/**
 * RouterStrategy — Pluggable Routing Strategy System
 *
 * Inspired by ClawRouter commit 14c83c258 "refactor: extract routing into pluggable RouterStrategy system".
 * Provides a RouterStrategy interface and built-in implementations:
 *   - RulesStrategy (default): wraps the existing 6-factor scoring engine
 *   - CostStrategy: always picks cheapest available model
 *   - LatencyStrategy: prioritizes low p95 latency with reliability weighting
 *   - SLAStrategy: prefers candidates that satisfy latency/error/cost SLOs
 *   - LKGPStrategy: tries last known good provider first
 */

import type { ProviderCandidate, ScoredProvider } from "./scoring.ts";
import { scorePool } from "./scoring.ts";
import { getTaskFitness } from "./taskFitness.ts";
import { clamp01 } from "../../utils/number.ts";

export interface SlaRoutingPolicy {
  targetP95Ms?: number;
  maxErrorRate?: number;
  maxCostPer1MTokens?: number;
  hardConstraints?: boolean;
}

export interface RoutingContext {
  taskType: string;
  requestHasTools?: boolean;
  requestHasVision?: boolean;
  estimatedInputTokens?: number;
  lastKnownGoodProvider?: string;
  lkgpEnabled?: boolean;
  sla?: SlaRoutingPolicy;
}

export interface RoutingDecision {
  provider: string;
  model: string;
  strategy: string;
  reason: string;
  candidatesConsidered: number;
  finalScore: number;
}

export interface RouterStrategy {
  readonly name: string;
  readonly description: string;
  select(pool: ProviderCandidate[], context: RoutingContext): RoutingDecision;
}

// ── RulesStrategy: wraps 6-factor scoring engine ────────────────────────────

class RulesStrategyImpl implements RouterStrategy {
  readonly name = "rules";
  readonly description =
    "6-factor weighted scoring: quota, health, cost, latency, taskFit, stability";

  select(pool: ProviderCandidate[], context: RoutingContext): RoutingDecision {
    const eligible = pool.filter((c) => c.circuitBreakerState !== "OPEN");
    const ranked: ScoredProvider[] = scorePool(
      eligible.length > 0 ? eligible : pool,
      context.taskType,
      undefined,
      getTaskFitness
    );
    const best = ranked[0];
    if (!best) throw new Error("[RulesStrategy] No candidates to score");
    return {
      provider: best.provider,
      model: best.model,
      strategy: this.name,
      reason: `RulesStrategy: score=${best.score.toFixed(3)} (quota=${best.factors.quota.toFixed(2)}, health=${best.factors.health.toFixed(2)}, cost=${best.factors.costInv.toFixed(2)}, taskFit=${best.factors.taskFit.toFixed(2)})`,
      candidatesConsidered: ranked.length,
      finalScore: best.score,
    };
  }
}

// ── CostStrategy: always picks cheapest healthy provider ─────────────────────

class CostStrategyImpl implements RouterStrategy {
  readonly name = "cost";
  readonly description = "Always selects cheapest available provider (by costPer1MTokens)";

  select(pool: ProviderCandidate[], context: RoutingContext): RoutingDecision {
    const healthy = pool.filter((c) => c.circuitBreakerState !== "OPEN");
    const candidates = healthy.length > 0 ? healthy : pool;
    const sorted = [...candidates].sort((a, b) => a.costPer1MTokens - b.costPer1MTokens);
    const best = sorted[0];
    if (!best) throw new Error("[CostStrategy] No candidates available");
    return {
      provider: best.provider,
      model: best.model,
      strategy: this.name,
      reason: `CostStrategy: cheapest at $${best.costPer1MTokens.toFixed(3)}/1M tokens`,
      candidatesConsidered: candidates.length,
      finalScore: best.costPer1MTokens === 0 ? 1.0 : 1 / best.costPer1MTokens,
    };
  }
}

// ── LatencyStrategy: prioritize low latency + reliability ───────────────────

class LatencyStrategyImpl implements RouterStrategy {
  readonly name = "latency";
  readonly description = "Prioritizes lowest p95 latency with reliability weighting";

  select(pool: ProviderCandidate[], context: RoutingContext): RoutingDecision {
    const healthy = pool.filter((c) => c.circuitBreakerState !== "OPEN");
    const candidates = healthy.length > 0 ? healthy : pool;
    const sorted = [...candidates].sort((a, b) => {
      const aPenalty = a.errorRate * 1000;
      const bPenalty = b.errorRate * 1000;
      return a.p95LatencyMs + aPenalty - (b.p95LatencyMs + bPenalty);
    });
    const best = sorted[0];
    if (!best) throw new Error("[LatencyStrategy] No candidates available");

    const latencyScore = best.p95LatencyMs > 0 ? Math.max(0.001, 10_000 / best.p95LatencyMs) : 1;
    const reliability = Math.max(0, 1 - best.errorRate);
    const finalScore = latencyScore * 0.7 + reliability * 0.3;

    return {
      provider: best.provider,
      model: best.model,
      strategy: this.name,
      reason: `LatencyStrategy: p95=${best.p95LatencyMs}ms, errorRate=${(best.errorRate * 100).toFixed(2)}%`,
      candidatesConsidered: candidates.length,
      finalScore,
    };
  }
}

// ── SLAStrategy: favor targets that meet latency/error/cost SLOs ───────────

const DEFAULT_SLA_TARGET_P95_MS = 2_000;
const DEFAULT_SLA_MAX_ERROR_RATE = 0.05;

function toPositiveFinite(value: unknown): number | undefined {
  const numericValue = Number(value);
  return Number.isFinite(numericValue) && numericValue > 0 ? numericValue : undefined;
}

function toFiniteRate(value: unknown): number | undefined {
  const numericValue = Number(value);
  return Number.isFinite(numericValue) && numericValue >= 0 ? Math.min(1, numericValue) : undefined;
}

function inverseNormalized(value: number, maxValue: number): number {
  if (!Number.isFinite(value) || value < 0) return 0;
  if (!Number.isFinite(maxValue) || maxValue <= 0) return 1;
  return clamp01(1 - value / maxValue);
}

function scoreAtOrBelowThreshold(value: number, threshold: number): number {
  // A zero threshold is an intentional zero-tolerance policy.
  if (threshold <= 0) return value === 0 ? 1 : 0;
  return clamp01(threshold / Math.max(value, 0.000_001));
}

function getHealthScore(candidate: ProviderCandidate): number {
  if (candidate.circuitBreakerState === "CLOSED") return 1;
  if (candidate.circuitBreakerState === "HALF_OPEN") return 0.5;
  return 0;
}

function getSlaViolationScore(candidate: ProviderCandidate, policy: Required<SlaRoutingPolicy>) {
  let violation = candidate.circuitBreakerState === "OPEN" ? 1 : 0;

  if (candidate.p95LatencyMs > policy.targetP95Ms) {
    violation += (candidate.p95LatencyMs - policy.targetP95Ms) / policy.targetP95Ms;
  }

  if (candidate.errorRate > policy.maxErrorRate) {
    violation +=
      policy.maxErrorRate > 0
        ? (candidate.errorRate - policy.maxErrorRate) / policy.maxErrorRate
        : candidate.errorRate;
  }

  if (policy.maxCostPer1MTokens > 0 && candidate.costPer1MTokens > policy.maxCostPer1MTokens) {
    violation +=
      (candidate.costPer1MTokens - policy.maxCostPer1MTokens) / policy.maxCostPer1MTokens;
  }

  return violation;
}

class SLAStrategyImpl implements RouterStrategy {
  readonly name = "sla-aware";
  readonly description =
    "Selects the provider most likely to satisfy latency, error-rate, and cost SLOs";

  select(pool: ProviderCandidate[], context: RoutingContext): RoutingDecision {
    const healthy = pool.filter((c) => c.circuitBreakerState !== "OPEN");
    const candidates = healthy.length > 0 ? healthy : pool;
    if (candidates.length === 0) throw new Error("[SLAStrategy] No candidates available");

    const maxCost = Math.max(...candidates.map((c) => c.costPer1MTokens), 0.001);
    const maxStdDev = Math.max(...candidates.map((c) => c.latencyStdDev), 0.001);
    const policy: Required<SlaRoutingPolicy> = {
      targetP95Ms: toPositiveFinite(context.sla?.targetP95Ms) ?? DEFAULT_SLA_TARGET_P95_MS,
      maxErrorRate: toFiniteRate(context.sla?.maxErrorRate) ?? DEFAULT_SLA_MAX_ERROR_RATE,
      maxCostPer1MTokens: toPositiveFinite(context.sla?.maxCostPer1MTokens) ?? 0,
      hardConstraints: context.sla?.hardConstraints === true,
    };

    const scored = candidates
      .map((candidate) => {
        const latencyScore = scoreAtOrBelowThreshold(candidate.p95LatencyMs, policy.targetP95Ms);
        const errorScore = scoreAtOrBelowThreshold(candidate.errorRate, policy.maxErrorRate);
        const costScore =
          policy.maxCostPer1MTokens > 0
            ? scoreAtOrBelowThreshold(candidate.costPer1MTokens, policy.maxCostPer1MTokens)
            : inverseNormalized(candidate.costPer1MTokens, maxCost);
        const stabilityScore = inverseNormalized(candidate.latencyStdDev, maxStdDev);
        const healthScore = getHealthScore(candidate);
        const violationScore = getSlaViolationScore(candidate, policy);

        return {
          candidate,
          violationScore,
          score:
            latencyScore * 0.35 +
            errorScore * 0.35 +
            healthScore * 0.15 +
            costScore * 0.1 +
            stabilityScore * 0.05,
        };
      })
      .sort((a, b) => {
        if (policy.hardConstraints) {
          return a.violationScore - b.violationScore || b.score - a.score;
        }
        return b.score - a.score;
      });

    const best = scored[0];
    if (!best) throw new Error("[SLAStrategy] No candidates available after scoring");

    const anyCompliant = scored.some((entry) => entry.violationScore === 0);
    const fallbackNote = !anyCompliant ? "; no candidate met all SLA constraints" : "";
    return {
      provider: best.candidate.provider,
      model: best.candidate.model,
      strategy: this.name,
      reason: `SLAStrategy: p95=${best.candidate.p95LatencyMs}ms/${policy.targetP95Ms}ms, errorRate=${(best.candidate.errorRate * 100).toFixed(2)}%/${(policy.maxErrorRate * 100).toFixed(2)}%, cost=$${best.candidate.costPer1MTokens.toFixed(3)}/1M${fallbackNote}`,
      candidatesConsidered: candidates.length,
      finalScore: best.score,
    };
  }
}

// ── LKGPStrategy: tries last known good provider first ───────────────────────

class LKGPStrategyImpl implements RouterStrategy {
  readonly name = "lkgp";
  readonly description = "Tries last known good provider first, then falls back to rules";

  select(pool: ProviderCandidate[], context: RoutingContext): RoutingDecision {
    if (context.lkgpEnabled === false) {
      return getStrategy("rules").select(pool, context);
    }

    if (context.lastKnownGoodProvider) {
      const candidates = pool.filter(
        (c) => c.provider === context.lastKnownGoodProvider && c.circuitBreakerState !== "OPEN"
      );
      if (candidates.length > 0) {
        const best = candidates[0];
        return {
          provider: best.provider,
          model: best.model,
          strategy: this.name,
          reason: `LKGP: using last known good provider ${best.provider}`,
          candidatesConsidered: 1,
          finalScore: 1.0,
        };
      }
    }

    // Fallback to rules strategy
    return getStrategy("rules").select(pool, context);
  }
}

// ── Registry ──────────────────────────────────────────────────────────────────

const strategyRegistry = new Map<string, RouterStrategy>();

const rulesStrategy = new RulesStrategyImpl();
const costStrategy = new CostStrategyImpl();
const latencyStrategy = new LatencyStrategyImpl();
const slaStrategy = new SLAStrategyImpl();
const lkgpStrategy = new LKGPStrategyImpl();

strategyRegistry.set("rules", rulesStrategy);
strategyRegistry.set("cost", costStrategy);
strategyRegistry.set("eco", costStrategy); // alias
strategyRegistry.set("latency", latencyStrategy);
strategyRegistry.set("fast", latencyStrategy); // alias
strategyRegistry.set("sla-aware", slaStrategy);
strategyRegistry.set("sla", slaStrategy); // alias
strategyRegistry.set("lkgp", lkgpStrategy);

export function getStrategy(name: string): RouterStrategy {
  const strategy = strategyRegistry.get(name);
  if (!strategy) {
    console.warn(`[RouterStrategy] Strategy '${name}' not found, falling back to 'rules'`);
    return rulesStrategy;
  }
  return strategy;
}

export function registerStrategy(name: string, strategy: RouterStrategy): void {
  if (strategyRegistry.has(name)) {
    console.warn(`[RouterStrategy] Overwriting strategy '${name}'`);
  }
  strategyRegistry.set(name, strategy);
}

export function listStrategies(): Array<{ name: string; description: string }> {
  return [...strategyRegistry.entries()].map(([name, s]) => ({ name, description: s.description }));
}

export function selectWithStrategy(
  pool: ProviderCandidate[],
  context: RoutingContext,
  strategyName = "rules"
): RoutingDecision {
  return getStrategy(strategyName).select(pool, context);
}