edr-platform/apps/edr-freight-api/src/modules/exchange-settings/exchange-settings.service.ts

import { Injectable, Logger } from "@nestjs/common";
import { InjectRepository } from "@nestjs/typeorm";
import { Repository } from "typeorm";

import { ExchangeSetting } from "./entities/exchange-setting.entity";

/**
 * Rate used before the row exists and before the first successful CBE fetch —
 * the CBE USD transactional selling rate on 2026-08-04.
 */
const SEED_FALLBACK_RATE = 162.4165;

/** Health of the CBE feed, as surfaced to the backoffice. */
export interface ExchangeFeedStatus {
  /** Rate most recently observed, whatever its source. */
  rate: number | null;
  /** `live` means CBE answered; `stored`/`default` mean it is failing. */
  source: "live" | "stored" | null;
  /** ISO timestamp of the last successful fetch. */
  lastSuccessAt: string | null;
  /** Message from the most recent failure, cleared on success. */
  lastError: string | null;
}

/**
 * Owns the single `exchange_settings` row: the USD→ETB fallback used when the
 * CBE endpoint is unreachable.
 *
 * The live CBE rate is always preferred. This value is only read on failure,
 * and every successful fetch overwrites it, so it tracks the last known good
 * rate rather than drifting into a stale constant.
 */
@Injectable()
export class ExchangeSettingsService {
  private readonly logger = new Logger(ExchangeSettingsService.name);

  /**
   * Feed health, recorded from the exchange provider's callbacks rather than
   * read off an injected `ExchangeService`. The provider is registered several
   * times (bookings, contracts, warehouses), so no single instance sees every
   * fetch — and injecting one here would be circular, since those
   * registrations inject *this* service.
   */
  private feed: ExchangeFeedStatus = {
    rate: null,
    source: null,
    lastSuccessAt: null,
    lastError: null,
  };

  constructor(
    @InjectRepository(ExchangeSetting)
    private readonly repository: Repository<ExchangeSetting>,
  ) {}

  /** Health of the CBE feed as last observed by any provider instance. */
  getFeedStatus(): ExchangeFeedStatus {
    return { ...this.feed };
  }

  /** The settings row, created at the seed rate on first access. */
  async get(): Promise<ExchangeSetting> {
    const existing = await this.repository.findOne({ where: {} });
    if (existing) return existing;

    return this.repository.save(
      this.repository.create({
        fallbackRate: SEED_FALLBACK_RATE,
        fallbackSource: "AUTO",
        lastSyncedAt: null,
      }),
    );
  }

  /**
   * Reads the stored fallback for the exchange provider. Returns `null` on any
   * failure so the provider falls through to its own static default rather
   * than propagating a database error into a pricing call.
   */
  async loadFallbackRate(): Promise<number | null> {
    // Only reached when the live fetch failed, so this call is itself the
    // signal that the feed is down.
    try {
      const { fallbackRate } = await this.get();
      const usable = Number.isFinite(fallbackRate) && fallbackRate > 0;
      this.feed = {
        ...this.feed,
        rate: usable ? fallbackRate : this.feed.rate,
        source: "stored",
        lastError: this.feed.lastError ?? "CBE endpoint unreachable",
      };
      return usable ? fallbackRate : null;
    } catch (err) {
      const message = (err as Error).message;
      this.feed = { ...this.feed, source: "stored", lastError: message };
      this.logger.warn(`Could not read stored exchange fallback: ${message}`);
      return null;
    }
  }

  /**
   * Records a freshly fetched live rate as the new fallback. Marked `AUTO`,
   * overwriting a manual entry — a manual rate is a stopgap for while CBE is
   * down, so a working CBE feed takes precedence again.
   */
  async saveFallbackRate(rate: number): Promise<void> {
    // Only called after a successful fetch, so the feed is confirmed healthy.
    this.feed = {
      rate,
      source: "live",
      lastSuccessAt: new Date().toISOString(),
      lastError: null,
    };

    const current = await this.get();
    await this.repository.update(current.id, {
      fallbackRate: rate,
      fallbackSource: "AUTO",
      lastSyncedAt: new Date(),
      updatedById: null,
    });
    this.logger.log(`Exchange fallback synced from CBE: ${rate} ETB/USD`);
  }

  /** Operator sets the fallback by hand, e.g. during a prolonged CBE outage. */
  async setManualRate(
    rate: number,
    updatedById?: string | null,
  ): Promise<ExchangeSetting> {
    const current = await this.get();
    await this.repository.update(current.id, {
      fallbackRate: rate,
      fallbackSource: "MANUAL",
      updatedById: updatedById ?? null,
    });
    this.logger.warn(
      `Exchange fallback set manually to ${rate} ETB/USD by ${updatedById ?? "unknown user"}`,
    );
    return this.get();
  }
}