// kafka — 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"
	"net/http"
	"sync"
	"time"

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

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

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

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

	httpClient := &http.Client{Timeout: timeout}

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

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

// Search executes the request against local engines (in parallel) and
// optionally the upstream instance for unported engines.
//
// Individual engine failures are reported as unresponsive_engines rather
// than aborting the entire search.
//
// If a Valkey cache is configured and contains a cached response for this
// request, the cached result is returned without hitting any engines.
func (s *Service) Search(ctx context.Context, req SearchRequest) (SearchResponse, error) {
	// Check cache first.
	if s.cache != nil {
		cacheKey := cache.Key(req)
		if cached, hit := s.cache.Get(ctx, cacheKey); hit {
			return cached, nil
		}
	}

	merged, err := s.executeSearch(ctx, req)
	if err != nil {
		return SearchResponse{}, err
	}

	// Store in cache.
	if s.cache != nil {
		cacheKey := cache.Key(req)
		s.cache.Set(ctx, cacheKey, merged)
	}

	return merged, nil
}

// executeSearch runs the actual engine queries and merges results.
func (s *Service) executeSearch(ctx context.Context, req SearchRequest) (SearchResponse, error) {
	localEngineNames, upstreamEngineNames, _ := s.planner.Plan(req)

	// Run all local engines concurrently.
	type engineResult struct {
		name string
		resp contracts.SearchResponse
		err  error
	}

	localResults := make([]engineResult, 0, len(localEngineNames))

	var wg sync.WaitGroup
	var mu sync.Mutex

	for _, name := range localEngineNames {
		eng, ok := s.localEngines[name]
		if !ok {
			mu.Lock()
			localResults = append(localResults, engineResult{
				name: name,
				resp: unresponsiveResponse(req.Query, name, "engine_not_registered"),
			})
			mu.Unlock()
			continue
		}

		wg.Add(1)
		go func(name string, eng engines.Engine) {
			defer wg.Done()

			r, err := eng.Search(ctx, req)

			mu.Lock()
			defer mu.Unlock()

			if err != nil {
				localResults = append(localResults, engineResult{
					name: name,
					resp: unresponsiveResponse(req.Query, name, err.Error()),
				})
				return
			}
			localResults = append(localResults, engineResult{name: name, resp: r})
		}(name, eng)
	}

	wg.Wait()

	// Collect successful responses and determine upstream fallbacks.
	responses := make([]contracts.SearchResponse, 0, len(localResults)+1)
	upstreamSet := map[string]bool{}
	for _, e := range upstreamEngineNames {
		upstreamSet[e] = true
	}

	for _, lr := range localResults {
		responses = append(responses, lr.resp)

		// If a local engine returned nothing (e.g. qwant anti-bot), fall back
		// to upstream if available.
		if shouldFallbackToUpstream(lr.name, lr.resp) && !upstreamSet[lr.name] {
			upstreamEngineNames = append(upstreamEngineNames, lr.name)
			upstreamSet[lr.name] = true
		}
	}

	// Upstream proxy for unported (or fallback) engines.
	if s.upstreamClient != nil && len(upstreamEngineNames) > 0 {
		r, err := s.upstreamClient.SearchJSON(ctx, req, upstreamEngineNames)
		if err != nil {
			// Upstream failure is treated as a single unresponsive engine entry.
			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
}