#!/usr/bin/env python
""" commit-msg hook for git.

Hooks checks and alters commit message content based
on given rules.

This uses concept of project tagging, if there environmental variable
PROJECT_TAGS defined or hard coded TAGS variable then it takes
it's value as possible tag words.

Example:
    PROJECT_TAGS=ISSUES, anotherissue

Commits themselves should use convention::

    'TAG-1234: Commit message'

To us branch as basis for commit message creation. Then they should be in format::

    TAG-1234_some_optional_text


This should be installed as commit-msg into $GIT_DIR/hooks/ folder. Like
my_project/.git/hooks/commit-msg
"""
import sys
import os
import subprocess


TAGS = None


def get_tags():
    """ get project tags from PROJECT_TAGS environmental variable
    """
    tags = TAGS or os.getenv('PROJECT_TAGS', '')
    return tags.split(',')


def get_current_branch_name():
    return subprocess.check_output(['git', 'rev-parse', '--abbrev-ref', 'HEAD'])


def _get_commit_message_file():
    # The first argument is the filename where lies commit message
    return open(sys.argv[1], 'r+')


def read_commit_message():
    """ Read original commit message from file
    """
    with _get_commit_message_file() as f:
        return f.read().strip()


def write_commit_message(message):
    """ Write new commit message to file
    """
    with _get_commit_message_file() as f:
        f.seek(0)
        f.write(message)
        f.truncate()


def parse_tag(message):
    """ parse possible tag from message
    """
    return message.split('-')[0].strip()


def parse_tag_and_num_from_branch(branch_name):
    return branch_name.split('_')[0].strip()


def parse_tag_from_tagnum(tagnum):
    return tagnum.split('-')[0].strip()


def is_tagged(message):
    """ check if commit message has already a tag in it.
    """
    tags = get_tags()
    possible_tag = parse_tag(message)
    return any(possible_tag == tag for tag in tags)


def get_tagnum_from_branch():
    tags = get_tags()
    branch_name = get_current_branch_name()
    branch_tagnum = parse_tag_and_num_from_branch(branch_name)

    if parse_tag_from_tagnum(branch_tagnum) in tags:
        return branch_tagnum


def write_tagnum(message, tag=None):
    """ write tagnum to commit message.

    add tag if exists
    """
    if tag is not None:
        write_commit_message("{}: {}".format(tag, message))


def alter_message(message):
    """ Alter message content if needed

    tag is not added if it cannot be parsed from branch,
    or it already exists.
    """
    if not is_tagged(message):
        write_tagnum(message, get_tagnum_from_branch())


def run():
    message = read_commit_message()
    alter_message(message)


if __name__ == '__main__':
    try:
        run()
        sys.exit(0)
    except Exception as e:
        print(str(e))
        sys.exit(1)