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

[ -t 0 ] && { echo "$0 must be run as a filter." >&2; exit 1; }

mfile="$1"; [ -z "$1" ] && { echo "No image file specified!" >&2; exit 1; }

BLOCKSIZE=`sed -nE 's/^#BLOCKSIZE=([0-9]+)$/\1/p' "$mfile"`
[ -z "$BLOCKSIZE" ] && { echo "Mapfile does not specify blocksize!" >&2; exit 1; }

map="$(grep -v ^\# "$mfile" \
	| awk '/[a-f0-9]{64}/{curfile=$2;next}{print curfile, $0}' \
	| sort -unk 3)"
nitems=`wc -l <<<"$map"` 

shopt -s expand_aliases
alias dd="dd bs=$BLOCKSIZE status=none"

{
pos=0
for (( i = 0; i < $nitems; i++ )) do
	read hash lblock block lengt _ < <(sed $((i+1))'q;d' <<<"$map")
	dd if=/dev/stdin count=$((block-pos))
	dd if=/dev/stdin count=$((lengt-1)) >/dev/null
	dd if=pool/$hash count=$((lengt-1)) skip=$lblock
	pos=$((block+lengt-1))
done
dd if=/dev/stdin
} > "${2:-/dev/stdout}"