/**
 * Sub-route policy — FR-024.
 *
 * Pure policy function that evaluates whether a tool-result turn is
 * eligible for same-provider economical sub-routing without breaking
 * the session pin.
 *
 * Rules (FR-024, exhaustive):
 *   1. Turn must be a tool_result.
 *   2. Payload must be below the configurable size threshold (default 2 KB).
 *   3. An economical-cloud model on the same provider as the pin must exist
 *      and be healthy.
 *   4. The economical model must not be the pinned model itself.
 *
 * When all conditions are met the caller may dispatch to the economical
 * model. The session pin record is unchanged — the pin holds.
 */

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

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

export type SubRouteReason =
  | 'eligible'
  | 'not_tool_result'
  | 'payload_exceeds_threshold'
  | 'pinned_model_not_in_fleet'
  | 'no_same_provider_economical';

export interface SubRouteEvaluation {
  readonly eligible: boolean;
  readonly subRouteModel?: ModelProfile;
  readonly pinnedModel?: ModelProfile;
  readonly reason: SubRouteReason;
}

export interface SubRoutePolicyConfig {
  /** Max payload size (bytes or token estimate) for sub-routing. Default 2048. */
  readonly sizeThreshold?: number;
}

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

const DEFAULT_SIZE_THRESHOLD = 2048;

/**
 * Select the lowest-cost healthy model from candidates (SP-085).
 */
export function selectLowestCostModel(
  candidates: readonly ModelProfile[],
): ModelProfile | undefined {
  let best: ModelProfile | undefined;
  let bestCost = Infinity;

  for (const model of candidates) {
    if (model.healthy === false) continue;
    const cost = model.pricing.fallback_cost_per_1m;
    if (cost < bestCost) {
      bestCost = cost;
      best = model;
    }
  }

  return best;
}

// ─── Policy ───────────────────────────────────────────────────────────────────

/**
 * Evaluate sub-route eligibility for a single request against a session pin.
 *
 * Designed for the hot path — no allocation beyond the return object,
 * no I/O, deterministic.
 */
export function evaluateSubRoutePolicy(
  request: RoutingRequest,
  pin: SessionPin,
  fleet: readonly ModelProfile[],
  config?: SubRoutePolicyConfig,
): SubRouteEvaluation {
  const threshold = config?.sizeThreshold ?? DEFAULT_SIZE_THRESHOLD;

  if (request.turn_type !== 'tool_result') {
    return { eligible: false, reason: 'not_tool_result' };
  }

  const payloadSize =
    request.estimated_input_tokens ?? request.prompt_text.length;

  if (payloadSize > threshold) {
    return { eligible: false, reason: 'payload_exceeds_threshold' };
  }

  const pinnedModel = fleet.find((m) => m.id === pin.pinned_model_id);
  if (!pinnedModel) {
    return { eligible: false, reason: 'pinned_model_not_in_fleet' };
  }

  const candidates = fleet.filter(
    (m) =>
      m.tier === 'economical-cloud' &&
      m.provider === pinnedModel.provider &&
      m.id !== pin.pinned_model_id &&
      m.healthy !== false,
  );

  const econModel = selectLowestCostModel(candidates);

  if (!econModel) {
    return { eligible: false, reason: 'no_same_provider_economical' };
  }

  return {
    eligible: true,
    subRouteModel: econModel,
    pinnedModel,
    reason: 'eligible',
  };
}