/**
  Principle: keep geometry, roles, and constructors separate

  Right now you have three conceptual layers:

  - Vocabulary / ontology
    What is a block, pixel, role, level, channel?
    → Tiling.ts

  - Pure geometry / index sets
    What does a 4×4 grid mean at the type level?
    → grid definitions, cartesian products

  - Role-specific constructors
    How do we construct “payload pixels”, “link pixels”, “echo pixels”?
    → payload/link/echo constructors

  This mirrors exactly how you think about the system:

  - tiling is the substrate
  - grids are pure geometry
  - pixel constructors are semantic overlays

  This file answers one question only:

  “Given a payload block and a coordinate, what is the payload pixel?”

  That’s perfect spec granularity.
*/
import type {
  BlockAssignment,
  TypedTiledPixel,
  PixelAssignment,
} from "./tiling.ts";
import type { Grid4x4 } from "./grid.ts";

/**
 * Construct a single payload pixel inside a Level-2 block.
 *
 * This is a PURE TYPE-LEVEL constructor:
 *  - no runtime values
 *  - no side effects
 *
 * It represents one "fat pixel" contribution to a HASSE value cell.
 */
export type PayloadPixel<
  B extends BlockAssignment<2>,
  P extends readonly [number, number]
> =
  PixelAssignment<2> & {
    owner: B["owner"];
    level: 2;
    block: B["block"];
    role: B["role"];
    pixel: TypedTiledPixel<2> & {
      block: B["block"];
      index: { ix: P[0]; iy: P[1] };
      role: "Payload";
      channel: "Data";
    };
  };

/* ----------------------------------------------------- */
/* 2. Union → tuple helpers                              */
/* ----------------------------------------------------- */

type UnionToIntersection<U> =
  (U extends unknown ? (x: U) => void : never) extends
  (x: infer I) => void ? I : never;

type UnionToTuple<U, T extends unknown[] = []> =
  [U] extends [never]
    ? T
    : UnionToIntersection<U extends unknown ? (u: U) => void : never> extends
        (u: infer I) => void
      ? UnionToTuple<Exclude<U, I>, [...T, I]>
      : T;

/* ----------------------------------------------------- */
/* 3. All payload pixels for a payload block              */
/* ----------------------------------------------------- */

/**
 * All payload pixels for a Level-2 payload block.
 *
 * This is the 4×4 “fat pixel” surface of a HASSE value cell.
 */
export type PayloadPixelsForBlock<
  B extends BlockAssignment<2>
> =
  UnionToTuple<
    Grid4x4 extends infer P
      ? P extends readonly [number, number]
        ? PayloadPixel<B, P>
        : never
      : never
  >;