export type BreakpointDefinition = Readonly<Record<string, number>>;

/** Validate declaration order once, when an app creates its responsive contract. */
export function breakpointEntries<const T extends BreakpointDefinition>(definition: T) {
  const entries = Object.entries(definition) as [keyof T & string, number][];
  if (!entries.length) throw new Error('Breakpoints must declare at least one threshold.');
  let previous = -1;
  for (const [name, threshold] of entries) {
    if (!name || !Number.isFinite(threshold) || threshold < 0) {
      throw new Error(`Breakpoint "${name}" must have a finite, non-negative threshold.`);
    }
    if (threshold <= previous) {
      throw new Error('Breakpoint thresholds must be strictly ascending in declaration order.');
    }
    previous = threshold;
  }
  return entries;
}

/** Largest satisfied name, with `base` as the deterministic below-range fallback. */
export function breakpointAt<Name extends string>(
  entries: readonly (readonly [Name, number])[],
  width: number
): Name | 'base' {
  let current: Name | 'base' = 'base';
  for (const [name, threshold] of entries) {
    if (width < threshold) break;
    current = name;
  }
  return current;
}