/**
 * Cache-warmup economics — FR-008 rule #4, SC-006.
 *
 * Decides whether switching providers justifies the cache-warmup penalty.
 * A provider switch invalidates upstream KV-cache, so the router must
 * compare the warmup cost against projected savings from the candidate
 * model before breaking a pin.
 *
 * When warmup cost exceeds projected savings the pin holds (User Story 4,
 * scenario 4).
 */

import type { ModelProfile, SessionPin } from '../types/index.js';

// ─── Types ────────────────────────────────────────────────────────────────────

export interface CacheEconomicsConfig {
  /**
   * Multiplier applied to estimated_input_tokens to approximate
   * the token cost of re-prompting a cold provider cache.
   * Default: 1.15 (15 % warmup overhead).
   */
  readonly warmupCostMultiplier?: number;

  /**
   * Minimum per-1M-token savings required to justify a provider switch.
   * Prevents switching for negligible price differences.
   * Default: 0.10 (USD).
   */
  readonly minSavingsThreshold?: number;

  /**
   * Number of remaining turns to amortize savings over.
   * Higher values favor switching; lower values favor keeping the pin.
   * Default: 5.
   */
  readonly projectedRemainingTurns?: number;
}

export interface CacheEconomicsResult {
  readonly shouldSwitch: boolean;
  readonly warmupCostUsd: number;
  readonly projectedSavingsUsd: number;
  readonly reason: string;
}

// ─── Defaults ─────────────────────────────────────────────────────────────────

const DEFAULT_WARMUP_COST_MULTIPLIER = 1.15;
const DEFAULT_MIN_SAVINGS_THRESHOLD = 0.10;
const DEFAULT_PROJECTED_REMAINING_TURNS = 5;

// ─── Evaluation ───────────────────────────────────────────────────────────────

/**
 * Evaluate whether switching from the currently-pinned model to a candidate
 * on a different provider is justified by projected cost savings.
 *
 * Returns `shouldSwitch: false` when:
 * - Candidate is on the same provider (no cache penalty).
 *   Caller should handle same-provider switches without this check.
 * - Warmup cost exceeds projected savings.
 * - Savings fall below the minimum threshold.
 *
 * Returns `shouldSwitch: true` only when projected savings meaningfully
 * exceed the cache-warmup penalty.
 */
export function evaluateCacheEconomics(
  pin: SessionPin,
  pinnedModel: ModelProfile,
  candidate: ModelProfile,
  estimatedInputTokens: number,
  config?: CacheEconomicsConfig,
): CacheEconomicsResult {
  const warmupMultiplier =
    config?.warmupCostMultiplier ?? DEFAULT_WARMUP_COST_MULTIPLIER;
  const minSavings =
    config?.minSavingsThreshold ?? DEFAULT_MIN_SAVINGS_THRESHOLD;
  const remainingTurns =
    config?.projectedRemainingTurns ?? DEFAULT_PROJECTED_REMAINING_TURNS;

  if (pinnedModel.provider === candidate.provider) {
    return {
      shouldSwitch: false,
      warmupCostUsd: 0,
      projectedSavingsUsd: 0,
      reason: 'same_provider_no_cache_penalty',
    };
  }

  const tokenCountM = estimatedInputTokens / 1_000_000;

  const warmupTokens = estimatedInputTokens * (warmupMultiplier - 1);
  const warmupCostUsd =
    (warmupTokens / 1_000_000) * candidate.pricing.fallback_cost_per_1m;

  const perTurnSavings =
    (pinnedModel.pricing.fallback_cost_per_1m -
      candidate.pricing.fallback_cost_per_1m) *
    tokenCountM;

  const projectedSavingsUsd = perTurnSavings * remainingTurns;

  if (projectedSavingsUsd < minSavings) {
    return {
      shouldSwitch: false,
      warmupCostUsd,
      projectedSavingsUsd,
      reason: 'savings_below_threshold',
    };
  }

  if (warmupCostUsd >= projectedSavingsUsd) {
    return {
      shouldSwitch: false,
      warmupCostUsd,
      projectedSavingsUsd,
      reason: 'warmup_exceeds_savings',
    };
  }

  return {
    shouldSwitch: true,
    warmupCostUsd,
    projectedSavingsUsd,
    reason: 'switch_justified',
  };
}