// Functional core for LLM performance metrics calculation

export interface TurnMetrics {
  turnId: string;
  inputTokens: number;
  outputTokens: number;
  durationMs: number;
  timeToFirstTokenMs?: number;
}

export interface PromptMetrics {
  provider: string;
  model: string;
  turnCount: number;
  inputTokens: number;
  outputTokens: number;
  totalTokens: number;
  prefillTokensPerSec: number;
  generationTokensPerSec: number;
  combinedTokensPerSec: number;
  totalDurationMs: number;
  timeToFirstTokenMs?: number;
  turns: TurnMetrics[];
}

export interface MetricLogEntry {
  timestamp: string;
  provider: string;
  model: string;
  turnCount: number;
  inputTokens: number;
  outputTokens: number;
  totalTokens: number;
  prefillTokensPerSec: number;
  generationTokensPerSec: number;
  combinedTokensPerSec: number;
  totalDurationMs: number;
  timeToFirstTokenMs?: number;
}

/**
 * Calculate metrics for a single turn
 */
export function calculateTurnMetrics(params: {
  turnId: string;
  inputTokens: number;
  outputTokens: number;
  durationMs: number;
  timeToFirstTokenMs?: number;
}): TurnMetrics {
  return {
    turnId: params.turnId,
    inputTokens: params.inputTokens,
    outputTokens: params.outputTokens,
    durationMs: params.durationMs,
    timeToFirstTokenMs: params.timeToFirstTokenMs,
  };
}

/**
 * Aggregate multiple turn metrics into prompt-level metrics
 */
export function aggregatePromptMetrics(params: {
  provider: string;
  model: string;
  turnMetrics: TurnMetrics[];
}): PromptMetrics {
  const { provider, model, turnMetrics } = params;

  if (turnMetrics.length === 0) {
    return {
      provider,
      model,
      turnCount: 0,
      inputTokens: 0,
      outputTokens: 0,
      totalTokens: 0,
      prefillTokensPerSec: 0,
      generationTokensPerSec: 0,
      combinedTokensPerSec: 0,
      totalDurationMs: 0,
      turns: [],
    };
  }

  // Sum tokens across all turns
  const inputTokens = turnMetrics.reduce((sum, t) => sum + t.inputTokens, 0);
  const outputTokens = turnMetrics.reduce((sum, t) => sum + t.outputTokens, 0);
  const totalTokens = inputTokens + outputTokens;

  // Sum duration across all turns
  const totalDurationMs = turnMetrics.reduce((sum, t) => sum + t.durationMs, 0);
  const totalDurationSec = totalDurationMs / 1000;

  // Time to first token is from the first turn
  const timeToFirstTokenMs = turnMetrics[0]?.timeToFirstTokenMs;

  // Calculate tokens per second
  // Prefill: input tokens / TTFT duration (prefill phase)
  // Generation: output tokens / (totalDuration - TTFT) (generation phase)
  // Combined: total tokens / total duration
  // When TTFT is unavailable, prefill and generation phases cannot be separated,
  // so we set them to 0 and only report combined.
  const ttftSec = timeToFirstTokenMs !== undefined ? timeToFirstTokenMs / 1000 : undefined;
  const generationDurationSec = timeToFirstTokenMs !== undefined
    ? (totalDurationMs - timeToFirstTokenMs) / 1000
    : undefined;

  const prefillTokensPerSec = (ttftSec && ttftSec > 0) ? inputTokens / ttftSec : 0;
  const generationTokensPerSec = (generationDurationSec !== undefined && generationDurationSec > 0)
    ? outputTokens / generationDurationSec
    : 0;
  const combinedTokensPerSec = totalDurationSec > 0 ? totalTokens / totalDurationSec : 0;

  return {
    provider,
    model,
    turnCount: turnMetrics.length,
    inputTokens,
    outputTokens,
    totalTokens,
    prefillTokensPerSec,
    generationTokensPerSec,
    combinedTokensPerSec,
    totalDurationMs,
    timeToFirstTokenMs,
    turns: turnMetrics,
  };
}

/**
 * Format metrics for TUI display
 */
export function formatMetricsForDisplay(metrics: PromptMetrics): string {
  const lines: string[] = [];

  // Header with provider/model
  lines.push(`📊 Performance: ${metrics.provider}/${metrics.model}`);

  if (metrics.turnCount === 0) {
    lines.push("  No turns recorded");
    return lines.join("\n");
  }

  // Format duration display
  const durationSec = metrics.totalDurationMs / 1000;
  const durationDisplay = durationSec >= 60
    ? `${(durationSec / 60).toFixed(1)}m`
    : `${durationSec.toFixed(1)}s`;

  // Prefill metrics (only when TTFT was available)
  if (metrics.prefillTokensPerSec > 0) {
    lines.push(
      `  Prefill: ${metrics.inputTokens.toLocaleString()} tokens @ ${metrics.prefillTokensPerSec.toFixed(1)} tok/s`
    );
  }

  // Generation metrics (only when TTFT was available)
  if (metrics.generationTokensPerSec > 0) {
    lines.push(
      `  Generation: ${metrics.outputTokens.toLocaleString()} tokens @ ${metrics.generationTokensPerSec.toFixed(1)} tok/s`
    );
  }

  // Combined metrics
  lines.push(
    `  Combined: ${metrics.totalTokens.toLocaleString()} tokens @ ${metrics.combinedTokensPerSec.toFixed(1)} tok/s (${durationDisplay} total)`
  );

  // Time to first token
  if (metrics.timeToFirstTokenMs !== undefined) {
    lines.push(`  TTFT: ${metrics.timeToFirstTokenMs.toFixed(0)}ms`);
  }

  // Turn count
  if (metrics.turnCount > 1) {
    lines.push(`  Turns: ${metrics.turnCount}`);
  }

  return lines.join("\n");
}

/**
 * Convert PromptMetrics to JSONL log entry
 */
export function toLogEntry(metrics: PromptMetrics): MetricLogEntry {
  return {
    timestamp: new Date().toISOString(),
    provider: metrics.provider,
    model: metrics.model,
    turnCount: metrics.turnCount,
    inputTokens: metrics.inputTokens,
    outputTokens: metrics.outputTokens,
    totalTokens: metrics.totalTokens,
    prefillTokensPerSec: Math.round(metrics.prefillTokensPerSec * 100) / 100,
    generationTokensPerSec: Math.round(metrics.generationTokensPerSec * 100) / 100,
    combinedTokensPerSec: Math.round(metrics.combinedTokensPerSec * 100) / 100,
    totalDurationMs: metrics.totalDurationMs,
    timeToFirstTokenMs: metrics.timeToFirstTokenMs,
  };
}