#!/usr/bin/env python3

import argparse
import os
import re
import subprocess
import sys
from typing import List, Dict

import logging

logging.basicConfig(format="[%(asctime)s]%(levelname)s:%(message)s", level=logging.INFO)

# regular expression to find the name of the main branch on the remote
re_match_remote_branch = re.compile(r"ref: refs/heads/(?P<branch>\S+)\tHEAD")

# never delete any branches or references with one of these names
immortal_ref = ["main", "master", "HEAD"]

# that's how my remotes are usually named, and in that order of preference.
preferred_remotes = ["origin", "github", "work"]


class GitConfig(object):
    """Represent the configuration for the git repository."""

    def __init__(self) -> None:
        self.guess_remote()
        self.guess_primary_branch()
        self.remote_ref = f"{self.remote_name}/{self.primary_branch}"
        self.me = os.getenv("USER")

    def guess_remote(self) -> None:
        """Guess the name and URL for the remote repository.

        If the name of the remote is from the list of preferred remote, we
        return the name and URL.

        If we don't have a remote set, throw an exception.
        If we don't find any remote, throw an exception.
        """
        candidates = subprocess.run(
            ["git", "config", "--get-regexp", "remote\.[a-z0-9]+.url"],
            capture_output=True,
            check=True,
            encoding="utf-8",
        ).stdout.splitlines()

        if len(candidates) == 0:
            raise ValueError("No remote is defined.")

        remotes = dict()

        for candidate in candidates:
            parts = candidate.split(" ")
            remote = parts[0].split(".")[1]
            url = parts[1]
            remotes[remote] = url

        for remote in preferred_remotes:
            if remote in remotes:
                self.remote_name = remote
                self.remote_url = remotes[remote]
                return

        raise ValueError("can't find the preferred remote.")

    def guess_primary_branch(self) -> None:
        """Guess the primary branch on the remote.

        If we can't figure out the default branch, thrown an exception.
        """
        remote_head = subprocess.run(
            ["git", "ls-remote", "--symref", self.remote_name, "HEAD"],
            capture_output=True,
            check=True,
            encoding="utf-8",
        ).stdout.splitlines()

        for rh in remote_head:
            m = re_match_remote_branch.match(rh)
            if m:
                self.primary_branch = m.group("branch")
                return

        raise ValueError(
            f"can't find the name of the remote branch for {self.remote_name}"
        )


def is_git_repository() -> bool:
    """Check if we are inside a git repository.

    Return True if we are, false otherwise."""
    res = subprocess.run(
        ["git", "rev-parse", "--show-toplevel"], check=False, capture_output=True
    )
    return not res.returncode


def fetch(remote: str):
    """Fetch updates from the remote repository."""
    subprocess.run(["git", "fetch", remote, "--prune"], capture_output=True, check=True)


def ref_sha(ref: str) -> str:
    """Get the sha from a ref."""
    res = subprocess.run(
        ["git", "show-ref", ref], capture_output=True, check=True, encoding="utf-8"
    )
    return res.stdout.rstrip()


def get_branches(options: List[str]) -> List[str]:
    """Get a list of branches."""
    return subprocess.run(
        ["git", "branch", "--format", "%(refname:short)"] + options,
        capture_output=True,
        check=True,
        encoding="utf-8",
    ).stdout.splitlines()


def ref_tree(ref: str) -> str:
    """Get the reference from a tree."""
    return subprocess.run(
        ["git", "rev-parse", f"{ref}^{{tree}}"],
        check=True,
        capture_output=True,
        encoding="utf-8",
    ).stdout.rstrip()


def rebase_local_branches(config: GitConfig, local_rebase_tree_id: dict) -> None:
    """Try to rebase the local branches that have been not been merged."""
    for branch in get_branches(["--list", "--no-merged"]):
        _rebase_local_branch(branch, config, local_rebase_tree_id)


def _rebase_local_branch(
    branch: str, config: GitConfig, local_rebase_tree_id: dict
) -> None:
    res = subprocess.run(
        [
            "git",
            "merge-base",
            "--is-ancestor",
            config.remote_ref,
            branch,
        ],
        check=False,
        capture_output=True,
    )
    if res.returncode == 0:
        logging.info(
            f"local branch {branch} is already a descendant of {config.remote_ref}."
        )
        local_rebase_tree_id[branch] = ref_tree(branch)
        return

    logging.info(f"local branch {branch} will be rebased on {config.remote_ref}.")
    subprocess.run(
        ["git", "checkout", "--force", branch], check=True, capture_output=True
    )
    res = subprocess.run(
        ["git", "rebase", config.remote_ref], check=True, capture_output=True
    )
    if res.returncode == 0:
        logging.info(f"local branch {branch} has been rebased")
        local_rebase_tree_id[branch] = ref_tree(branch)
    else:
        logging.error(f"failed to rebase local branch {branch}.")
        subprocess.run(["git", "rebase", "--abort"], check=True)
        subprocess.run(
            ["git", "checkout", "--force", config.primary_branch], check=True
        )
        subprocess.run(["git", "reset", "--hard"], check=True)


def rebase_remote_branches(
    config: GitConfig, local_rebase_tree_id: dict, main_sha: str
) -> None:
    for branch in get_branches(
        ["--list", "-r", f"{config.me}/*", "--no-merged", config.remote_ref]
    ):
        _rebase_remote_branches(branch, config, local_rebase_tree_id, main_sha)


def _rebase_remote_branches(
    branch: str, config: GitConfig, local_rebase_tree_id: dict, main_sha: str
) -> None:
    remote, head = branch.split("/")
    if head in immortal_ref:
        return

    res = subprocess.run(
        ["git", "merge-base", "--is-ancestor", config.remote_ref, branch],
        check=False,
        capture_output=True,
    )
    if res.returncode == 0:
        logging.info(
            f"local branch {branch} is already a descendant of {config.remote_ref}."
        )
        return

    logging.info(f"remote branch {branch} will be rebased on {config.remote_ref}.")

    sha = ref_sha(branch)
    subprocess.run(["git", "checkout", "--force", sha], capture_output=True, check=True)
    res = subprocess.run(
        ["git", "rebase", config.remote_ref],
        capture_output=True,
        check=True,
    )
    if res.returncode == 0:
        new_sha = ref_sha("--head")
        short_sha = new_sha[0:8]
        logging.info(f"remote branch {branch} at {sha} rebased to {new_sha}.")
        if new_sha == main_sha:
            logging.info(f"remote branch {branch}, when rebased, is already merged!")
            logging.info(f"would run `git push {remote} :{head}'")
        elif new_sha == sha:
            logging.info(f"remote branch {branch}, when rebased, is unchanged!")
        elif ref_tree(new_sha) == local_rebase_tree_id.get(head, ""):
            logging.info(f"remote branch {branch}, when rebased, same as local branch!")
            logging.info(f"would run `git push --force-with-lease {remote} {head}'")
        else:
            logging.info(
                f"remote branch {branch} has been rebased to create {short_sha}!"
            )
            logging.info(
                f"would run `git push --force-with-lease {remote} {new_sha}:{head}'"
            )
    else:
        logging.error(f"failed to rebase remote branch {branch}.")
        subprocess.run(["git", "rebase", "--abort"], check=True)
        subprocess.run(
            ["git", "checkout", "--force", config.primary_branch], check=True
        )
        subprocess.run(["git", "reset", "--hard"], check=True)


def destroy_remote_merged_branches(config: GitConfig, dry_run: bool) -> None:
    """Destroy remote branches that have been merged."""
    for branch in get_branches(
        ["--list", "-r", f"{config.me}/*", "--merged", config.remote_ref]
    ):
        remote, head = branch.split("/")
        if head in immortal_ref:
            continue
        logging.info(f"remote branch {branch} has been merged")
        if dry_run:
            logging.info(f"would have run git push {remote} :{head}")
        else:
            subprocess.run(
                ["git", "push", remote, f":{head}"], check=True, encoding="utf-8"
            )


def destroy_local_merged_branches(config: GitConfig, dry_run: bool) -> None:
    """Destroy local branches that have been merged."""
    for branch in get_branches(["--list", "--merged", config.remote_ref]):
        if branch in immortal_ref:
            continue

        logging.info(f"local branch {branch} has been merged")
        if dry_run:
            logging.info(f"would have run git branch --delete --force {branch}")
        else:
            subprocess.run(
                ["git", "branch", "--delete", "--force", branch],
                check=True,
                encoding="utf-8",
            )


def workdir_is_clean() -> bool:
    """Check the git workdir is clean."""
    res = subprocess.run(
        ["git", "status", "--porcelain"],
        check=True,
        capture_output=True,
        encoding="utf-8",
    ).stdout.splitlines()
    return not len(res)


def main(dry_run: bool) -> bool:
    if not is_git_repository():
        logging.error("error: run this inside a git repository")
        return False

    if not workdir_is_clean():
        logging.error("the git workdir is not clean, commit or stash your changes.")
        return False

    config = GitConfig()

    # what's our current sha ?
    origin_main_sha = ref_sha(config.remote_ref)

    # let's get everything up to date
    fetch(config.remote_name)

    # let's get the new sha
    main_sha = ref_sha(config.remote_ref)

    if origin_main_sha != main_sha:
        logging.info(f"we started with {origin_main_sha} and now we have {main_sha}")

    local_rebase_tree_id: Dict[str, str] = dict()

    # try to rebase local branches that have been not been merged
    rebase_local_branches(config, local_rebase_tree_id)

    # try to rebase remote branches that have been not been merged
    rebase_remote_branches(config, local_rebase_tree_id, main_sha)

    # let's checkout to main now and see what left to do
    subprocess.run(
        ["git", "checkout", "--force", config.primary_branch],
        check=True,
        capture_output=True,
    )

    # branches on the remote that have been merged can be destroyed.
    destroy_remote_merged_branches(config, dry_run)

    # local branches that have been merged can be destroyed.
    destroy_local_merged_branches(config, dry_run)

    # TODO: restore to the branch I was on before ?
    return True


if __name__ == "__main__":
    parser = argparse.ArgumentParser(
        description="delete local and remote branches that have been merged."
    )
    parser.add_argument(
        "--dry-run",
        action=argparse.BooleanOptionalAction,
        help="when set to True, do not execute the destructive actions",
        default=True,
    )
    args = parser.parse_args()

    if not main(args.dry_run):
        sys.exit(1)