#!/usr/bin/env bash

# exit if a command fails
set -o errexit

# exit if required variables are not set
set -o nounset

# number of threads to use
if [[ ${DEBUG} -eq 1 ]]; then
  parallel_thread_count=1
  export oxipng_thread_count=1
else
  parallel_thread_count=4
  export oxipng_thread_count=4
fi

# make sure the correct number of arguments are passed; if not, output syntax and exit
if [ "$#" -ne 1 ]; then
  echo -e "\nUsage: ${0} <temporary directory>\n"
  exit 1
fi

# set base directory (passed from invoking command)
base_dir="${1}"

# store current directory
current_dir="$(pwd)"
export current_dir
if [[ ${DEBUG} -eq 1 ]]; then
  echo "DEBUG: Current directory is ${current_dir}"
fi

# create matching filesystem hierarchy in tempdir
temp_dir="${base_dir}$(readlink -f .)"
export temp_dir
mkdir -p "${temp_dir}"
if [[ ${DEBUG} -eq 1 ]]; then
  echo "DEBUG: Created temporary directory ${temp_dir} for ${current_dir}"
fi

# create temporary file to store pending commands
commands="${temp_dir}/commands.txt"
if [[ ${DEBUG} -eq 1 ]]; then
  echo "DEBUG: Storing commands for ${current_dir} in ${commands}"
fi

# attempt to optimize png, and embed checksum if successful
function optimize_png() {
  local file_name
  file_name="${1}"

  local stored_checksum
  stored_checksum="$(exiftool -s3 -Comment "${file_name}")"

  local current_checksum
  current_checksum="$(identify -verbose "${file_name}" | grep signature | awk '{print $2}')"

  local new_file
  new_file="${temp_dir}/${file_name}"

  local old_file
  old_file="${current_dir}/${file_name}"

  if [ "${current_checksum}" == "${stored_checksum}" ]; then
    if [[ ${DEBUG} -eq 1 ]]; then
      echo "DEBUG: Stored and calculated checksums match for ${file_name}"
    fi

    return
  fi

  if oxipng --opt 2 --quiet --alpha --keep="tEXt,display" --zopfli --out "${new_file}" --threads "${oxipng_thread_count}" "${old_file}"; then
    # calculate size improvements
    old_size=$(du -b "${old_file}" | awk '{print $1}')
    old_size_human=$(du -h "${old_file}" | awk '{print $1}')
    new_size=$(du -b "${new_file}" | awk '{print $1}')
    new_size_human=$(du -h "${new_file}" | awk '{print $1}')
    new_percentage=$(echo "scale=2; ${new_size}/${old_size}*100" | bc)
    rounded_percentage=$(printf "%.0f" "${new_percentage}")

    # calculate new image data checksum
    new_checksum=$(identify -verbose "${new_file}" | grep signature | awk '{print $2}')

    # update metadata
    exiftool -Comment="${new_checksum}" "${new_file}" >/dev/null 2>&1

    # output stats
    echo "Reduced file size of ${old_file} from ${old_size_human} to ${new_size_human} (${rounded_percentage}%)"

    # discard changes if in debug mode
    if [[ ${DEBUG} -eq 1 ]]; then
      echo "DEBUG: Not modifying ${file_name}."
      sleep 1

      return
    fi

    # replace with updated file
    mv "${new_file}" "${old_file}"
  fi
}
export -f optimize_png

# generate list of commands to run
for input_file in *.png; do
  if [ -f "${input_file}" ]; then
    echo "optimize_png \"${input_file}\"" >> "${commands}"
  fi
done

# run commands in parallel
if [ -s "${commands}" ]; then
  parallel --line-buffer --no-notice --jobs "${parallel_thread_count}" < "${commands}"
fi