#!/bin/bash

abs() {
  [[ $[ $@ ] -lt 0 ]] && echo "$[ ($@) * -1 ]" || echo "$[ $@ ]"
}

DIMENSIONS=( $(identify "${1}" | cut -d\  -f3 | sed "s/x/ /g") )
POWERS=( 16 32 64 128 )

for POWER in ${POWERS[@]}; do
  WIDEDIFS+=( $(abs "${DIMENSIONS[0]}-${POWER}")  )
  HITEDIFS+=( $(abs "${DIMENSIONS[1]}-${POWER}")  )
done

IFS=$'\n'
WIDESORT=( $(sort -n <<<"${WIDEDIFS[*]}"))
HITESORT=( $(sort -n <<<"${HITEDIFS[*]}"))
unset IFS

for SIZE in $((DIMENSIONS[0] - WIDESORT[0])) $((DIMENSIONS[0] + WIDESORT[0])); do
  (( SIZE > 0 && (SIZE & (SIZE - 1)) == 0 )) && RESIZE=${SIZE}
done

TEMPFILE=$(mktemp)
convert "${1}" -resize ${RESIZE}x${RESIZE}\! ${TEMPFILE}
png2icns ${1/%png/icns} ${TEMPFILE}
rm ${TEMPFILE}