pingpong

#!/bin/zsh

readonly program="$(basename "${0}")"

function depends_on {
  local -r all_deps=("${@}")
  local missing_deps=()

  for dep in "${all_deps[@]}"; do
    hash "${dep}" 2> /dev/null || missing_deps+=("${dep}")
  done

  if [[ "${#missing_deps[@]}" -gt 0 ]]; then
    echo 'Missing required tools:' >&2
    printf '  %s\n' "${missing_deps[@]}" >&2
    exit 1
  fi
}

function get_output_path_ext {
  local -r ext="${1}"
  local -r input_path="${2}"
  local output_path="${3}"

  if [[ -n "${output_path}" ]]; then
    [[ "${output_path##*.}" == "${ext}" ]] && echo "${output_path}" || echo "${output_path}${ext}"
  else
    output_path="${input_path%.*}${ext}"

    while [[ -e "${output_path}" ]]; do
      output_path="${output_path%.*}_$(date -u +'%Y.%m.%d.%H%M%S')${ext}"
    done

    echo "${output_path}"
  fi
}

function usage {
  echo "
    Stitch a video with its reversed version, for continuous loops

    Usage:
      ${program} [options] <file>

    Options:
      -o, --output-file <file>               File to output to. Default is saving next to the input file with same name and date.
      -h, --help                             Show this help.
  " | sed -E 's/^ {4}//'
}

args=()
while [[ "${1}" ]]; do
  case "${1}" in
    -h | --help)
      usage
      exit 0
      ;;
    -o | --output-file)
      readonly given_output_path="${2}"
      shift
      ;;
    --)
      shift
      args+=("${@}")
      break
      ;;
    -*)
      echo "Unrecognised option: ${1}"
      exit 1
      ;;
    *)
      args+=("${1}")
      ;;
  esac
  shift
done
set -- "${args[@]}"

if [[ "${#}" -eq 0 ]]; then
  usage
  exit 1
fi

readonly input_file="${1}"
readonly input_file_ext="$([[ "${input_file}" == *.* ]] && echo ".${input_file##*.}" || echo '')"
readonly output_file="$(get_output_path_ext "${input_file_ext}" "${input_file}" "${given_output_path}")"

depends_on 'ffmpeg'

ffmpeg -i "${input_file}" -an -filter_complex "[0]reverse[r];[0][r]concat" "${output_file}"