#!/bin/bash

set -euo pipefail

function show_usage {
	cat <<-END
	usage: $0 [OPTIONS...] input_file output_file

	Converts a PDF file to a series of JPEG images using convert to
	reduce the quality and then merges back the result in a single pdf 

	options:
		-q, --quality    Set the JPEG quality (default: 75)
		-v, --verbose    Enable verbose output
		-h, --help       Display this help message
END
	exit 1
}

quality=75
verbose=false

while [[ $# -gt 0 ]]
do
	key="$1"

	case $key in
		-q|--quality)
			quality="$2"
			shift # past argument
			shift # past value
			;;
		-v|--verbose)
			verbose=true
			shift # past argument
			;;
		-h|--help)
			show_usage
			;;
		-*)
			echo "Unknown option: $key"
			show_usage
			;;
		*)
			break
			;;
	esac
done

if [ -z "$1" ] || [ -z "$2" ]; then
	show_usage
fi

input_file="$1"
output_file="$2"

temp_pages_dir="$(mktemp -d)"

function logf {
	if [ "$verbose" = true ]; then
		printf "$@"
	fi
}

logf "Converting \"$input_file\" to \"$output_file\"\n"

page_count=$(pdfinfo "$input_file" | grep Pages | awk '{print $2}')

logf "Processing $page_count pages\n"

start=$(date +%s.%N)
for i in $(seq 0 $((page_count-1))); do
	iter_start=$(date +%s.%N)

	convert -density 300 -quality "$quality" "${input_file}[$i]" "$temp_pages_dir/$i.jpg"

	# Compute estimated remaining time
  end=$(date +%s.%N)
  elapsed=$(echo "$end - $start" | bc)
  iter_elapsed=$(echo "$end - $iter_start" | bc)
  remaining=$(echo "($elapsed / ($i + 1)) * ($page_count - $i)" | bc)
  remaining_fmt=$(printf "%02d:%02d:%02d" $((remaining/3600)) $((remaining/60)) $((remaining%60)))

	logf "Compressed page $((i+1)) in $(printf '%.2f' "$iter_elapsed")s (estimated remaining time $remaining_fmt)\n"
done

logf "Merging compressed pages to a single PDF\n"
convert "$temp_pages_dir"/*.jpg "$output_file"

rm -r "$temp_pages_dir"

logf "Done\n"