// SEMANTIC SCHOLAR HANDLER
//
// Uses S2's Graph API to fetch paper metadata via DOI.
// 
// STRATEGY:
// - requires valid DOI in URL or DOI.org redirect
// - batch API for efficiency (up to 500 papers per request)
// - positional matching: response[i] maps to URLs[i]
// - rate limited to 100ms per request (configurable with API key)
//
// AUTH:
// - S2_API_KEY environment variable increases rate limits
// - Without key: public limits apply
package main

import (
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"strings"
)

const (
	semScholarPaperDOIFmtTitle    = "https://api.semanticscholar.org/graph/v1/paper/DOI:%s?fields=title"
	semScholarPaperDOIFmtFull     = "https://api.semanticscholar.org/graph/v1/paper/DOI:%s?fields=title,abstract"
	semScholarBatchURLTitle       = "https://api.semanticscholar.org/graph/v1/paper/batch?fields=title"
	semScholarBatchURLFull        = "https://api.semanticscholar.org/graph/v1/paper/batch?fields=title,abstract"
)

// escapeDOI URL-encodes a DOI for safe use in API endpoints.
// DOIs contain forward slashes which must be escaped for the URL path.
// Example: "10.1234/abcd5678" -> "10.1234/abcd5678" (already safe in this case)
func escapeDOI(doi string) string {
	parts := strings.SplitN(doi, "/", 2)
	if len(parts) != 2 {
		return url.PathEscape(doi)
	}
	return url.PathEscape(parts[0]) + "/" + url.PathEscape(parts[1])
}

// S2BatchResponseItem represents a Semantic Scholar batch API response item
type S2BatchResponseItem struct {
	PaperID  string `json:"paperId"`
	Title    string `json:"title"`
	Abstract string `json:"abstract"`
}

// fetchSemanticScholar fetches content for a single DOI via Semantic Scholar.
func fetchSemanticScholar(ctx context.Context, config *Config, urlStr string) (*Article, error) {
	doi := getDOI(urlStr)
	if doi == "" {
		return nil, fmt.Errorf("Semantic Scholar: URL doesn't contain valid DOI: %s", urlStr)
	}

	// rate limit
	if err := config.HTTP.RateLimitS2(ctx); err != nil {
		return nil, err
	}

	escapedDOI := escapeDOI(doi)
	
	// choose the appropriate URL based on whether we need content
	var apiURL string
	if config.WithContent {
		apiURL = fmt.Sprintf(semScholarPaperDOIFmtFull, escapedDOI)
	} else {
		apiURL = fmt.Sprintf(semScholarPaperDOIFmtTitle, escapedDOI)
	}

	req, err := http.NewRequestWithContext(ctx, "GET", apiURL, nil)
	if err != nil {
		return nil, fmt.Errorf("failed to construct Semantic Scholar request: %w", err)
	}
	req.Header.Set("Accept", "application/json")
	if config.S2APIKey != "" {
		req.Header.Set("x-api-key", config.S2APIKey)
	}

	resp, err := config.HTTP.Do(req)
	if err != nil {
		return nil, fmt.Errorf("failed to make request to Semantic Scholar API: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("Semantic Scholar API returned non-200 status for DOI %s: %s", doi, resp.Status)
	}

	body, err := io.ReadAll(resp.Body)
	if err != nil {
		return nil, fmt.Errorf("failed to read Semantic Scholar API response body: %w", err)
	}

	var s2 struct {
		Title    string `json:"title"`
		Abstract string `json:"abstract"`
	}
	if err := json.Unmarshal(body, &s2); err != nil {
		return nil, fmt.Errorf("failed to unmarshal Semantic Scholar JSON for DOI %s: %w", doi, err)
	}

	title := normalizeSpace(s2.Title)
	content := normalizeSpace(s2.Abstract)

	// del content if not requested
	if !config.WithContent {
		content = ""
	}

	if title == "" {
		return nil, fmt.Errorf("no title found for DOI %s", doi)
	}

	return &Article{
		Title:   title,
		Content: content,
		URL:     urlStr,
	}, nil
}

// fetchSemanticScholarBatch fetches a batch of papers from the S2 API.
func fetchSemanticScholarBatch(ctx context.Context, config *Config, urls []string) ([]*Article, error) {
	if len(urls) == 0 {
		return nil, nil
	}

	// rate limit
	if err := config.HTTP.RateLimitS2(ctx); err != nil {
		return nil, err
	}

	// extract DOIs from URLs, maintaining order for pos matching
	validURLs := make([]string, 0, len(urls))
	s2IDs := make([]string, 0, len(urls))

	for _, urlStr := range urls {
		doi := getDOI(urlStr)
		if doi != "" {
			validURLs = append(validURLs, urlStr)
			s2IDs = append(s2IDs, "DOI:"+doi)
		}
	}

	if len(s2IDs) == 0 {
		return nil, nil
	}

	requestBody, err := json.Marshal(map[string][]string{"ids": s2IDs})
	if err != nil {
		return nil, fmt.Errorf("failed to marshal S2 batch request body: %w", err)
	}

	// choose the appropriate URL based on whether we need content
	var batchURL string
	if config.WithContent {
		batchURL = semScholarBatchURLFull
	} else {
		batchURL = semScholarBatchURLTitle
	}

	req, err := http.NewRequestWithContext(ctx, "POST", batchURL, bytes.NewReader(requestBody))
	if err != nil {
		return nil, fmt.Errorf("failed to create S2 batch request: %w", err)
	}
	req.Header.Set("Content-Type", "application/json")
	if config.S2APIKey != "" {
		req.Header.Set("x-api-key", config.S2APIKey)
	}

	resp, err := config.HTTP.Do(req)
	if err != nil {
		return nil, fmt.Errorf("S2 batch request failed: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("S2 batch API returned non-200 status: %s", resp.Status)
	}

	var responseItems []*S2BatchResponseItem
	if err := json.NewDecoder(resp.Body).Decode(&responseItems); err != nil {
		return nil, fmt.Errorf("failed to decode S2 batch response: %w", err)
	}

	var articles []*Article
	// match responses positionally to input URLs
	for i, item := range responseItems {
		if i >= len(validURLs) {
			break
		}
		if item == nil {
			continue
		}

		title := normalizeSpace(item.Title)
		if title != "" {
			content := normalizeSpace(item.Abstract)
			
			// skip content if not requested
			if !config.WithContent {
				content = ""
			}
			
			articles = append(articles, &Article{
				Title:   title,
				Content: content,
				URL:     validURLs[i],
			})
		}
	}

	return articles, nil
}