#!/usr/bin/env sh
# compat: +ash +bash +dash +zsh

__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.
    ### * `-m` -- manual: 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.
    ### * `-n` -- dry-run: don't actually change anything.
    ### * `-v` -- verbose: print changes before doing them.
    ###
    ### 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_v __permit_x || return

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

    __permit_v=0 # verbosity
    __permit_n=0 # dry-run
    __permit_m=e # mode
    __permit_a=0 # automatic

    unset __permit_stop || return
    for __permit_f; do
        shift
        case "${__permit_stop+:}$__permit_f" in
        (/\?|-h|-help|--help)
            printf 'usage: permit %s\n' "$__permit_h"
            return 0;;
        (--) __permit_stop=1;;
        (-?*)
            while __permit_f="${__permit_f#?}"; [ -n "$__permit_f" ]; do
                case "${__permit_f}" in

                (v*) __permit_v=1;; # verbosity
                (n*) __permit_n=1;; # dry-run
                (a*) __permit_a=1;; # automatic
                (m*) __permit_a=0;; # manual
                (e*) __permit_m=e;; # everything
                (f*) __permit_m=f;; # files only
                (d*) __permit_m=d;; # directories only

                (*)
                    printf >&2 'permit: unknown flag: -%.1s\n' "$__permit_f"
                    return 64;;
                esac
            done;;
        (*) set -- "$@" "$__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

    [ "$__permit_v" = 0 ] || [ "$#" = 0 ] || printf >&2 'notice: +x %s\n' "$@"
    [ "$__permit_n" = 1 ] || [ "$#" = 0 ] || chmod +x -- "$@" || __permit_x=$?
    return "$__permit_x"
}

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