#!/usr/bin/env bash
set -euo pipefail

usage() {
  echo "Usage: $(basename "$0") <source.png> [output.icns]" >&2
}

gen-png() {
  local src="$1"
  local px="$2"
  local dpi="$3"
  local out="$4"

  # Resize longest side to <px>, then pad to a square <px>x<px> with transparency.
  local tmp_resized
  tmp_resized="${out}.tmp.png"

  # Scale preserving aspect ratio; longest side becomes <px>.
  sips -Z "$px" "$src" --out "$tmp_resized" > /dev/null

  # Pad to exact square dimensions centered on transparent background.
  sips --padToHeightWidth "$px" "$px" \
    "$tmp_resized" --out "$out" > /dev/null

  rm -f "$tmp_resized" || true

  # Set DPI metadata.
  sips -s dpiHeight "$dpi" -s dpiWidth "$dpi" "$out" > /dev/null
}

get-image-max-dim() {
  local src="$1"
  local w h

  # Capture width/height; avoid exiting on failure due to set -e/pipefail.
  w=$(sips -g pixelWidth "$src" 2> /dev/null |
    awk '/pixelWidth/ {print $2}') || true
  h=$(sips -g pixelHeight "$src" 2> /dev/null |
    awk '/pixelHeight/ {print $2}') || true

  if [[ -z "$w" || -z "$h" ]]; then
    echo "Failed to read image dimensions with sips: $src" >&2
    return 1
  fi

  if ((w > h)); then
    echo "$w"
  else
    echo "$h"
  fi
}

verify-png() {
  local src="$1"
  local fmt
  fmt=$(sips -g format "$src" 2> /dev/null |
    awk '/format/ {print tolower($2)}') || true
  if [[ -z "$fmt" ]]; then
    echo "Unable to determine image format with sips: $src" >&2
    return 1
  fi
  if [[ "$fmt" != "png" ]]; then
    echo "Input must be a PNG image: $src" >&2
    return 1
  fi
}

make-iconset() {
  local src="$1"
  local dest_dir="$2"

  mkdir -p "$dest_dir"

  # Sizes to generate (@1x). @2x files are double the pixels.
  local sizes=(16 32 128 256 512)

  # Determine maximum source dimension to avoid upscaling.
  local src_max
  if ! src_max=$(get-image-max-dim "$src"); then
    return 1
  fi

  local generated
  generated=0

  for sz in "${sizes[@]}"; do
    # @1x
    local out1="${dest_dir}/icon_${sz}x${sz}.png"
    if ((sz <= src_max)); then
      gen-png "$src" "$sz" 72 "$out1"
      generated=$((generated + 1))
    fi

    # @2x (double pixels, higher DPI)
    local dsz=$((sz * 2))
    local out2="${dest_dir}/icon_${sz}x${sz}@2x.png"
    if ((dsz <= src_max)); then
      gen-png "$src" "$dsz" 144 "$out2"
      generated=$((generated + 1))
    fi
  done

  if ((generated == 0)); then
    echo "Source image too small; no icon sizes generated: $src" >&2
    return 1
  fi
}

cleanup() {
  local path="$1"

  if [[ -z "$path" || ! -d "$path" ]]; then
    return 0
  fi

  # Remove generated PNGs inside the temp directory
  find "$path" -type f -name '*.png' -delete 2> /dev/null || true

  # Remove generated `*.iconset` inside the temp directory
  find "$path" -type d -name '*.iconset' -delete 2> /dev/null || true

  # Attempt to remove the temp directory if now empty
  rmdir "$path" 2> /dev/null || true
}

main() {
  if [[ $# -lt 1 || $# -gt 2 ]]; then
    usage
    exit 1
  fi

  local src="$1"
  local out="${2:-${src%.*}.icns}"

  if [[ ! -f "$src" ]]; then
    echo "Source file not found: $src" >&2
    exit 1
  fi

  if ! command -v sips > /dev/null 2>&1; then
    echo "This script requires 'sips' (macOS) but it wasn't found." >&2
    exit 1
  fi

  if ! command -v iconutil > /dev/null 2>&1; then
    echo "This script requires 'iconutil' (macOS) but it wasn't found." >&2
    exit 1
  fi

  local outdir
  outdir="$(dirname "$out")"
  mkdir -p "$outdir"

  # Create a temporary working directory
  tmp_root="$(mktemp -d 2> /dev/null || mktemp -d -t png2icns)"
  trap 'cleanup "$tmp_root"' EXIT

  # Verify input is PNG; we do not accept other formats.
  if ! verify-png "$src"; then
    exit 1
  fi

  local iconset_dir
  iconset_dir="${tmp_root}/icon.iconset"

  make-iconset "$src" "$iconset_dir"

  # Build `.icns` from the generated iconset
  if ! iconutil -c icns -o "$out" "$iconset_dir" 2> /dev/null; then
    echo "Failed to create .icns with iconutil" >&2
    exit 1
  fi

  echo "Done. ICNS written to: $out"
}

main "$@"