##################################
# shellcheck shell=sh enable=all #
##################################

which() (
    if [ $# -gt 0 ]; then
        while [ $# -gt 0 ]; do
            case "${1}" in
            --)
                shift

                break

                ;;
            -i)
                searchonlypath=1

                shift

                ;;
            -a)
                locateall="${locateall-1}"

                shift

                ;;
            -s | -sa | -as)
                locateall=
                besilent=1

                shift

                ;;
            -ia | -ai)
                searchonlypath=1
                locateall="${locateall-1}"

                shift

                ;;
            -is | -si | -ias | -isa | -ais | -asi | -sia | -sai)
                searchonlypath=1
                locateall=
                besilent=1

                shift

                ;;
            -*)
                headline="invalid option — ${1#-}"
                exitstatus=2

                set --

                break

                ;;
            *)
                break

                ;;
            esac
        done
    fi

    if [ $# -gt 0 ]; then
        for command; do
            located=

            [ -n "${searchonlypath-}" ] || {
                cmd="$(LC_ALL=C command -v -- "${command}" 2>/dev/null)"

                if [ -z "${cmd}" ]; then
                    exitstatus=1

                    continue
                elif [ "${cmd}" = "${command}" ] || [ ! -x "${cmd}" ]; then
                    [ "${cmd}" = "${command}" ] || {
                        cmd="${cmd#*\'}"
                        cmd="${cmd%\'}"
                    }

                    located=1

                    [ -n "${besilent-}" ] || printf "%s\n" "${cmd}"

                    [ -n "${locateall-}" ] || continue
                fi
            }

            IFS=:

            #############################
            # shellcheck disable=SC2086 #
            #############################
            for path in ${PATH}; do
                if [ -x "${path}/${command}" ]; then
                    located=1

                    [ -n "${besilent-}" ] || printf "%s/%s\n" "${path}" "${command}"

                    [ -n "${locateall-}" ] || break
                fi
            done

            [ -n "${located}" ] || exitstatus=1
        done

        return "${exitstatus-0}"
    fi

    printf "which: %s\n\n" "${headline-locates command(s) and reports to standard output}" >&2
    printf "%s\n" "USAGE" >&2
    printf "    %s\n" "which [-ias] command …" >&2
    printf "%s\n" "OPTIONS" >&2
    printf "    %s     %s\n" "-i" "ignore shell builtins, functions and aliases" >&2
    printf "    %s     %s\n" "-a" "print all matches" >&2
    printf "    %s     %s\n" "-s" "silently return 0 if all commands are located or 1 otherwise" >&2
    printf "%s\n" "EXIT STATUS" >&2
    printf "    %s      %s\n" 0 "all commands are located" >&2
    printf "    %s      %s\n" 1 "failed to locate some command(s)" >&2
    printf "    %s      %s\n" 2 "an invalid option is specified" >&2

    return "${exitstatus-1}"
)