/** A single color role in the fixed isotype brand palette, or an empty cell. */
export type IsotypeColorKey = 'purple' | 'blue' | 'magenta' | 'yellow' | 'green' | 'red' | 'brown';

export type IsotypeCell = IsotypeColorKey | 'blank';

/** Row-major 4x4 grid (16 cells) — a tuple so every pattern is guaranteed complete at compile time. */
export type IsotypePattern = readonly [
  IsotypeCell,
  IsotypeCell,
  IsotypeCell,
  IsotypeCell,
  IsotypeCell,
  IsotypeCell,
  IsotypeCell,
  IsotypeCell,
  IsotypeCell,
  IsotypeCell,
  IsotypeCell,
  IsotypeCell,
  IsotypeCell,
  IsotypeCell,
  IsotypeCell,
  IsotypeCell,
];

/**
 * Fixed Xertica brand hex values for the `xertica` theme, provided directly
 * by design. These are intentionally NOT CSS custom properties — an isotype
 * is a brand identity mark, like a logo, and must not reskin when the
 * active UI theme changes.
 */
export const ISOTYPE_COLORS: Record<IsotypeColorKey, string> = {
  purple: '#5A4A96',
  blue: '#1899AF',
  magenta: '#C45BAA',
  yellow: '#FAF338',
  green: '#2E8B5A',
  red: '#DE5B48',
  brown: '#2A2415',
};

/** Widen this union as new isotypes join the series. */
export type IsotypePatternId = 'core';

export const ISOTYPE_PATTERNS: Record<IsotypePatternId, IsotypePattern> = {
  core: [
    'purple', 'blue', 'blue', 'blue',
    'magenta', 'blank', 'yellow', 'green',
    'magenta', 'yellow', 'yellow', 'green',
    'magenta', 'red', 'red', 'brown',
  ],
};

export const ISOTYPE_PATTERN_IDS = Object.keys(ISOTYPE_PATTERNS) as IsotypePatternId[];

/** Deterministic PRNG (mulberry32) — the same seed always produces the same sequence. */
export function mulberry32(seed: number): () => number {
  let a = seed >>> 0;
  return () => {
    a |= 0;
    a = (a + 0x6d2b79f5) | 0;
    let t = Math.imul(a ^ (a >>> 15), 1 | a);
    t = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
  };
}

/**
 * Returns a copy of the given pattern with exactly one additional non-blank
 * cell (chosen deterministically from `seed`) swapped to `'blank'`. Pure —
 * the same `(patternId, seed)` pair always returns the same result, and a
 * cell that's already blank in the base pattern is never re-chosen.
 */
export function getIsotypeVariant(patternId: IsotypePatternId, seed: number): IsotypePattern {
  const base = ISOTYPE_PATTERNS[patternId];
  const candidateIndices = base.reduce<number[]>(
    (acc, cell, i) => (cell === 'blank' ? acc : [...acc, i]),
    []
  );
  const rng = mulberry32(seed);
  const chosen = candidateIndices[Math.floor(rng() * candidateIndices.length)];
  return base.map((cell, i) => (i === chosen ? 'blank' : cell)) as unknown as IsotypePattern;
}