export class ValidationError extends Error {
  status = 400;
}

export interface ValidatedScreenshotRequest {
  pagePath: string;
  qs: URLSearchParams;
}

const MAX_REPEATED_PARAMS = 40;
const MAX_VALUE_LENGTH = 500;
const NUMERIC_RE = /^-?(?:\d+|\d*\.\d+)$/;
const PATH_RE = /^\/(?:invite\/[A-Za-z0-9]{1,20})?$/;
const SAFE_VALUE_RE = /^[^\u0000-\u001f\u007f]+$/;
const REPEATED_KEYS = ['filter', 'school', 'crime', 'ethnicity', 'poi', 'tt'] as const;

type Query = Record<string, unknown>;

function validationError(message: string): never {
  throw new ValidationError(message);
}

function firstString(query: Query, key: string): string | undefined {
  const value = query[key];
  if (value == null) return undefined;
  if (Array.isArray(value)) {
    validationError(`${key} must not be repeated`);
  }
  if (typeof value !== 'string') {
    validationError(`${key} must be a string`);
  }
  return value || undefined;
}

function repeatedStrings(query: Query, key: string): string[] {
  const value = query[key];
  if (value == null) return [];
  const values = Array.isArray(value) ? value : [value];
  if (values.length > MAX_REPEATED_PARAMS) {
    validationError(`${key} has too many values`);
  }
  return values.filter((item): item is string => {
    if (typeof item !== 'string') {
      validationError(`${key} values must be strings`);
    }
    return item.length > 0;
  });
}

function assertSafeValue(key: string, value: string): void {
  if (value.length > MAX_VALUE_LENGTH) {
    validationError(`${key} is too long`);
  }
  if (!SAFE_VALUE_RE.test(value)) {
    validationError(`${key} contains invalid characters`);
  }
}

function appendBoundedNumber(
  qs: URLSearchParams,
  query: Query,
  key: string,
  min: number,
  max: number,
): void {
  const value = firstString(query, key);
  if (value == null) return;
  if (value.length > 40 || !NUMERIC_RE.test(value)) {
    validationError(`${key} must be a number`);
  }
  const numeric = Number(value);
  if (!Number.isFinite(numeric) || numeric < min || numeric > max) {
    validationError(`${key} is out of range`);
  }
  qs.set(key, value);
}

export function buildScreenshotRequest(query: Query): ValidatedScreenshotRequest {
  const qs = new URLSearchParams();

  appendBoundedNumber(qs, query, 'lat', -90, 90);
  appendBoundedNumber(qs, query, 'lon', -180, 180);
  appendBoundedNumber(qs, query, 'zoom', 0, 22);

  const tab = firstString(query, 'tab');
  if (tab != null) {
    if (tab !== 'area' && tab !== 'properties') {
      validationError('tab is invalid');
    }
    qs.set('tab', tab);
  }

  const og = firstString(query, 'og');
  if (og != null) {
    if (og !== '1') {
      validationError('og is invalid');
    }
    qs.set('og', og);
  }

  for (const key of REPEATED_KEYS) {
    for (const value of repeatedStrings(query, key)) {
      assertSafeValue(key, value);
      qs.append(key, value);
    }
  }

  const pagePath = firstString(query, 'path') ?? '/';
  if (!PATH_RE.test(pagePath)) {
    validationError('path is invalid');
  }

  return { pagePath, qs };
}