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

    # note that $zero can be empty, but must be set.
    if [ $# -lt 3 ] || [ -z "$1" ] || [ -z "$2" ] || [ "${3+x}" != x ]; then
        printf "$usage" argc >&2
        return 2
    fi

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

    case "$cond" in
    '-eq') many='exactly';;
#   '-lt') many='fewer than';;
#   '-gt') many='more than';;
    '-le') many='at most';;
    '-ge') many='at least';;
    *)
        printf "$usage" argc >&2
        return 2;;
    esac

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

    if [ ! $# "$cond" "$want" ]; then
        local err="expected $many $want argument$plural, got"
        printf '%s: %s %s\n' "$zero" "$err" "$#" >&2
        return 1
    fi

    return 0
}

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