#!/bin/sh

print_padding() {
    [ $1 -eq 0 ] || dd if=/dev/zero bs=$1 count=1 2>/dev/null
}

print_tar_header() {
    local checksum=$1; shift

    while [ -n "$1" ]; do
        local width=$1; shift
        local format=$1; shift
        local value=$1; shift

        # replace checksum (the only field with width 7)
        if [ $width = 7 ]; then
            format='%06o'
            value=$checksum
        fi

        local str="$(printf "$format" "$value")"

        printf "%.${width}s" "$str"
        [ ${#str} -ge $width ] || print_padding $((width - (${#str} % width)))
    done

    return $checksum
}

checksum_tar_header() {
    local checksum=0

    while [ -n "$1" ]; do
        local width=$1; shift
        local format=$1; shift
        local value=$1; shift

        for byte in $(printf "$format" "$value" | hexdump -n $width -ve '1/1 "%u "'); do
            checksum=$((checksum + $byte))
        done
    done

    return $checksum
}

make_tar_header() {
    checksum_tar_header "$@"
    print_tar_header $? "$@"
}

make_tar_member() {
    local name=$1
    local content=$2
    local mode=644
    local uid=1000
    local gid=1000
    local size=${#content}
    local mtime=$(date +%s -r "/tmp/t/$name")
    local type=0
    local link=""
    local username="jow"
    local groupname="jow"

    # member header
    make_tar_header \
        100 "%.100s" "$name"      \
          8   "%07d"  $mode       \
          8   "%07o"  $uid        \
          8   "%07o"  $gid        \
         12  "%011o"  $size       \
         12  "%011o"  $mtime      \
          7    "%7s"  ""          \
          1    "%1s"  ""          \
          1     "%d"  $type       \
        100 "%.100s" "$link"      \
          8   "%-7s" "ustar"      \
         32  "%.32s" "$username"  \
         32  "%.32s" "$groupname"

    # header padding
    print_padding 183

    # content data
    printf "%s" "$content"

    # content padding
    print_padding $((512 - (size % 512)))
}


{
    make_tar_member "example.txt" "Hello there"
    make_tar_member "example-2.txt" "Blah blah"

    # alternatively, instead of the dd, run a `tar -c ...` here
    print_padding 1024
} | gzip > /tmp/test.tar.gz