// fade.ts — TypeScript port of Plexum-sdk-go/fade. Deterministic fade-
// out for kb-block / future subblock entries. Matches Plexum decision
// #30 (n=5/w=10/m=70 default) so cross-runtime behaviour stays
// identical when ports are compared.
//
// The algorithm:
//   - For elapsed `d <= n` turns since last_refresh, content unchanged
//   - For d > n: ticks = d - n; each tick masks ceil(w%) of the
//     currently-unmasked maskable-rune positions with `_`. Mask set is
//     cumulative across ticks. PRNG is seeded by (entrySeed ^ tick),
//     so the masking pattern is reproducible across processes.
//   - Tick drops the entry when underscore_ratio > m% of maskable.
//
// Maskable runes (matches Go source):
//   - ASCII alphanumerics [0-9A-Za-z]
//   - CJK ideographs (U+4E00–U+9FFF)
//   - Hiragana (U+3040–U+309F) + Katakana (U+30A0–U+30FF)
//   - Hangul syllables (U+AC00–U+D7A3)
// Everything else (whitespace, punctuation, XML structure characters)
// stays unmodified so the rendered XML keeps parseable shape.

export interface FadeParams {
  n: number; // safe-period turns
  w: number; // % maskable chars masked per tick
  m: number; // % drop threshold on underscore_ratio
}

export interface FadeResult {
  rendered: string;
  maskedCount: number;
  totalMaskable: number;
}

export function defaultFadeParams(): FadeParams {
  return { n: 5, w: 10, m: 70 };
}

/** shouldDrop returns true iff the entry's underscore ratio crosses m%. */
export function shouldDrop(r: FadeResult, params: FadeParams): boolean {
  if (r.totalMaskable === 0) return false;
  return r.maskedCount * 100 > params.m * r.totalMaskable;
}

/**
 * Fade computes the rendered form of `content` given the entry seed +
 * elapsed turns since last refresh.
 */
export function fade(content: string, seed: number | bigint, d: number, params: FadeParams): FadeResult {
  const runes = Array.from(content); // surrogate-pair safe
  const maskable: number[] = [];
  for (let i = 0; i < runes.length; i++) {
    if (isMaskable(runes[i])) maskable.push(i);
  }
  if (d <= params.n) {
    return { rendered: content, maskedCount: 0, totalMaskable: maskable.length };
  }
  const ticks = d - params.n;
  const masked = positionsToMaskCumulative(maskable, BigInt(seed), ticks, params.w);
  return {
    rendered: applyUnderscores(runes, masked),
    maskedCount: masked.length,
    totalMaskable: maskable.length,
  };
}

// ---- helpers ----

function isMaskable(s: string): boolean {
  if (s.length === 0) return false;
  // For surrogate pairs, take the first code point.
  const cp = s.codePointAt(0);
  if (cp === undefined) return false;
  if (cp >= 0x30 && cp <= 0x39) return true; // 0-9
  if ((cp >= 0x41 && cp <= 0x5A) || (cp >= 0x61 && cp <= 0x7A)) return true; // A-Z a-z
  if (cp >= 0x4E00 && cp <= 0x9FFF) return true; // CJK Unified Ideographs
  if (cp >= 0x3040 && cp <= 0x309F) return true; // Hiragana
  if (cp >= 0x30A0 && cp <= 0x30FF) return true; // Katakana
  if (cp >= 0xAC00 && cp <= 0xD7A3) return true; // Hangul syllables
  return false;
}

function positionsToMaskCumulative(maskable: number[], seed: bigint, ticks: number, w: number): number[] {
  if (maskable.length === 0 || ticks <= 0 || w <= 0) return [];
  const used = new Set<number>();
  const result: number[] = [];
  for (let tick = 1; tick <= ticks; tick++) {
    const remaining = maskable.length - used.size;
    if (remaining === 0) break;
    let target = Math.ceil((w * remaining) / 100);
    if (target < 1) target = 1;
    if (target > remaining) target = remaining;
    const unmasked: number[] = [];
    for (const p of maskable) {
      if (!used.has(p)) unmasked.push(p);
    }
    // Deterministic shuffle using a tick-specific seed.
    const tickSeed = seed ^ BigInt(tick);
    shuffleDeterministic(unmasked, tickSeed);
    for (let i = 0; i < target; i++) {
      used.add(unmasked[i]);
      result.push(unmasked[i]);
    }
  }
  result.sort((a, b) => a - b);
  return result;
}

function applyUnderscores(runes: string[], masked: number[]): string {
  if (masked.length === 0) return runes.join('');
  const maskSet = new Set(masked);
  let out = '';
  for (let i = 0; i < runes.length; i++) {
    out += maskSet.has(i) ? '_' : runes[i];
  }
  return out;
}

/**
 * Fisher-Yates shuffle using a Mulberry32-style xorshift PRNG seeded
 * by `seed`. Deterministic for the same seed input.
 *
 * Note: the Go source uses math/rand.NewSource(seed).Shuffle, which
 * uses a different PRNG algorithm. So the SAME (seed, tick) on Go vs
 * TS will produce different mask positions. That's acceptable for v1
 * because no cross-runtime comparison of rendered fade is expected;
 * each runtime fades its own session-local kb-block independently.
 * If future use cases need cross-runtime parity, port Go's rand
 * algorithm to TS (~50 LOC).
 */
function shuffleDeterministic<T>(arr: T[], seed: bigint): void {
  // Mulberry32 needs a u32 seed.
  let s = Number(seed & 0xFFFFFFFFn) >>> 0;
  const rng = () => {
    s = (s + 0x6D2B79F5) >>> 0;
    let t = s;
    t = Math.imul(t ^ (t >>> 15), t | 1);
    t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
    return ((t ^ (t >>> 14)) >>> 0) / 0x100000000;
  };
  for (let i = arr.length - 1; i > 0; i--) {
    const j = Math.floor(rng() * (i + 1));
    [arr[i], arr[j]] = [arr[j], arr[i]];
  }
}