import type {
    FontWeight,
    UnifiedFont,
} from '../../model';

/** Valid font weight values (100-900 in increments of 100) */
const SIZES = [100, 200, 300, 400, 500, 600, 700, 800, 900];

/**
 * Gets the URL for a font file at a specific weight
 *
 * Constructs the appropriate URL for loading a font file based on
 * the font object and requested weight. Handles variable fonts and
 * provides fallbacks for static fonts.
 *
 * @param font - Unified font object containing style URLs
 * @param weight - Font weight (100-900)
 * @returns URL string for the font file, or undefined if not found
 * @throws Error if weight is not a valid value (100-900)
 *
 * @example
 * ```ts
 * const url = getFontUrl(roboto, 700); // Returns URL for Roboto Bold
 *
 * // Variable fonts: backend maps weight to VF URL
 * const vfUrl = getFontUrl(inter, 450); // Returns variable font URL
 *
 * // Fallback for missing weights
 * const fallback = getFontUrl(font, 900); // Falls back to regular/400 if 900 missing
 * ```
 */
export function getFontUrl(font: UnifiedFont, weight: number): string | undefined {
    if (!SIZES.includes(weight)) {
        throw new Error(`Invalid weight: ${weight}`);
    }

    const weightKey = weight.toString() as FontWeight;

    // Try exact match (backend maps weight to VF URL for variable fonts)
    if (font.styles.variants?.[weightKey]) {
        return font.styles.variants[weightKey];
    }

    // Fallbacks for static fonts when exact weight is missing
    return font.styles.regular || font.styles.variants?.['400'] || font.styles.variants?.['regular'];
}