#!/usr/bin/env sh
# YES_ZSH YES_BASH YES_DASH YES_ASH

__permit_hex() {
    if xxd -l 0 2>/dev/null; then
        __permit_hex() { exec xxd -p -l4 -- "$@"; }
    else
        __permit_hex() { exec hexdump -ve '1 1 "%02x"' -n4 -- "$@"; }
    fi
    __permit_hex "$@"
}

permit() { ### @- conditionally set executable permissions on each of its arguments.
    ### flags:
    ### * `-a` -- automatic: skip any files whose contents do not begin with
    ###           with one of several signatures. this does not affect directories.
    ### * `-A` -- turn off automatic mode. (default)
    ### * `-e` -- everything: consider both regular files and directories. (default)
    ### * `-f` -- files: skip any arguments that are not regular files.
    ### * `-d` -- directories: skip any arguments that are not directories.
    ###
    ### the `-e`, `-f`, and `-d` flags all override one another, and any of them
    ### can be combined with `-a`. arguments that are neither regular files nor
    ### directories (such as symlinks) are always skipped. arguments that are
    ### already executable by the current user are skipped. arguments that do
    ### not appear to refer to an existing file are passed through to chmod.
    ### directories are never recursed.
    unset __permit_4 __permit_a __permit_f __permit_h __permit_m __permit_n __permit_x || return

    __permit_h="[-a | -A] [-e | -f | -d] [--] {files...}"
    if [ $# = 0 ]; then
        printf >&2 'usage: permit %s\n' "$__permit_h"
        return 64;
    fi

    __permit_m=e # everything
    __permit_a=0 # automatic

    __permit_n=$#
    while [ $((__permit_n-=1)) -ge 0 ]; do
        __permit_f="$1"
        case "$__permit_f" in

        (/\?|-h|-help|--help)
            printf 'usage: permit %s\n' "$__permit_h"
            return 0;;

        (--)
            shift
            while [ $((__permit_n-=1)) -ge 0 ]; do
                set -- "$@" "$1"
                shift
            done
            break;;

        (-*)
            shift
            case "${__permit_f#-}" in
            (a) __permit_a=1;; # automatic (on)
            (A) __permit_a=0;; # automatic (off)
            (e) __permit_m=e;; # everything
            (f) __permit_m=f;; # files only
            (d) __permit_m=d;; # directories only
            (?|-*)
                printf >&2 'permit: %s\n' "unknown flag: $__permit_f"
                return 64;;
            (*)
                set -- "${__permit_f%"${__permit_f#-?}"}" "-${__permit_f#-?}" "$@"
                : $((__permit_n+=2))
                continue;;
            esac;;

        (*)
            shift
            set -- "$@" "$__permit_f";;
        esac
    done

    if [ $# = 0 ]; then
        printf >&2 'usage: permit %s\n' "$__permit_h"
        return 64;
    fi

    __permit_x=
    for __permit_f; do
        [ -n "$__permit_x" ] || { __permit_x=0; set --; }
        if [ -x "$__permit_f" ]; then
            : # pass
        elif ! [ -e "$__permit_f" ]; then
            set -- "$@" "$__permit_f"
        elif [ "$__permit_a" = 1 ] && ! [ -d "$__permit_f" ]; then
            __permit_4="$(__permit_hex "$__permit_f")" || __permit=$?
            case "$__permit_4" in (7f454c46) false;; (2321*) false;; (4d5a*) false;;
            esac || set -- "$@" "$__permit_f" # runs when a case matches
        elif [ "-$__permit_m" "$__permit_f" ]; then
            set -- "$@" "$__permit_f"
        fi
    done

    [ "$#" = 0 ] || chmod +x -- "$@" || __permit_x=$?
    return "$__permit_x"
}

[ -n "${preload+-}" ] || permit "$@"