"""Logic related to generated a stream of unique symbols for macros to use.

Exposes this functionality as the `gen_sym` function.
"""

from macropy.core.macros import *

@register(injected_vars)
def gen_sym(tree, **kw):
    """Create a generator that creates symbols which are not used in the given
    `tree`. This means they will be hygienic, i.e. it guarantees that they will
    not cause accidental shadowing, as long as the scope of the new symbol is
    limited to `tree` e.g. by a lambda expression or a function body"""
    @Walker
    def name_finder(tree, collect, **kw):
        if type(tree) is Name:
            collect(tree.id)
        if type(tree) is Import:
            names = [x.asname or x.name for x in tree.names]
            map(collect, names)
        if type(tree) is ImportFrom:
            names = [x.asname or x.name for x in tree.names]
            map(collect, names)
        if type(tree) in (FunctionDef, ClassDef):
            collect(tree.name)

    found_names = set(name_finder.collect(tree))

    def name_for(name="sym"):

        if name not in found_names:
            found_names.add(name)
            return name
        offset = 1
        while name + str(offset) in found_names:

            offset += 1
        found_names.add(name + str(offset))
        return name + str(offset)
    return name_for