#!/usr/bin/env python
# vim: et

from __future__ import with_statement

import os
import sys
import commands
import exceptions

def nearest_git_objects(path):
    dotgit = os.path.join(path, ".git")
    objects = os.path.join(path, "objects")
    if os.path.isdir(dotgit):
        return nearest_git_objects(dotgit)
    elif os.path.isdir(objects):
        return objects
    raise exceptions.RuntimeError("Could not find git dir from " + path)

def alt_path(git_path):
    return os.path.join(git_path, "info", "alternates")

def read_objs(git_path):
    fn = alt_path(git_path)
    alts = set()
    if os.path.exists(fn):
        with open(fn) as f:
            alts = set([l.strip() for l in f.readlines()])

    return alts

def setup_alternates(from_objects, to_objects):
    alts = read_objs(from_objects)
    alts.add(to_objects)
    with open(alt_path(from_objects), "w") as f:
        f.write("\n".join(alts) + "\n")

def here():
    (e, o) = commands.getstatusoutput("git rev-parse --git-dir")
    if e != 0:
        raise exceptions.RuntimeError("This is not a git repo.")
    return nearest_git_objects(o.strip())

def setup_new_alternate(where):
    alt = nearest_git_objects(where)
    loc = here()
    print loc, "->", alt
    setup_alternates(loc, alt)

def display_alternates():
    p = here()
    alts = read_objs(p)
    print "Alternates for %s (%d):" % (p, len(alts))
    for s in sorted(alts):
        print "   %s" % s

if __name__ == '__main__':
    if len(sys.argv) < 2:
        display_alternates()
    else:
        setup_new_alternate(sys.argv[1])