// samsa — a privacy-respecting metasearch engine
// Copyright (C) 2026-present metamorphosis-dev
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program. If not, see <https://www.gnu.org/licenses/>.

package search

import (
	"context"
	"encoding/json"
	"fmt"
	"sync"
	"time"

	"github.com/metamorphosis-dev/samsa/internal/cache"
	"github.com/metamorphosis-dev/samsa/internal/config"
	"github.com/metamorphosis-dev/samsa/internal/contracts"
	"github.com/metamorphosis-dev/samsa/internal/engines"
	"github.com/metamorphosis-dev/samsa/internal/httpclient"
	"github.com/metamorphosis-dev/samsa/internal/upstream"
)

type ServiceConfig struct {
	UpstreamURL       string
	HTTPTimeout       time.Duration
	Cache             *cache.Cache
	CacheTTLOverrides map[string]time.Duration
	EnginesConfig     *config.Config
}

type Service struct {
	upstreamClient *upstream.Client
	planner        *engines.Planner
	localEngines   map[string]engines.Engine
	engineCache    *cache.EngineCache
}

func NewService(cfg ServiceConfig) *Service {
	timeout := cfg.HTTPTimeout
	if timeout <= 0 {
		timeout = 10 * time.Second
	}

	httpClient := httpclient.NewClient(timeout)

	var up *upstream.Client
	if cfg.UpstreamURL != "" {
		c, err := upstream.NewClient(cfg.UpstreamURL, timeout)
		if err == nil {
			up = c
		}
	}

	var engineCache *cache.EngineCache
	if cfg.Cache != nil {
		engineCache = cache.NewEngineCache(cfg.Cache, cfg.CacheTTLOverrides)
	}

	return &Service{
		upstreamClient: up,
		planner:        engines.NewPlannerFromEnv(),
		localEngines:   engines.NewDefaultPortedEngines(httpClient, cfg.EnginesConfig),
		engineCache:    engineCache,
	}
}

// derefString returns the string value of a *string, or "" if nil.
func derefString(s *string) string {
	if s == nil {
		return ""
	}
	return *s
}

// Search executes the request against local engines (in parallel) and
// optionally the upstream instance for unported engines.
func (s *Service) Search(ctx context.Context, req SearchRequest) (SearchResponse, error) {
	queryHash := cache.QueryHash(
		req.Query,
		int(req.Pageno),
		int(req.Safesearch),
		req.Language,
		derefString(req.TimeRange),
	)

	localEngineNames, upstreamEngineNames, _ := s.planner.Plan(req)

	// Phase 1: Parallel cache lookups — classify each engine as fresh/stale/miss
	type cacheResult struct {
		engine       string
		cached       cache.CachedEngineResponse
		hit          bool
		fresh        *contracts.SearchResponse // nil if no fresh response
		fetchErr     error
		unmarshalErr bool // true if hit but unmarshal failed (treat as miss)
	}

	cacheResults := make([]cacheResult, len(localEngineNames))

	var lookupWg sync.WaitGroup
	for i, name := range localEngineNames {
		lookupWg.Add(1)
		go func(i int, name string) {
			defer lookupWg.Done()

			result := cacheResult{engine: name}

			if s.engineCache != nil {
				cached, ok := s.engineCache.Get(ctx, name, queryHash)
				if ok {
					result.hit = true
					result.cached = cached
					if !s.engineCache.IsStale(cached, name) {
						// Fresh cache hit — deserialize and use directly
						var resp contracts.SearchResponse
						if err := json.Unmarshal(cached.Response, &resp); err == nil {
							result.fresh = &resp
						} else {
							// Unmarshal failed — treat as cache miss (will fetch fresh synchronously)
							result.unmarshalErr = true
							result.hit = false // treat as miss
						}
					}
					// If stale: result.fresh stays zero, result.cached has stale data
				}
			}

			cacheResults[i] = result
		}(i, name)
	}
	lookupWg.Wait()

	// Phase 2: Fetch fresh for misses and stale entries
	var fetchWg sync.WaitGroup
	for i, name := range localEngineNames {
		cr := cacheResults[i]

		// Fresh hit — nothing to do in phase 2
		if cr.hit && cr.fresh != nil {
			continue
		}

		// Stale hit — return stale immediately, refresh in background
		if cr.hit && len(cr.cached.Response) > 0 && s.engineCache != nil && s.engineCache.IsStale(cr.cached, name) {
			fetchWg.Add(1)
			go func(name string) {
				defer fetchWg.Done()
				eng, ok := s.localEngines[name]
				if !ok {
					return
				}
				freshResp, err := eng.Search(ctx, req)
				if err != nil {
					s.engineCache.Logger().Debug("background refresh failed", "engine", name, "error", err)
					return
				}
				s.engineCache.Set(ctx, name, queryHash, freshResp)
			}(name)
			continue
		}

		// Cache miss — fetch fresh synchronously
		if !cr.hit {
			fetchWg.Add(1)
			go func(i int, name string) {
				defer fetchWg.Done()

				eng, ok := s.localEngines[name]
				if !ok {
					cacheResults[i] = cacheResult{
						engine:   name,
						fetchErr: fmt.Errorf("engine not registered: %s", name),
					}
					return
				}

				freshResp, err := eng.Search(ctx, req)
				if err != nil {
					cacheResults[i] = cacheResult{
						engine:   name,
						fetchErr: err,
					}
					return
				}

				// Cache the fresh response
				if s.engineCache != nil {
					s.engineCache.Set(ctx, name, queryHash, freshResp)
				}

				cacheResults[i] = cacheResult{
					engine: name,
					fresh:  &freshResp,
					hit:    false,
				}
			}(i, name)
		}
	}
	fetchWg.Wait()

	// Phase 3: Collect responses for merge
	responses := make([]contracts.SearchResponse, 0, len(cacheResults))

	for _, cr := range cacheResults {
		if cr.fetchErr != nil {
			responses = append(responses, unresponsiveResponse(req.Query, cr.engine, cr.fetchErr.Error()))
			continue
		}
		// Use fresh data if available (fresh hit or freshly fetched), otherwise use stale cached
		if cr.fresh != nil {
			responses = append(responses, *cr.fresh)
		} else if cr.hit && len(cr.cached.Response) > 0 {
			var resp contracts.SearchResponse
			if err := json.Unmarshal(cr.cached.Response, &resp); err == nil {
				responses = append(responses, resp)
			}
		}
	}

	// Upstream proxy for unported (or fallback) engines.
	// ... rest of the existing code is UNCHANGED ...
	if s.upstreamClient != nil && len(upstreamEngineNames) > 0 {
		r, err := s.upstreamClient.SearchJSON(ctx, req, upstreamEngineNames)
		if err != nil {
			responses = append(responses, contracts.SearchResponse{
				Query:               req.Query,
				UnresponsiveEngines: [][2]string{{"upstream", err.Error()}},
			})
		} else {
			responses = append(responses, r)
		}
	}

	if len(responses) == 0 {
		return emptyResponse(req.Query), nil
	}

	merged := MergeResponses(responses)
	if merged.Query == "" {
		merged.Query = req.Query
	}
	return merged, nil
}

// unresponsiveResponse returns a zero-result response marking the engine as unresponsive.
func unresponsiveResponse(query, engine, reason string) contracts.SearchResponse {
	return contracts.SearchResponse{
		Query:               query,
		NumberOfResults:     0,
		Results:             []contracts.MainResult{},
		Answers:             []map[string]any{},
		Corrections:         []string{},
		Infoboxes:           []map[string]any{},
		Suggestions:         []string{},
		UnresponsiveEngines: [][2]string{{engine, reason}},
	}
}

// emptyResponse returns a valid empty response with stable empty slices.
func emptyResponse(query string) contracts.SearchResponse {
	return contracts.SearchResponse{
		Query:               query,
		NumberOfResults:     0,
		Results:             []contracts.MainResult{},
		Answers:             []map[string]any{},
		Corrections:         []string{},
		Infoboxes:           []map[string]any{},
		Suggestions:         []string{},
		UnresponsiveEngines: [][2]string{},
	}
}

func shouldFallbackToUpstream(engineName string, r contracts.SearchResponse) bool {
	if engineName != "qwant" {
		return false
	}
	return len(r.Results) == 0 && len(r.Answers) == 0 && len(r.Infoboxes) == 0
}