_get_empty_treeref()
{
  git mktree </dev/null
}

_make_pcommit()
{
  local timestamp; timestamp=$(date +'%s %z')
  git hash-object -t commit -w --stdin <<-_commit_data
	tree $(_get_empty_treeref)
	author TopGit <> $timestamp
	committer "$author"
	x-topgit-data-node yes

	TopGit data node

	This commit is used internally by TopGit. Please just ignore it.

	TopGit data follow:
	$(cat)
	_commit_data
}

_append_parentref_to_HEAD()
{
  local parent; parent="$1"
  local ref; ref=$(git cat-file commit HEAD |
                     sed -e "/^parent/aparent ${parent}" |
                     git hash-object -t commit -w --stdin)
  git update-ref HEAD $ref
}

_remove_parentref_from_HEAD()
{
  local parent; parent="$1"
  local ref; ref=$(git cat-file commit HEAD |
                     grep -v "^parent ${parent}" |
                     git hash-object -t commit -w --stdin)
  git update-ref HEAD $ref
}

# returns 0 if $1 is a valid TopGit pcommit
_is_pcommit()
{
  local pcommit; pcommit="$1"
  git cat-file commit $pcommit | grep -q '^x-topgit-data-node yes$'
}

# returns the ref of the pcommit attached to $1
# returns 1 when none is found
_find_pcommit()
{
  local commit; commit="$1"
  local parentnr; parentnr=1

  while :; do
    ref=$(tg_ds_resolve_commit "${commit}^${parentnr}") || break
    _is_pcommit "$ref" && printf "$ref\n" && return 0
    parentnr=$(($parentnr + 1))
  done
  return 1
}

# expects data on stdin and adds to HEAD
tg_ds_add_data()
{
  local author; author=$(git cat-file commit HEAD | sed -rne 's,^author ,,p')
  _append_parentref_to_HEAD $(_make_pcommit "$author")
}

# removes data from HEAD
tg_ds_remove_data()
{
  local pcommit; pcommit=$(_find_pcommit HEAD) || return   # no error
  _remove_parentref_from_HEAD $pcommit
}

# lists data on a given commit
tg_ds_list_data()
{
  local commit; commit="${1:-HEAD}"
  ref=$(tg_ds_resolve_commit $commit) || die "tg_ds_list_data: invalid commit: $commit"
  local pcommit; pcommit=$(_find_pcommit $commit) || return 1
  git cat-file commit $pcommit | sed -e '0,/^TopGit data follow:$/d'
}