stream-json-formatter.ts

/**
 * @license
 * Copyright 2025 Google LLC
 * SPDX-License-Identifier: Apache-2.0
 */

import type {
  JsonStreamEvent,
  ModelStreamStats,
  StreamStats,
} from './types.js';
import type { SessionMetrics } from '../telemetry/uiTelemetry.js';

/**
 * Formatter for streaming JSON output.
 * Emits newline-delimited JSON (JSONL) events to stdout in real-time.
 */
export class StreamJsonFormatter {
  /**
   * Formats a single event as a JSON string with newline (JSONL format).
   * @param event - The stream event to format
   * @returns JSON string with trailing newline
   */
  formatEvent(event: JsonStreamEvent): string {
    return JSON.stringify(event) + '\n';
  }

  /**
   * Emits an event directly to stdout in JSONL format.
   * @param event - The stream event to emit
   */
  emitEvent(event: JsonStreamEvent): void {
    process.stdout.write(this.formatEvent(event));
  }

  /**
   * Converts SessionMetrics to simplified StreamStats format.
   * Includes per-model token breakdowns and aggregated totals.
   * @param metrics - The session metrics from telemetry
   * @param durationMs - The session duration in milliseconds
   * @returns Simplified stats for streaming output
   */
  convertToStreamStats(
    metrics: SessionMetrics,
    durationMs: number,
  ): StreamStats {
    const {
      totalTokens,
      inputTokens,
      outputTokens,
      cached,
      input,
      models,
    } = Object.entries(metrics.models).reduce(
      (acc, [modelName, modelMetrics]) => {
        const modelStats: ModelStreamStats = {
          total_tokens: modelMetrics.tokens.total,
          input_tokens: modelMetrics.tokens.prompt,
          output_tokens: modelMetrics.tokens.candidates,
          cached: modelMetrics.tokens.cached,
          input: modelMetrics.tokens.input,
        };

        acc.models[modelName] = modelStats;
        acc.totalTokens += modelStats.total_tokens;
        acc.inputTokens += modelStats.input_tokens;
        acc.outputTokens += modelStats.output_tokens;
        acc.cached += modelStats.cached;
        acc.input += modelStats.input;

        return acc;
      },
      {
        totalTokens: 0,
        inputTokens: 0,
        outputTokens: 0,
        cached: 0,
        input: 0,
        models: {} as Record<string, ModelStreamStats>,
      },
    );

    return {
      total_tokens: totalTokens,
      input_tokens: inputTokens,
      output_tokens: outputTokens,
      cached,
      input,
      duration_ms: durationMs,
      tool_calls: metrics.tools.totalCalls,
      models,
    };
  }
}