function remove_children(element) {
    Array.from(element.children).forEach(child => {
	element.removeChild(child);
    });
}

function trigger_change_event(element) {
    evt = new Event("change");
    element.dispatchEvent(evt);
}

function setup_groups(group_data) {
    elt = document.getElementById("group");
    remove_children(elt);
    the_groups = group_data.reduce(
	function(acc, row) {
	    grouping = row[2].slice(7).trim();
	    if(acc[grouping] === undefined) {
		acc[grouping] = [];
	    }
	    acc[grouping].push([row[0], row[1]]);
	    return acc;
	},
	{});
    for(grouping in the_groups) {
	optgrp = document.createElement("optgroup");
	optgrp.setAttribute("label", grouping);
	the_groups[grouping].forEach(group => {
	    opt = document.createElement("option");
	    opt.setAttribute("value", group[0]);
	    opt.appendChild(document.createTextNode(group[1]));
	    optgrp.appendChild(opt);
	});
	elt.appendChild(optgrp);
    }
    trigger_change_event(elt);
}

function setup_types(type_data) {
    elt = document.getElementById("type");
    remove_children(elt);
    the_types = type_data.reduce(function(acc, row) {
	grp = row[2];
	if(acc[grp] === undefined) {
	    acc[grp] = [];
	}
	acc[grp].push([row[0], row[1]]);
	return acc;
    }, {});
    for(type_group in the_types) {
	optgrp = document.createElement("optgroup");
	optgrp.setAttribute("label", type_group);
	the_types[type_group].forEach(type => {
	    opt = document.createElement("option");
	    opt.setAttribute("value", type[0]);
	    opt.appendChild(document.createTextNode(type[1]));
	    optgrp.appendChild(opt);
	});
	elt.appendChild(optgrp);
    }
    trigger_change_event(elt);
}

function setup_datasets(dataset_data) {
    console.info("DATASET DATA:", dataset_data);
    elt = document.getElementById("dataset");
    remove_children(elt);
    dataset_data.forEach(dataset => {
	opt = document.createElement("option");
	opt.setAttribute("value", dataset[0]);
	opt.appendChild(document.createTextNode(
	    "[" + dataset[1] + "] " + dataset[2]));
	elt.appendChild(opt);
    });
    trigger_change_event(elt);
}

function menu_contents() {
    return JSON.parse(
	document.getElementsByTagName("form")[0].getAttribute(
	    "data-menu-content"));
}

function update_menu(event) {
    menu = menu_contents();

    species_elt = document.getElementById("species");
    group_elt = document.getElementById("group");
    type_elt = document.getElementById("type");
    dataset_elt = document.getElementById("dataset");

    if(event.target == species_elt) {
	setup_groups(menu["groups"][species_elt.value]);
    }

    if(event.target == group_elt) {
	setup_types(menu["types"][species_elt.value][group_elt.value]);
    }

    if(event.target == type_elt) {
	setup_datasets(
	    menu["datasets"][species_elt.value][group_elt.value][type_elt.value]
	);
    }
}