import { formatIpv4, parseCidr } from "@add-ideas/toolbox-helpers";

const MAX_REQUIREMENTS = 1_024;
const MAX_NAME_LENGTH = 120;

export interface VlsmRequirement {
  name: string;
  hosts: number;
}

export interface VlsmAllocation extends VlsmRequirement {
  requestedIndex: number;
  prefixLength: number;
  network: string;
  firstUsable: string;
  lastUsable: string;
  broadcast: string;
  usableHosts: number;
  addresses: number;
}

export interface VlsmPlan {
  base: string;
  allocations: VlsmAllocation[];
  usedAddresses: number;
  freeAddresses: number;
  warnings: string[];
}

function prefixForHosts(hosts: number): number {
  if (!Number.isSafeInteger(hosts) || hosts < 1 || hosts > 4_294_967_294)
    throw new Error(
      "Host requirements must be integers from 1 to 4,294,967,294.",
    );
  const addressBits = Math.ceil(Math.log2(hosts + 2));
  return 32 - Math.max(2, addressBits);
}

function align(value: bigint, block: bigint): bigint {
  const remainder = value % block;
  return remainder === 0n ? value : value + block - remainder;
}

export function parseVlsmRequirements(source: string): VlsmRequirement[] {
  if (source.length > 256 * 1024)
    throw new Error("Requirements exceed the 256 KiB limit.");
  const requirements: VlsmRequirement[] = [];
  for (const [index, raw] of source
    .replaceAll("\r\n", "\n")
    .split("\n")
    .entries()) {
    const line = raw.trim();
    if (!line || line.startsWith("#")) continue;
    if (requirements.length >= MAX_REQUIREMENTS)
      throw new Error(
        `More than ${MAX_REQUIREMENTS} subnet requirements were supplied.`,
      );
    const separator = line.lastIndexOf(",");
    if (separator < 1)
      throw new Error(`Line ${index + 1}: expected name,hosts.`);
    const name = line.slice(0, separator).trim();
    const hostsText = line.slice(separator + 1).trim();
    if (!name || name.length > MAX_NAME_LENGTH)
      throw new Error(
        `Line ${index + 1}: name must contain 1–${MAX_NAME_LENGTH} characters.`,
      );
    if (!/^\d+$/u.test(hostsText))
      throw new Error(`Line ${index + 1}: hosts must be a positive integer.`);
    const hosts = Number(hostsText);
    prefixForHosts(hosts);
    requirements.push({ name, hosts });
  }
  if (requirements.length === 0)
    throw new Error("Enter at least one name,hosts requirement.");
  return requirements;
}

export function planVlsm(
  baseCidr: string,
  requirements: readonly VlsmRequirement[],
): VlsmPlan {
  if (requirements.length === 0 || requirements.length > MAX_REQUIREMENTS)
    throw new Error(
      `Supply between 1 and ${MAX_REQUIREMENTS} subnet requirements.`,
    );
  const base = parseCidr(baseCidr);
  if (base.version !== 4)
    throw new Error(
      "The VLSM planner currently supports IPv4 base networks only.",
    );

  const sorted = requirements
    .map((requirement, requestedIndex) => ({
      ...requirement,
      requestedIndex,
      prefixLength: prefixForHosts(requirement.hosts),
    }))
    .sort(
      (left, right) =>
        left.prefixLength - right.prefixLength ||
        left.requestedIndex - right.requestedIndex,
    );

  let cursor = base.network.value;
  const endExclusive = base.network.value + base.size;
  const allocations: VlsmAllocation[] = [];
  for (const requirement of sorted) {
    if (requirement.prefixLength < base.prefixLength)
      throw new Error(
        `${requirement.name} needs /${requirement.prefixLength}, which is larger than ${base.canonical}.`,
      );
    const addresses = 2 ** (32 - requirement.prefixLength);
    const block = BigInt(addresses);
    const network = align(cursor, block);
    const after = network + block;
    if (after > endExclusive)
      throw new Error(
        `${requirement.name} does not fit. Allocation stopped before producing a partial plan.`,
      );
    allocations.push({
      name: requirement.name,
      hosts: requirement.hosts,
      requestedIndex: requirement.requestedIndex,
      prefixLength: requirement.prefixLength,
      network: `${formatIpv4(network)}/${requirement.prefixLength}`,
      firstUsable: formatIpv4(network + 1n),
      lastUsable: formatIpv4(after - 2n),
      broadcast: formatIpv4(after - 1n),
      usableHosts: addresses - 2,
      addresses,
    });
    cursor = after;
  }

  const used = allocations.reduce((total, item) => total + item.addresses, 0);
  const baseSize = Number(base.size);
  return {
    base: base.canonical,
    allocations,
    usedAddresses: used,
    freeAddresses: baseSize - used,
    warnings: [
      "Allocations are largest-first and use conventional network/broadcast reservations; point-to-point /31 semantics are not assumed.",
      "Free-address count includes alignment gaps and is not itself a contiguous remainder guarantee.",
    ],
  };
}