cleanpath in zsh

#!/usr/bin/env nix-shell
#! nix-shell -i zsh -p zsh --pure

#
# Remove empty and duplicate entries from a colon-separated path.
#
# For example: ":a:b:b:::c:a::" --> "a:b:c"
#

# Ensure that the script uses only built-in shell commands.
PATH=''

delete_duplicates() {
    if [[ ${#entries} -ne 0 ]]; then
        local i=1
        while [[ ${i} -lt ${#entries} ]]; do
            local j=
            (( j=${i} + 1 ))
            while [[ ${j} -le ${#entries} ]]; do
                if [[ "${entries[${i}]}" == "${entries[${j}]}" ]]; then
                    # Delete the duplicate entry.
                    entries[${j}]=()
                else
                    (( j=${j} + 1 ))
                fi
            done
            (( i=${i} + 1 ))
        done
    fi
}

if [[ ${#} != 1 ]]; then
    echo "${0}: command line error" 1>&2
    echo "Usage: ${0} PATH" 1>&2
    exit_status=1
else
    dirty_path="${1}"

    # Split the path at ':' characters.
    entries=(${(s/:/)dirty_path})

    delete_duplicates

    # Join the path with ':' characters.
    clean_path="${(j/:/)entries}"

    echo -n "${clean_path}"

    exit_status=0
fi

exit ${exit_status}