import type { ExportResult } from "@opentelemetry/core";
import { ExportResultCode } from "@opentelemetry/core";
import type { LogRecordExporter, ReadableLogRecord } from "@opentelemetry/sdk-logs";
import { SeverityNumber } from "@opentelemetry/api-logs";
import {
  ATTR_SERVICE_NAME,
  ATTR_SERVICE_VERSION,
} from "@opentelemetry/semantic-conventions";

const SEVERITY_TEXT: Record<number, string> = {
  [SeverityNumber.DEBUG]: "debug",
  [SeverityNumber.DEBUG2]: "debug",
  [SeverityNumber.DEBUG3]: "debug",
  [SeverityNumber.DEBUG4]: "debug",
  [SeverityNumber.INFO]: "info",
  [SeverityNumber.INFO2]: "info",
  [SeverityNumber.INFO3]: "info",
  [SeverityNumber.INFO4]: "info",
  [SeverityNumber.WARN]: "warn",
  [SeverityNumber.WARN2]: "warn",
  [SeverityNumber.WARN3]: "warn",
  [SeverityNumber.WARN4]: "warn",
  [SeverityNumber.ERROR]: "error",
  [SeverityNumber.ERROR2]: "error",
  [SeverityNumber.ERROR3]: "error",
  [SeverityNumber.ERROR4]: "error",
  [SeverityNumber.FATAL]: "fatal",
  [SeverityNumber.FATAL2]: "fatal",
  [SeverityNumber.FATAL3]: "fatal",
  [SeverityNumber.FATAL4]: "fatal",
};

/**
 * Exports OTel log records as newline-delimited JSON to stdout.
 *
 * Output format:
 * {"timestamp":"2026-02-23T12:00:00.000Z","level":"info","message":"Server started","service":"atbb-appview","port":3000}
 *
 * Compatible with standard log aggregation tools (ELK, Grafana Loki, Datadog, etc.).
 */
export class StructuredLogExporter implements LogRecordExporter {
  export(
    records: ReadableLogRecord[],
    resultCallback: (result: ExportResult) => void
  ): void {
    try {
      for (const record of records) {
        const entry = this.formatRecord(record);
        process.stdout.write(JSON.stringify(entry) + "\n");
      }
      resultCallback({ code: ExportResultCode.SUCCESS });
    } catch {
      resultCallback({ code: ExportResultCode.FAILED });
    }
  }

  shutdown(): Promise<void> {
    return Promise.resolve();
  }

  private formatRecord(record: ReadableLogRecord): Record<string, unknown> {
    const resourceAttrs = record.resource?.attributes ?? {};
    const logAttrs = record.attributes ?? {};

    // Build the base log entry
    const entry: Record<string, unknown> = {
      timestamp: this.hrTimeToISO(record.hrTime),
      level: SEVERITY_TEXT[record.severityNumber ?? SeverityNumber.INFO] ?? "info",
      message: record.body ?? "",
    };

    // Add service metadata from resource
    const service = resourceAttrs[ATTR_SERVICE_NAME];
    if (service) {
      entry.service = service;
    }
    const version = resourceAttrs[ATTR_SERVICE_VERSION];
    if (version) {
      entry.version = version;
    }
    const environment = resourceAttrs["deployment.environment.name"];
    if (environment) {
      entry.environment = environment;
    }

    // Spread user-provided attributes into the top level
    for (const [key, value] of Object.entries(logAttrs)) {
      if (value !== undefined && value !== null) {
        entry[key] = value;
      }
    }

    return entry;
  }

  private hrTimeToISO(hrTime: [number, number]): string {
    const ms = hrTime[0] * 1000 + hrTime[1] / 1_000_000;
    return new Date(ms).toISOString();
  }
}