import { constants } from "node:fs";
import { open } from "node:fs/promises";

const admissionFailure = (code: string, kind: "file-bytes", measured: number, limit: number) =>
  Object.assign(Error(code), { metadataAdmission: { kind, measured, limit } });

/** Admission estimates, not a hard V8 heap limit. A process/cgroup ceiling is
 * still required. Charge input, parse trees and derived indexes before reading
 * or parsing. Each live candidate retains its reservation until explicit close. */
export class CandidateMetadataBudget {
  readonly limits: Readonly<{ bytes: number; ownerBytes: number; candidates: number }>;
  private bytes = 0;
  private scopes = 0;
  private readonly owners = new Map<string, number>();
  // A cutover holds both the serving and proposed installation plus transient
  // parsing space. The Reader with checked Query Trees retains ~176 MiB per
  // installation; its replacement also needs transient parse/index space.
  // Keep the aggregate ceiling independent and leave room for parsing overlap.
  constructor(limits = { bytes: 768 * 1024 * 1024, ownerBytes: 512 * 1024 * 1024, candidates: 128 }) {
    for (const value of Object.values(limits))
      if (!Number.isSafeInteger(value) || value < 1) throw Error("INVALID_CANDIDATE_METADATA_BUDGET");
    if (limits.ownerBytes > limits.bytes) throw Error("INVALID_CANDIDATE_METADATA_BUDGET");
    this.limits = Object.freeze({ ...limits });
  }
  snapshot() {
    return { bytes: this.bytes, candidates: this.scopes, owners: this.owners.size, limits: this.limits };
  }
  ownedBytes(owner: string) {
    return this.owners.get(owner) ?? 0;
  }
  acquire(owner: string) {
    if (!owner || owner.length > 4096) throw Error("INVALID_CANDIDATE_METADATA_OWNER");
    if (this.scopes >= this.limits.candidates) throw Error("CANDIDATE_METADATA_LIMIT");
    this.scopes++;
    let bytes = 0,
      closed = false;
    const charge = (amount: number) => {
      if (closed) throw Error("CANDIDATE_METADATA_CLOSED");
      if (!Number.isSafeInteger(amount) || amount < 0) throw Error("INVALID_CANDIDATE_METADATA_CHARGE");
      const owned = this.owners.get(owner) ?? 0;
      if (this.bytes + amount > this.limits.bytes || owned + amount > this.limits.ownerBytes)
        throw Error("CANDIDATE_METADATA_LIMIT");
      bytes += amount;
      this.bytes += amount;
      this.owners.set(owner, owned + amount);
    };
    const subtract = (amount: number) => {
      bytes -= amount;
      this.bytes -= amount;
      const remaining = (this.owners.get(owner) ?? 0) - amount;
      if (remaining) this.owners.set(owner, remaining);
      else this.owners.delete(owner);
    };
    return {
      get bytes() {
        return bytes;
      },
      charge,
      /** Opaque, idempotent transient reservation; callers cannot subtract an
       * arbitrary amount or release another candidate's charges. */
      reserve: (amount: number) => {
        charge(amount);
        let released = false;
        return () => {
          if (closed || released) return;
          released = true;
          subtract(amount);
        };
      },
      release: () => {
        if (closed) return;
        closed = true;
        subtract(bytes);
        this.scopes--;
      },
    };
  }
}
export type CandidateMetadataScope = ReturnType<CandidateMetadataBudget["acquire"]>;
export const candidateMetadataBudget = new CandidateMetadataBudget();

// Estimate JSON allocation without constructing its tree. Object/array headers,
// value slots and strings get explicit weights, with a multiplier for checked
// indexes/derived representations. Indentation must not multiply tree charges.
function jsonAllocation(bytes: Uint8Array): number {
  let estimate = 0;
  for (let i = 0; i < bytes.length; i++) {
    const c = bytes[i]!;
    if (c === 123) estimate += 64;
    else if (c === 91) estimate += 32;
    else if (c === 34) {
      const start = ++i;
      while (i < bytes.length && bytes[i] !== 34) {
        if (bytes[i] === 92) i++;
        i++;
      }
      estimate += 32 + 2 * (i - start);
    } else if (c === 45 || (c >= 48 && c <= 57) || c === 116 || c === 102 || c === 110) {
      estimate += 16;
      while (i + 1 < bytes.length) {
        const next = bytes[i + 1]!;
        if (next === 9 || next === 10 || next === 13 || next === 32 || next === 44 || next === 93 || next === 125)
          break;
        i++;
      }
    }
  }
  return estimate * 2;
}

/** Immutable regular files only. Fixed allocation plus an extra byte catches
 * growth; readFile() would allocate before the admission check can intervene. */
export async function readCandidateMetadata(
  path: string,
  scope: CandidateMetadataScope,
  limit: number,
): Promise<{ bytes: Uint8Array; releaseInput: () => void; releaseTree: () => void }> {
  const handle = await open(path, constants.O_RDONLY | constants.O_NOFOLLOW | constants.O_NONBLOCK);
  let releaseInput: (() => void) | undefined;
  try {
    const stat = await handle.stat();
    if (!stat.isFile() || !Number.isSafeInteger(stat.size)) throw Error("CANDIDATE_METADATA_FILE_LIMIT");
    if (stat.size > limit) throw admissionFailure("CANDIDATE_METADATA_FILE_LIMIT", "file-bytes", stat.size, limit);
    // Reserve the input buffer and UTF-16 decoding before allocating either.
    releaseInput = scope.reserve((stat.size + 1) * 3);
    const bytes = Buffer.alloc(stat.size + 1);
    let offset = 0;
    while (offset < bytes.length) {
      const read = await handle.read(bytes, offset, bytes.length - offset, offset);
      if (!read.bytesRead) break;
      offset += read.bytesRead;
    }
    if (offset !== stat.size) throw Error("CANDIDATE_METADATA_CHANGED");
    const content = bytes.subarray(0, offset);
    // No JSON.parse/decoded tree exists until its shape-based charge succeeds.
    const releaseTree = scope.reserve(jsonAllocation(content));
    return { bytes: content, releaseInput, releaseTree };
  } catch (error) {
    releaseInput?.();
    throw error;
  } finally {
    await handle.close();
  }
}