/**
 * @license
 * Copyright (c) 2017 The Polymer Project Authors. All rights reserved.
 * This code may only be used under the BSD style license found at
 * http://polymer.github.io/LICENSE.txt
 * The complete set of authors may be found at
 * http://polymer.github.io/AUTHORS.txt
 * The complete set of contributors may be found at
 * http://polymer.github.io/CONTRIBUTORS.txt
 * Code distributed by Google as part of the polymer project is also
 * subject to an additional IP rights grant found at
 * http://polymer.github.io/PATENTS.txt
 */
import { isPrimitive } from '../lib/parts.js';
import { directive } from '../lit-html.js';
const _state = new WeakMap();
// Effectively infinity, but a SMI.
const _infinity = 0x7fffffff;
/**
 * Renders one of a series of values, including Promises, to a Part.
 *
 * Values are rendered in priority order, with the first argument having the
 * highest priority and the last argument having the lowest priority. If a
 * value is a Promise, low-priority values will be rendered until it resolves.
 *
 * The priority of values can be used to create placeholder content for async
 * data. For example, a Promise with pending content can be the first,
 * highest-priority, argument, and a non_promise loading indicator template can
 * be used as the second, lower-priority, argument. The loading indicator will
 * render immediately, and the primary content will render when the Promise
 * resolves.
 *
 * Example:
 *
 *     const content = fetch('./content.txt').then(r => r.text());
 *     html`${until(content, html`<span>Loading...</span>`)}`
 */
export const until = directive((...args) => (part) => {
    let state = _state.get(part);
    if (state === undefined) {
        state = {
            lastRenderedIndex: _infinity,
            values: [],
        };
        _state.set(part, state);
    }
    const previousValues = state.values;
    let previousLength = previousValues.length;
    state.values = args;
    for (let i = 0; i < args.length; i++) {
        // If we've rendered a higher-priority value already, stop.
        if (i > state.lastRenderedIndex) {
            break;
        }
        const value = args[i];
        // Render non-Promise values immediately
        if (isPrimitive(value) ||
            typeof value.then !== 'function') {
            part.setValue(value);
            state.lastRenderedIndex = i;
            // Since a lower-priority value will never overwrite a higher-priority
            // synchronous value, we can stop processing now.
            break;
        }
        // If this is a Promise we've already handled, skip it.
        if (i < previousLength && value === previousValues[i]) {
            continue;
        }
        // We have a Promise that we haven't seen before, so priorities may have
        // changed. Forget what we rendered before.
        state.lastRenderedIndex = _infinity;
        previousLength = 0;
        Promise.resolve(value).then((resolvedValue) => {
            const index = state.values.indexOf(value);
            // If state.values doesn't contain the value, we've re-rendered without
            // the value, so don't render it. Then, only render if the value is
            // higher-priority than what's already been rendered.
            if (index > -1 && index < state.lastRenderedIndex) {
                state.lastRenderedIndex = index;
                part.setValue(resolvedValue);
                part.commit();
            }
        });
    }
});
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