#!/bin/bash
# I, Ian Kelling, follow the GNU license recommendations at
# https://www.gnu.org/licenses/license-recommendations.en.html. They
# recommend that small programs, < 300 lines, be licensed under the
# Apache License 2.0. This file contains or is part of one or more small
# programs. If a small program grows beyond 300 lines, I plan to switch
# its license to GPL.

# Copyright 2024 Ian Kelling

# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at

#     http://www.apache.org/licenses/LICENSE-2.0

# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.


_lnf_existing_link() {
  local target dest_file dest_dir
  target="$1"
  dest_file="$2"
  dest_dir="$3"
  if [[ -L $dest_file ]]; then
    if [[ $(readlink $dest_file) == "$target" ]]; then
      # Leave the link in place, but make sure it's
      # owner & group is as if we created it.
      # links all get 777 perms, so
      # we already know that is right.

      # test for setgid.
      if [[ $(stat -L -c%a "$dest_dir") == 2??? ]]; then
        grp=$(stat -L -c%g "$dest_dir") || return $?
      else
        grp=$(id -g) || return $?
      fi
      if [[ $EUID == 0 && $(stat -c%u "$dest_file") != 0 ]]; then
        chown -h 0:$grp "$dest_file" || return $?
      elif [[ $(stat -c%g "$dest_file") != "$grp" ]]; then
        chgrp -h $grp "$dest_file" || return $?
      fi
      do_exit=true
      return 0
    fi
    to_remove+=("$dest_file")
  elif [[ -e $dest_file ]]; then
    to_remove+=("$dest_file")
  fi
  to_link+=("$target")
}
lnf() {
  local help="Usage:
       lnf [OPTIONS] -T TARGET LINK_NAME     (1st form)
       lnf [OPTIONS] TARGET                  (2nd form)
       lnf [OPTIONS] TARGET... DIRECTORY     (3rd form)
Create symlinks forcefully

If the link already exists, make it's ownership be the same as if it was
newly created (only chown if we are root).  Removes existing files using
rm -rf. Create directory of link if needed. Slightly more restrictive
arguments than ln.

In the 1st form, create a link to TARGET with the name LINK_NAME.  In the 2nd
form, create a link to TARGET in the current directory.  In the 3rd form, create
links to each TARGET in DIRECTORY.

-n|--dry-run     Do verbose dry run.
-v|--verbose     Print commands which modify the filesystem.
-h|--help        Print help and exit.
"


  local temp nodir
  local verbose=false
  local dry_run=false
  local do_exit=false
  temp=$(getopt -l help,dry-run,verbose hnTv "$@") || usage 1
  eval set -- "$temp"
  while true; do
    case $1 in
      -n|--dry-run) dry_run=true; verbose=true; shift ;;
      -T) nodir=-T; shift ;;
      -v|--verbose) verbose=true; shift ;;
      -h|--help) echo "$help"; return 0 ;;
      --) shift; break ;;
      *) echo "$0: Internal error! unexpected args: $*" ; exit 1 ;;
    esac
  done

  if (( $# == 0 )); then
    echo "$help"
    return 1
  fi

  if [[ $nodir ]]; then
    if (( $# != 2 )); then
      echo "lnf: error: expected 2 arguments with -T flag. Got $#"
      return 1
    fi
  fi

  local reset_extglob=false
  ! shopt extglob >/dev/null && reset_extglob=true
  shopt -s extglob


  local -a to_remove to_link
  local ret prefix dest_dir grp target
  local mkdir=false

  if [[ $nodir ]]; then
    dest_file="$2"
    dest_dir="$(dirname "$dest_file")"
    _lnf_existing_link "$1" "$dest_file" "$dest_dir" || return $?
    if $do_exit; then return 0; fi
    if [[ ! -d $dest_dir ]]; then
      mkdir=true
      if [[ -e $dest_dir || -L $dest_dir ]]; then
        to_remove+=("$dest_dir")
      fi
    fi
    to_link+=("$dest_file")
  elif (( $# >= 2 )); then
    dest_dir="${!#}"
    if [[ -d $dest_dir ]]; then
      prefix="$dest_dir" # last arg
      for target in "${@:1:$(( $# - 1 ))}"; do # all but last arg
        # Remove 1 or more trailing slashes, using.
        dest_file="${target%%+(/)}"
        # remove any leading directory components, add prefix
        dest_file="$prefix/${target##*/}"
        _lnf_existing_link "$target" "$dest_file" "$dest_dir"
      done
    else
      to_link+=("${@:1:$(( $# - 1 ))}")
      mkdir=true
    fi
    if (( ${#to_link[@]} == 0 )); then
      return 0
    fi
    to_link+=("$dest_dir")
  elif  [[ $# -eq 1 ]]; then
    dest_file="${1##*/}"
    _lnf_existing_link "$1" "$dest_file" . || return $?
    if $do_exit; then return 0; fi
  fi
  if (( ${#to_remove[@]} >= 1 )); then
    if $verbose; then
      echo "lnf: rm -rf -- ${to_remove[*]}"
    fi
    if ! $dry_run; then
      rm -rf -- "${to_remove[@]}"
    fi
  fi

  $reset_extglob && shopt -u extglob

  if $mkdir; then
    if $verbose; then
      echo "lnf: mkdir -p $dest_dir"
    fi

    if ! $dry_run && ! mkdir -p "$dest_dir"; then
      echo "lnf error: failed to make directory $dest_dir"
      return 1
    fi
  fi

  if $verbose; then
    echo "lnf: ln -s $nodir -- ${to_link[*]}"
  fi
  if ! $dry_run; then
    ln -s $nodir -- "${to_link[@]}"
  fi
}
lnf "$@"