// CHAT-01 history limits (#1507, CHAT-02): at most 100 parts per page, 8 MiB
// of serialized UTF-8 per page, 64 blocks per part and 262144 characters per
// string. Oversize content splits into fragments and continuation parts; it is
// never clipped. Byte limits are measured on the serialized JSON, not on
// characters.

import { createHash } from "node:crypto";

export const LIMITS = Object.freeze({ parts: 100, pageBytes: 8 * 1024 * 1024, blocks: 64, chars: 262144 });

// Internal budgets that keep any single part well inside one page, so a page
// always holds at least one part.
export const FRAGMENT_BYTES = 1024 * 1024;
export const PART_BYTES = 4 * 1024 * 1024;

export const ID = /^[A-Za-z0-9][A-Za-z0-9._:-]{0,127}$/;

// A native value used as a CHAT-01 id. Values that do not fit the id pattern
// (OpenAI tool calls such as "call_x|fc_y") map to a stable hash.
export function safeId(value) {
  if (typeof value === "string" && ID.test(value)) return value;
  return "h-" + createHash("sha256").update(String(value)).digest("hex").slice(0, 40);
}

// Bytes one code point adds to a JSON string literal.
function jsonCost(cp) {
  if (cp === 0x22 || cp === 0x5c) return 2;
  if (cp < 0x20) return cp === 8 || cp === 9 || cp === 10 || cp === 12 || cp === 13 ? 2 : 6;
  if (cp < 0x80) return 1;
  if (cp < 0x800) return 2;
  if (cp >= 0xd800 && cp <= 0xdfff) return 6; // lone surrogate, escaped by JSON.stringify
  if (cp <= 0xffff) return 3;
  return 4;
}

// Splits a string into fragments of at most LIMITS.chars code points and
// FRAGMENT_BYTES of JSON. Never cuts a surrogate pair. "" is one fragment.
export function fragments(str) {
  const out = [];
  let start = 0, chars = 0, bytes = 0;
  for (let i = 0; i < str.length;) {
    const cp = str.codePointAt(i);
    const width = cp > 0xffff ? 2 : 1;
    const cost = jsonCost(cp);
    if (chars > 0 && (chars + 1 > LIMITS.chars || bytes + cost > FRAGMENT_BYTES)) {
      out.push(str.slice(start, i));
      start = i;
      chars = 0;
      bytes = 0;
    }
    chars += 1;
    bytes += cost;
    i += width;
  }
  out.push(str.slice(start));
  return out;
}

const bytesOf = (value) => Buffer.byteLength(JSON.stringify(value), "utf8");

// One native unit (a message, a compaction, a notice) becomes one or more
// CHAT-01 entries. `unit.blocks` holds logical blocks: { fields, key, value },
// where `value` is the string that may split and `key` names its field. A
// block with key null (an attachment) has no string and is one fragment.
export function unitParts(unit, base) {
  const items = [];
  unit.blocks.forEach((b, ordinal) => {
    if (b.key === null) {
      items.push({ ...b.fields, block: ordinal, fragment: 0, lastFragment: true });
      return;
    }
    const pieces = fragments(b.value);
    pieces.forEach((piece, k) => {
      items.push({ ...b.fields, [b.key]: piece, block: ordinal, fragment: k, lastFragment: k === pieces.length - 1 });
    });
  });
  const groups = [];
  let current = [], currentBytes = 0;
  for (const item of items) {
    const size = bytesOf(item) + 1;
    if (current.length && (current.length === LIMITS.blocks || currentBytes + size > PART_BYTES)) {
      groups.push(current);
      current = [];
      currentBytes = 0;
    }
    current.push(item);
    currentBytes += size;
  }
  groups.push(current);
  return groups.map((content, part) => ({
    version: 2,
    kind: "entry",
    id: safeId(`${unit.id}:${part}`),
    conversation: base.conversation,
    branch: base.branch,
    parent: unit.parent,
    execution: base.execution,
    nativeEntry: unit.nativeEntry,
    role: unit.role,
    request: null,
    content,
    part,
    lastPart: part === groups.length - 1,
    createdAt: unit.createdAt,
    message: unit.message,
  }));
}

// Takes entries from `start` while the page stays within LIMITS. `shell` is
// the page record with an empty `entries` array.
export function takePage(entries, sizes, start, shell) {
  let bytes = bytesOf(shell);
  let end = start;
  while (end < entries.length && end - start < LIMITS.parts) {
    const add = sizes[end] + (end > start ? 1 : 0);
    if (bytes + add > LIMITS.pageBytes) break;
    bytes += add;
    end += 1;
  }
  if (end === start && start < entries.length) throw new Error("internal: a single part exceeds the page byte limit");
  return end;
}

export { bytesOf };