/**
 * EPPP configuration environment adapter — [E00-S04-T04] no module reads
 * `process.env` except the configuration adapter.
 *
 * This module is the workspace's single owner of `process.env` reads.
 * `loadConfigFromEnv` maps the environment onto the validated config shape
 * (the E00-S04-T01 TypeBox/Ajv schema) and validates it with
 * `assertValidConfig` (the E00-S04-T02 startup validation) before returning
 * it, so every setting the application uses — `host`, `port`, `databaseUrl`,
 * `sessionSecret` — flows through the adapter and no other module reads
 * `process.env` directly (the issue's acceptance criteria: "no module reads
 * `process.env` directly except the configuration adapter", "all settings
 * flow through the adapter").
 *
 * The environment mapping (per the schema's documented environment sources,
 * `schema.ts`):
 *
 * - `HOST` → `host` — the interface the HTTP server binds, default `0.0.0.0`
 *   (the container default). The value is validated at this adapter boundary
 *   as a hostname or IP address (IPv4/IPv6) before it is used for binding or
 *   logged: an invalid `HOST` throws a field-specific `ConfigStartupError`
 *   naming `host`, so arbitrary env content is never echoed verbatim into the
 *   startup log (the issue's acceptance criterion: "HOST is validated at the
 *   adapter boundary as a hostname or IP address before it is used for
 *   binding or logged").
 * - `PORT` → `port` — integer in the valid TCP port range (1–65535), default
 *   `3000` (the Dockerfile `EXPOSE 3000` / compose `:3000` container port). A
 *   non-numeric or out-of-range override falls back to the default so a bad
 *   `PORT` cannot crash the process at startup (the behavior the server's
 *   entrypoint had before this adapter existed).
 * - `DATABASE_URL` → `databaseUrl` — optional; when absent the app has no
 *   startup migration run to wait for and reports ready immediately (the
 *   local non-container developer path, E00-S01-T06/E00-S03-T06).
 * - `EPPP_SESSION_SECRET` → `sessionSecret` — the required admin-session
 *   secret (Security-and-Operations §32/§26, ≥ 32 characters); a missing or
 *   too-short value fails startup with the field-specific error from
 *   `assertValidConfig`.
 *
 * `assertValidConfig` is what makes a missing required setting a startup
 * error: the adapter hands the mapped value to it, and it throws
 * `MissingRequiredSettingError` naming the missing field — the application
 * never boots with an invalid configuration.
 *
 * Rollback note from the issue: revert any module changes that read
 * `process.env`.
 */

import { isIP } from 'node:net';

import { assertValidConfig, ConfigStartupError } from './startup.js';
import type { Config } from './schema.js';

/**
 * Resolves the listen port from `PORT` (default 3000, matching the Dockerfile
 * `EXPOSE 3000` and the compose `:3000` container port). A non-numeric or
 * out-of-range override falls back to the default so a bad `PORT` value cannot
 * crash the process at startup.
 */
function resolvePort(raw: string | undefined): number {
  const port = Number(raw ?? 3000);
  return Number.isInteger(port) && port > 0 && port <= 65535 ? port : 3000;
}

/**
 * One RFC 1123 hostname label: 1–63 alphanumerics/hyphens, not starting or
 * ending with a hyphen.
 */
const HOSTNAME_LABEL = '[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?';

/**
 * A hostname: dot-separated RFC 1123 labels (e.g. `localhost`, `db`,
 * `api.internal.example`), at most 253 characters total.
 */
const HOSTNAME_PATTERN = new RegExp(`^(?:${HOSTNAME_LABEL}\\.)*${HOSTNAME_LABEL}$`);

/**
 * Resolves the bind interface from `HOST` (default `0.0.0.0` — the container
 * default). The value is validated at this adapter boundary as a hostname or
 * IP address (IPv4/IPv6, via `node:net` `isIP` or the RFC 1123 hostname
 * pattern) BEFORE it can be used for binding or logged: an invalid value
 * throws a field-specific `ConfigStartupError` naming `host`, so arbitrary
 * `HOST` content is never echoed verbatim into the startup log (the issue's
 * acceptance criterion — the server passes the validated value to
 * `server.listen`, and the startup log reflects the actual bind interface).
 *
 * Unlike `resolvePort` (which falls back to the default on a bad value), an
 * invalid `HOST` fails startup: an operator who sets `HOST=127.0.0.1` to
 * restrict network exposure must never silently get a different interface.
 */
function resolveHost(raw: string | undefined): string {
  if (raw === undefined) return '0.0.0.0';
  if (isIP(raw) !== 0 || (raw.length <= 253 && HOSTNAME_PATTERN.test(raw))) {
    return raw;
  }
  throw new ConfigStartupError(
    'invalid configuration: host: must be a valid hostname or IP address',
    ['host: must be a valid hostname or IP address'],
  );
}

/**
 * Maps the environment onto the validated configuration — the E00-S04-T04
 * environment adapter.
 *
 * Reads every setting from the given environment (defaulting to `process.env`
 * — this module is the workspace's single owner of `process.env` reads) and
 * returns the validated `Config`; an invalid environment fails fast with the
 * field-specific startup error, so a missing or malformed setting is a
 * startup error, never a silently-booted invalid configuration.
 *
 * @param env - the environment to read (defaults to `process.env`)
 * @returns the validated configuration
 * @throws {MissingRequiredSettingError} when a required setting is missing
 * @throws {ConfigStartupError} when the configuration violates the schema
 */
export function loadConfigFromEnv(env: NodeJS.ProcessEnv = process.env): Config {
  return assertValidConfig({
    host: resolveHost(env.HOST),
    port: resolvePort(env.PORT),
    databaseUrl: env.DATABASE_URL,
    sessionSecret: env.EPPP_SESSION_SECRET,
  });
}