export const VIEWPORT_DPR_SCHEMA_VERSION = 1 as const;
export const VIEWPORT_MAX_DPR = 2 as const;

export interface ViewportPixelMetrics {
  schemaVersion: typeof VIEWPORT_DPR_SCHEMA_VERSION;
  cssWidth: number;
  cssHeight: number;
  pixelRatio: number;
  backingWidth: number;
  backingHeight: number;
}

export interface ViewportNDC {
  x: number;
  y: number;
}

function finitePositive(value: number): boolean {
  return Number.isFinite(value) && value > 0;
}

export function resolveViewportPixelRatio(devicePixelRatio: number | undefined, maximum = VIEWPORT_MAX_DPR): number {
  if (!finitePositive(maximum)) throw new Error("VIEWPORT_DPR_INVALID");
  const observed = finitePositive(devicePixelRatio ?? 1) ? devicePixelRatio! : 1;
  return Math.min(observed, maximum);
}

export function resolveViewportPixelMetrics(cssWidth: number, cssHeight: number, devicePixelRatio: number | undefined, maximum = VIEWPORT_MAX_DPR): ViewportPixelMetrics {
  if (!finitePositive(cssWidth) || !finitePositive(cssHeight)) throw new Error("VIEWPORT_SIZE_INVALID");
  const pixelRatio = resolveViewportPixelRatio(devicePixelRatio, maximum);
  return {
    schemaVersion: VIEWPORT_DPR_SCHEMA_VERSION,
    cssWidth,
    cssHeight,
    pixelRatio,
    backingWidth: Math.max(1, Math.floor(cssWidth * pixelRatio)),
    backingHeight: Math.max(1, Math.floor(cssHeight * pixelRatio)),
  };
}

export function viewportNDC(clientX: number, clientY: number, bounds: { left: number; top: number; width: number; height: number }): ViewportNDC {
  if (![clientX, clientY, bounds.left, bounds.top, bounds.width, bounds.height].every(Number.isFinite) || bounds.width <= 0 || bounds.height <= 0) {
    throw new Error("VIEWPORT_BOUNDS_INVALID");
  }
  return {
    x: ((clientX - bounds.left) / bounds.width) * 2 - 1,
    y: -((clientY - bounds.top) / bounds.height) * 2 + 1,
  };
}