rc/sh/argc

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

argc() { ### @- validate the number of arguments in a function.
    ### ```sh
    ### # usage: myfunc() { argc $# -(eq|le|ge) [0-9] "$0" || return; }
    ###
    ### myfunc() {
    ###     # use one of the following:
    ###     argc $# -eq N "$0" || return
    ###     argc $# -le N "$0" || return
    ###     argc $# -ge N "$0" || return
    ###     # where N is an integer between 0 and 9.
    ### }
    ### ```

    local usage='usage: myfunc() { %s $# -(eq|le|ge) [0-9] "$0" || return; }\n'

    # note that $zero can be empty, but must be set.
    if [ $# -ne 4 ] || [ -z "$1" ] || [ -z "$2" ] || [ -z "$3" ]; then
        printf "$usage" argc >&2
        return 1
    fi

    local argc="$1"; shift
    local cond="$1"; shift
    local want="$1"; shift
    local zero="$1"; shift
    local many='' plural=''
    [ "$want" = 1 ] || plural='s'

    if [ "$argc" != "${argc%%[!0-9]*}" ]; then
        printf "$usage" argc >&2
        return 1
    fi

    case "$cond" in
    ('-eq') many='exactly';;
    ('-le') many='at most';;
    ('-ge') many='at least';;
    (*)
        printf "$usage" argc >&2
        return 1;;
    esac

    case "$want" in
    ([0-9]) :;; # no error, continue.
    (*)
        printf "$usage" argc >&2
        return 1;;
    esac

    if [ "$argc" "$cond" "$want" ] 2>/dev/null; then
        :
    else
        local err="expected $many $want argument$plural, got"
        printf '%s: %s %s\n' "$zero" "$err" "$argc" >&2
        return 64
    fi

    return 0
}

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