.new(expr, value)
    else
      expr
    end
  end

  def binary
    left = comparison

    if @current_token.type == TokenKinds::OPERATOR
      operator = @current_token.value
      advance
      right = comparison
      AST::Binary.new(operator, left, right)
    else
      left
    end
  end

  def comparison
    left = multiplication

    if @current_token.type == TokenKinds::OPERATOR &&
       %i[< > <= >=].include?(@current_token.type)
      operator = @current_token.value
      advance
      right = multiplication
      AST::Binary.new(operator, left, right)
    else
      left
    end
  end

  def addition
    left = multiplication

    if @current_token.type == TokenKinds::OPERATOR &&
       %i[+ -].include?(@current_token.value)
      operator = @current_token.value
      advance
      right = multiplication
      AST::Binary.new(operator, left, right)
    else
      left
    end
  end

  def multiplication
    left = unary

    if @current_token.type == TokenKinds::OPERATOR &&
       %i[* /].include?(@current_token.value)
      operator = @current_token.value
      advance
      right = unary
      AST::Binary.new(operator, left, right)
    else
      left
    end
  end

  def unary
    if @current_token.type == TokenKinds::OPERATOR &&
       %i[- !].include?(@current_token.value)
      operator = eat(TokenKinds::OPERATOR).value
      expr = primary
      AST::Unary.new(operator, expr)
    else
      primary
    end
  end

  def primary
    token = @current_token
    expr =
      case token.type
      when TokenKinds::NULL
        advance
        AST::Null.new
      when TokenKinds::BOOLEAN
        advance
        AST::Boolean.new(token.value)
      when TokenKinds::NUMBER
        advance
        AST::Number.new(token.value)
      when TokenKinds::STRING
        advance
        AST::String.new(token.value)
      when TokenKinds::ATOM
        advance
        AST::Atom.new(token.value)
      when TokenKinds::IDENTIFIER
        identifier
      when TokenKinds::LBRACKET
        array
      when TokenKinds::LBRACE
        hash
      when TokenKinds::LPAREN
        eat(TokenKinds::LPAREN)
        e = expression
        eat(TokenKinds::RPAREN)
        e
      when TokenKinds::FUNCTION
        function_expression
      else
        raise "Unexpected token #{token.type}"
      end

    if @current_token.type == TokenKinds::LPAREN
      args = arguments
      AST::FunctionCall.new(expr, args)
    elsif @current_token.type == TokenKinds::LBRACKET
      eat(TokenKinds::LBRACKET)
      key = expression
      eat(TokenKinds::RBRACKET)
      AST::Index.new(expr, key)
    else
      expr
    end
  end

  def function_expression
    eat(TokenKinds::FUNCTION)
    params = parameters
    body = block
    AST::FunctionDefinition.new(nil, params, body)
  end

  def identifier
    token = eat(TokenKinds::IDENTIFIER)
    AST::Identifier.new(token.value)
  end

  def array
    elements = Array.new
    eat(TokenKinds::LBRACKET)

    until @current_token.type == TokenKinds::RBRACKET
      elements << expression

      if @current_token.type == TokenKinds::COMMA
        eat(TokenKinds::COMMA)
      else
        break
      end
    end

    eat(TokenKinds::RBRACKET)

    AST::Array.new(elements)
  end

  def hash
    h = Hash.new
    eat(TokenKinds::LBRACE)

    until @current_token.type == TokenKinds::RBRACE
      key = eat(TokenKinds::ATOM).value
      eat(TokenKinds::ROCKET)
      value = expression

      h[key] = value

      if @current_token.type == TokenKinds::COMMA
        eat(TokenKinds::COMMA)
      else
        break
      end
    end

    eat(TokenKinds::RBRACE)

    AST::Hash.new(h)
  end

  def arguments
    args = Array.new
    eat(TokenKinds::LPAREN)

    until @current_token.type == TokenKinds::RPAREN
      args << expression

      if @current_token.type == TokenKinds::COMMA
        eat(TokenKinds::COMMA)
      else
        break
      end
    end

    eat(TokenKinds::RPAREN)

    args
  end

  def eat(type)
    token = @current_token

    if token.type == type
      advance
      token
    else
      if token.nil?
        raise "Unexpected #{token.type} - expected #{type}"
      else
        raise "Unexpected #{token.type} - expected #{type}"
      end
    end
  end

  def advance
    @current_token = @lexer.get_token
  end

  def at_end
    @current_token.type == TokenKinds::EOF
  end
end