// 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 engines

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"strings"
	"time"

	"github.com/metamorphosis-dev/kafka/internal/contracts"
)

const stackOverflowAPIBase = "https://api.stackexchange.com/2.3"

// StackOverflowEngine searches Stack Overflow via the public API.
// No API key is required, but providing one via STACKOVERFLOW_KEY env var
// or config raises the rate limit from 300 to 10,000 requests/day.
type StackOverflowEngine struct {
	client *http.Client
	apiKey string
}

func (e *StackOverflowEngine) Name() string { return "stackoverflow" }

func (e *StackOverflowEngine) Search(ctx context.Context, req contracts.SearchRequest) (contracts.SearchResponse, error) {
	if e == nil || e.client == nil {
		return contracts.SearchResponse{}, errors.New("stackoverflow engine not initialized")
	}
	q := strings.TrimSpace(req.Query)
	if q == "" {
		return contracts.SearchResponse{Query: req.Query}, nil
	}

	page := req.Pageno
	if page < 1 {
		page = 1
	}
	args := url.Values{}
	args.Set("order", "desc")
	args.Set("sort", "relevance")
	args.Set("site", "stackoverflow")
	args.Set("page", fmt.Sprintf("%d", page))
	args.Set("pagesize", "20")
	args.Set("filter", "!9_bDDxJY5")
	if e.apiKey != "" {
		args.Set("key", e.apiKey)
	}

	endpoint := stackOverflowAPIBase + "/search/advanced?" + args.Encode() + "&q=" + url.QueryEscape(q)

	httpReq, err := http.NewRequestWithContext(ctx, http.MethodGet, endpoint, nil)
	if err != nil {
		return contracts.SearchResponse{}, err
	}
	httpReq.Header.Set("User-Agent", "kafka/0.1 (compatible; +https://git.ashisgreat.xyz/penal-colony/kafka)")
	httpReq.Header.Set("Accept", "application/json")

	resp, err := e.client.Do(httpReq)
	if err != nil {
		return contracts.SearchResponse{}, err
	}
	defer resp.Body.Close()

	if resp.StatusCode == http.StatusTooManyRequests {
		return contracts.SearchResponse{
			Query:               req.Query,
			UnresponsiveEngines: [][2]string{{"stackoverflow", "rate_limited"}},
			Results:             []contracts.MainResult{},
			Answers:             []map[string]any{},
			Corrections:         []string{},
			Infoboxes:           []map[string]any{},
			Suggestions:         []string{},
		}, nil
	}

	if resp.StatusCode != http.StatusOK {
		io.Copy(io.Discard, io.LimitReader(resp.Body, 4*1024))
		return contracts.SearchResponse{}, fmt.Errorf("stackoverflow upstream error: status %d", resp.StatusCode)
	}

	body, err := io.ReadAll(io.LimitReader(resp.Body, 2*1024*1024))
	if err != nil {
		return contracts.SearchResponse{}, err
	}

	return parseStackOverflow(body, req.Query)
}

// soQuestion represents a question item from the Stack Exchange API.
type soQuestion struct {
	QuestionID    int           `json:"question_id"`
	Title         string        `json:"title"`
	Link          string        `json:"link"`
	Body          string        `json:"body"`
	Score         int           `json:"score"`
	AnswerCount   int           `json:"answer_count"`
	ViewCount     int           `json:"view_count"`
	Tags          []string      `json:"tags"`
	CreationDate  float64       `json:"creation_date"`
	Owner         *soOwner      `json:"owner"`
	AcceptedAnswerID *int       `json:"accepted_answer_id"`
	IsAnswered    bool          `json:"is_answered"`
}

type soOwner struct {
	Reputation int    `json:"reputation"`
	DisplayName string `json:"display_name"`
}

type soResponse struct {
	Items          []soQuestion `json:"items"`
	HasMore        bool         `json:"has_more"`
	QuotaRemaining int          `json:"quota_remaining"`
	QuotaMax       int          `json:"quota_max"`
}

func parseStackOverflow(body []byte, query string) (contracts.SearchResponse, error) {
	var resp soResponse
	if err := json.Unmarshal(body, &resp); err != nil {
		return contracts.SearchResponse{}, fmt.Errorf("stackoverflow JSON parse error: %w", err)
	}

	results := make([]contracts.MainResult, 0, len(resp.Items))
	for _, q := range resp.Items {
		if q.Link == "" {
			continue
		}

		// Strip HTML from the body excerpt.
		snippet := truncate(stripHTML(q.Body), 300)

		// Build a content string with useful metadata.
		content := snippet
		if q.Score > 0 {
			content = fmt.Sprintf("Score: %d", q.Score)
			if q.AnswerCount > 0 {
				content += fmt.Sprintf(" · %d answers", q.AnswerCount)
			}
			if q.ViewCount > 0 {
				content += fmt.Sprintf(" · %s views", formatCount(q.ViewCount))
			}
			if snippet != "" {
				content += "\n" + snippet
			}
		}

		// Append tags as category hint.
		if len(q.Tags) > 0 {
			displayTags := q.Tags
			if len(displayTags) > 5 {
				displayTags = displayTags[:5]
			}
			content += "\n[" + strings.Join(displayTags, "] [") + "]"
		}

		linkPtr := q.Link
		results = append(results, contracts.MainResult{
			Template:  "default",
			Title:     q.Title,
			Content:   content,
			URL:       &linkPtr,
			Engine:    "stackoverflow",
			Score:     float64(q.Score),
			Category:  "it",
			Engines:   []string{"stackoverflow"},
		})
	}

	return contracts.SearchResponse{
		Query:               query,
		NumberOfResults:    len(results),
		Results:             results,
		Answers:             []map[string]any{},
		Corrections:         []string{},
		Infoboxes:           []map[string]any{},
		Suggestions:         []string{},
		UnresponsiveEngines: [][2]string{},
	}, nil
}

// formatCount formats large numbers compactly (1.2k, 3.4M).
func formatCount(n int) string {
	if n >= 1_000_000 {
		return fmt.Sprintf("%.1fM", float64(n)/1_000_000)
	}
	if n >= 1_000 {
		return fmt.Sprintf("%.1fk", float64(n)/1_000)
	}
	return fmt.Sprintf("%d", n)
}

// truncate cuts a string to at most maxLen characters, appending "…" if truncated.
func truncate(s string, maxLen int) string {
	if len(s) <= maxLen {
		return s
	}
	return s[:maxLen] + "…"
}

// stackOverflowCreatedAt returns a time.Time from a Unix timestamp.
// Kept as a helper for potential future pubdate use.
func stackOverflowCreatedAt(unix float64) *string {
	t := time.Unix(int64(unix), 0).UTC()
	s := t.Format("2006-01-02")
	return &s
}