#!/bin/bash
# Usage: git sync
#
# Fetches new objects from origin remote named either "upstream" or "origin",
# then iterates over each local branch that corresponds to a remote branch and:
#
# - If the local branch is outdated, fast-forward it;
# - If the local branch contains unpushed work, warn about it;
# - If the branch seems merged and its upstream branch was deleted, delete it.
#
# If a local branch doesn't have any upstream configuration, but has a
# same-named branch on the remote, assume that's its upstream branch.
set -e

while [ $# -gt 0 ]; do
  case "$1" in
  -h | --help )
    sed -ne '/^#/!q;s/.\{1,2\}//;1d;p' < "$0"
    exit 0
    ;;
  * )
    "$0" --help >&2
    exit 1
    ;;
  esac
  shift 1
done

GIT_DIR="$(git rev-parse --git-dir)"

if [ -e "${GIT_DIR}/refs/remotes/upstream" ]; then
  ORIGIN="upstream"
else
  ORIGIN="origin"
fi

if [ -t 1 ]; then
  RED=$'\e[31m'
  LIGHT_RED=$'\e[31;1m'
  GREEN=$'\e[32m'
  LIGHT_GREEN=$'\e[32;1m'
  RESET=$'\e[0m'
else
  RED=""
  LIGHT_RED=""
  GREEN=""
  LIGHT_GREEN=""
  RESET=""
fi

main_branch() {
  local remote="$1"
  local head="$(cat "${GIT_DIR}/refs/remotes/${remote}/HEAD" 2>/dev/null)"
  if [[ $head == "ref: "* ]]; then
    echo "${head#ref: refs/remotes/${remote}/}"
  else
    return 1
  fi
}

has_upstream_configuration() {
  local branch="$1"
  local remote="$(git config "branch.${branch}.remote")"
  [ "$remote" = "$ORIGIN" ] || return 1
}

upstream_branch() {
  local branch="$1"
  local resolved=""
  if resolved="$(git rev-parse --symbolic-full-name "${branch}@{upstream}" 2>/dev/null)"; then
    echo "$resolved"
  else
    return 1
  fi
}

is_ancestor() {
  git merge-base --is-ancestor "$@" || (
    merge_base=$(git merge-base "$@") && [[ $(git cherry $2 $(git commit-tree $(git rev-parse $1^{tree}) -p $merge_base -m _)) == "-"* ]]
  )
}

git fetch "$ORIGIN" --prune --quiet --progress

MASTER="$(main_branch "$ORIGIN")" || MASTER="master"
CURRENT_BRANCH="$(git symbolic-ref --short --quiet HEAD || true)"

git branch --list | \
  while read -r local_branch; do
    local_branch="${local_branch#* }"
    remote_branch="refs/remotes/${ORIGIN}/${local_branch}"
    gone=""

    if has_upstream_configuration "$local_branch"; then
      remote_branch="$(upstream_branch "$local_branch")" || gone=1
    elif [ ! -e "${GIT_DIR}/${remote_branch}" ]; then
      remote_branch="" && gone=1
    fi

    if [ -n "$remote_branch" ]; then
      shas=( `git rev-parse "$local_branch" "$remote_branch"` )
      local_sha="${shas[0]}"
      remote_sha="${shas[1]}"

      if [ "$local_sha" = "$remote_sha" ]; then
        : # branch is up to date.
      elif is_ancestor "$local_branch" "$remote_branch"; then
        # Local branch is behind
        if [ "$local_branch" = "$CURRENT_BRANCH" ]; then
          git merge --ff-only --quiet "$remote_branch"
        else
          git update-ref "refs/heads/${local_branch}" "$remote_branch"
        fi
        echo "${GREEN}Updated branch ${LIGHT_GREEN}${local_branch}${RESET} (was ${local_sha:0:7})."
      else
        # Local branch is ahead or diverged.
        # TODO: Decide whether to try clean merge check + rebase here.
        echo "warning: \`$local_branch' seems to contain unpushed commits" >&2
      fi
    elif [ -n "$gone" ]; then
      # Upstream branch got deleted
      if is_ancestor "$local_branch" "${ORIGIN}/${MASTER}"; then
        if [ "$local_branch" = "$CURRENT_BRANCH" ]; then
          git checkout --quiet "$MASTER"
          CURRENT_BRANCH="$MASTER"
        fi
        local_sha="$(git rev-parse "$local_branch")"
        git branch -D "$local_branch" >/dev/null
        echo "${RED}Deleted branch ${LIGHT_RED}${local_branch}${RESET} (was ${local_sha:0:7})."
      else
        echo "warning: \`$local_branch' is neither present on $ORIGIN nor merged into $MASTER. Have you pushed it yet?" >&2
      fi
    fi
  done