kafka/internal/engines/crossref.go

// 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"
)

type CrossrefEngine struct {
	client *http.Client
}

func (e *CrossrefEngine) Name() string { return "crossref" }

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

	offset := 20 * (req.Pageno - 1)
	args := url.Values{}
	args.Set("query", q)
	args.Set("offset", fmt.Sprintf("%d", offset))

	endpoint := "https://api.crossref.org/works?" + args.Encode()
	httpReq, err := http.NewRequestWithContext(ctx, http.MethodGet, endpoint, nil)
	if err != nil {
		return contracts.SearchResponse{}, err
	}

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

	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
		body, _ := io.ReadAll(io.LimitReader(resp.Body, 16*1024))
		return contracts.SearchResponse{}, fmt.Errorf("crossref upstream error: status=%d body=%q", resp.StatusCode, string(body))
	}

	var api struct {
		Message struct {
			Items []crossrefItem `json:"items"`
		} `json:"message"`
	}

	if err := json.NewDecoder(resp.Body).Decode(&api); err != nil {
		return contracts.SearchResponse{}, err
	}

	results := make([]contracts.MainResult, 0, len(api.Message.Items))
	for _, item := range api.Message.Items {
		title := ""
		if len(item.Title) > 0 {
			title = strings.TrimSpace(item.Title[0])
		}

		content := strings.TrimSpace(item.Abstract)

		urlStr := strings.TrimSpace(item.URL)
		if urlStr == "" {
			urlStr = strings.TrimSpace(item.DOI)
		}

		pub := parseCrossrefDateParts(item.Published.DateParts)

		urlPtr := urlStr
		results = append(results, contracts.MainResult{
			Template: "default.html",
			Title:    title,
			Content:  content,
			URL:      &urlPtr,
			Pubdate:  pub,
			Engine:   "crossref",
			Score:    0,
			Category: "science",
			Priority: "",
			Positions: nil,
			Engines:   []string{"crossref"},
		})
	}

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

type crossrefItem struct {
	Type      string `json:"type"`
	Title     []string `json:"title"`
	URL       string   `json:"URL"`
	DOI       string   `json:"DOI"`
	Abstract  string   `json:"abstract"`
	Page      string   `json:"page"`
	Publisher string   `json:"publisher"`
	Subject   []string `json:"subject"`
	Published crossrefPublished `json:"published"`
}

type crossrefPublished struct {
	DateParts [][]int `json:"date-parts"`
}

func parseCrossrefDateParts(parts [][]int) *string {
	if len(parts) == 0 || len(parts[0]) == 0 {
		return nil
	}

	dp := parts[0]
	year := dp[0]
	month := 1
	day := 1
	if len(dp) >= 2 {
		month = dp[1]
	}
	if len(dp) >= 3 {
		day = dp[2]
	}

	t := time.Date(year, time.Month(month), day, 0, 0, 0, 0, time.UTC)
	formatted := t.Format("2006-01-02 00:00:00+0000")
	return &formatted
}