graph_drawing/line.py

from node import Leaf


def generate_line(root):
    """
    Print an expression tree in a single text line. Where needed, add
    parentheses.

    >>> from node import Node, Leaf
    >>> l0, l1 = Leaf(1), Leaf(2)
    >>> plus = Node('+', l0, l1)
    >>> print generate_line(plus)
    1 + 2

    >>> plus2 = Node('+', l0, l1)
    >>> times = Node('*', plus, plus2)
    >>> print generate_line(times)
    (1 + 2) * (1 + 2)

    >>> l2 = Leaf(3)
    >>> uminus = Node('-', l2)
    >>> times = Node('*', plus, uminus)
    >>> print generate_line(times)
    (1 + 2) * -3

    >>> exp = Leaf('x')
    >>> inf = Leaf('oo')
    >>> minus_inf = Node('-', inf)
    >>> integral = Node('int', exp, minus_inf, inf)
    >>> print generate_line(integral)
    int(x, -oo, oo)
    """

    operators = [
            ('+', '-'),
            ('*', '/', 'mod'),
            ('^', )
            ]

    max_pred = len(operators)

    def is_operator(node):
        """
        Check if a given node is an operator (otherwise, it's a function).
        """
        label = node.title()
        either = lambda a, b: a or b

        return reduce(either, map(lambda x: label in x, operators))

    def pred(node):
        """
        Get the precedence of an operator node.
        """
        # Check binary and n-ary operators
        if not isinstance(node, Leaf) and len(node) > 1:
            op = node.title()

            for i, group in enumerate(operators):
                if op in group:
                    return i

        # Unary operator and leaves have highest precedence
        return max_pred

    def traverse(node):
        """
        The expression tree is traversed using preorder traversal:
        1. Visit the root
        2. Traverse the subtrees in left-to-right order
        """
        if not node:
            return '<empty expression>'

        op = node.title()

        if not node.nodes:
            return op

        arity = len(node)

        if is_operator(node):
            if arity == 1:
                # Unary operator
                result = op + traverse(node[0])
            else:
                # N-ary operator
                node_pred = pred(node)
                result = ''
                sep = ' ' + op + ' '
                prev = None

                for child in node:
                    exp = traverse(child)

                    # Check if there is a precedence conflict.
                    # If so, add parentheses
                    child_pred = pred(child)

                    if child_pred < node_pred or \
                            (child_pred == node_pred and s != child.title()):
                        exp = '(' + exp + ')'

                    # a * b -> ab
                    # a * 2 -> a * 2
                    # a * (...) -> a(...)
                    # 2 * a -> 2a
                    if prev and not (op == '*' \
                                     and ((is_id(prev) and not is_int(child)) \
                                          or (is_int(prev) and is_id(child)))):
                        result += sep

                    result += exp
                    prev = child
        else:
            # Function
            result = op + '(' + ', '.join(map(traverse, node)) + ')'

        return result

    return traverse(root)


def is_id(node):
    return isinstance(node, Leaf) and not node.title().isdigit()


def is_int(node):
    return isinstance(node, Leaf) and node.title().isdigit()