/* @flow */

declare type StyleSheet = Object;

/**
 * The list of the well-known style properties which may not be numbers on Web
 * but must be numbers on React Native.
 *
 * @private
 */
const _WELL_KNOWN_NUMBER_PROPERTIES = [ 'height', 'width' ];

/* eslint-disable flowtype/space-before-type-colon */

/**
 * Create a style sheet using the provided style definitions.
 *
 * @param {StyleSheet} styles - A dictionary of named style definitions.
 * @param {StyleSheet} [overrides={}] - Optional set of additional (often
 * platform-dependent/specific) style definitions that will override the base
 * (often platform-independent) styles.
 * @returns {StyleSheet}
 */
export function createStyleSheet(styles: StyleSheet, overrides: StyleSheet = {})
        : StyleSheet {

/* eslint-enable flowtype/space-before-type-colon */

    const combinedStyles = {};

    for (const k of Object.keys(styles)) {
        combinedStyles[k]
            = _shimStyles({
                ...styles[k],
                ...overrides[k]
            });
    }

    return combinedStyles;
}

/**
 * Shims style properties to work correctly on native. Allows us to minimize the
 * number of style declarations that need to be set or overridden for specific
 * platforms.
 *
 * @param {StyleSheet} styles - An object which represents a stylesheet.
 * @private
 * @returns {StyleSheet}
 */
function _shimStyles<T: StyleSheet>(styles: T): T {
    // Certain style properties may not be numbers on Web but must be numbers on
    // React Native. For example, height and width may be expressed in percent
    // on Web but React Native will not understand them and we will get errors
    // (at least during development). Convert such well-known properties to
    // numbers if possible; otherwise, remove them to avoid runtime errors.
    for (const k of _WELL_KNOWN_NUMBER_PROPERTIES) {
        const v = styles[k];
        const typeofV = typeof v;

        if (typeofV !== 'undefined' && typeofV !== 'number') {
            const numberV = Number(v);

            if (Number.isNaN(numberV)) {
                delete styles[k];
            } else {
                styles[k] = numberV;
            }
        }
    }

    return styles;
}