// Package research serves the shared, public paper corpus. Account data and
// private collections do not belong to these anonymous read-only endpoints.
package research

import (
	"context"
	"database/sql"
	"encoding/json"
	"errors"
	"net/http"
	"net/url"
	"regexp"
	"strconv"
	"strings"
	"time"
	"unicode/utf8"
)

type Source struct {
	Label      string `json:"label"`
	URL        string `json:"url"`
	ExternalID string `json:"externalId"`
}

type Publication struct {
	Venue       string `json:"venue"`
	Year        int    `json:"year"`
	Track       string `json:"track"`
	Status      string `json:"status"`
	EvidenceURL string `json:"evidenceUrl,omitempty"`
}

type Paper struct {
	ID           string        `json:"id"`
	Title        string        `json:"title"`
	Abstract     *string       `json:"abstract"`
	Authors      []string      `json:"authors"`
	PublishedAt  *string       `json:"publishedAt"`
	Topics       []string      `json:"topics"`
	Sources      []Source      `json:"sources"`
	Publications []Publication `json:"publications,omitempty"`
	DOI          *string       `json:"doi,omitempty"`
	Journal      *string       `json:"journal,omitempty"`
	Comments     *string       `json:"comments,omitempty"`
}

type Filters struct {
	Query    string
	SearchIn string
	Topic    string
	Sort     string
	Cursor   string
	PageSize int
}

type Page struct {
	Items          []Paper  `json:"items"`
	NextCursor     string   `json:"nextCursor"`
	PreviousCursor string   `json:"previousCursor"`
	Topics         []string `json:"topics"`
	PageSize       int      `json:"pageSize"`
}

// Repository implementations must use a read-only transaction for each List,
// so its metadata and page describe the same database snapshot.
type Repository interface {
	List(context.Context, Filters) (Page, error)
	Get(context.Context, int64) (Paper, error)
}

type service struct {
	repository Repository
	projects   map[string]string
	slots      chan struct{}
	timeout    time.Duration
}

// Register adds only GET endpoints; no credentials or database errors are sent
// to anonymous callers. A nil repository leaves the existing server unchanged.
func Register(mux *http.ServeMux, repository Repository, projects map[string]string) {
	if repository == nil {
		return
	}
	s := &service{repository: repository, projects: projects, slots: make(chan struct{}, 2), timeout: 8 * time.Second}
	mux.HandleFunc("GET /projects/{project}/research/papers", s.serve)
	mux.HandleFunc("GET /projects/{project}/research/papers/{id}", s.serve)
}

func reply(w http.ResponseWriter, status int, value any) {
	w.Header().Set("Content-Type", "application/json")
	w.Header().Set("Cache-Control", "no-store")
	w.Header().Set("X-Content-Type-Options", "nosniff")
	w.WriteHeader(status)
	_ = json.NewEncoder(w).Encode(value)
}

func reject(w http.ResponseWriter, status int, code string) {
	reply(w, status, map[string]string{"error": code})
}

var topicPattern = regexp.MustCompile(`^[A-Za-z][A-Za-z0-9.-]{0,79}$`)
var positiveInteger = regexp.MustCompile(`^[1-9][0-9]*$`)

func parseFilters(raw string) (Filters, error) {
	f := Filters{SearchIn: "text", Sort: "recent", PageSize: 8}
	values, err := url.ParseQuery(raw)
	if err != nil {
		return f, err
	}
	invalid := errors.New("invalid query")
	for key, value := range values {
		if len(value) != 1 {
			return f, invalid
		}
		switch key {
		case "q":
			if !utf8.ValidString(value[0]) || utf8.RuneCountInString(value[0]) > 200 || len(strings.Fields(value[0])) > 20 || strings.ContainsRune(value[0], 0) {
				return f, invalid
			}
			f.Query = strings.TrimSpace(value[0])
		case "searchIn":
			if value[0] != "text" && value[0] != "author" {
				return f, invalid
			}
			f.SearchIn = value[0]
		case "topic":
			if value[0] != "" && !topicPattern.MatchString(value[0]) {
				return f, invalid
			}
			f.Topic = value[0]
		case "sort":
			if value[0] != "recent" && value[0] != "title" {
				return f, invalid
			}
			f.Sort = value[0]
		case "cursor":
			f.Cursor = value[0]
		case "pageSize":
			if !positiveInteger.MatchString(value[0]) {
				return f, invalid
			}
			n, err := strconv.Atoi(value[0])
			if err != nil || n > 50 {
				return f, invalid
			}
			f.PageSize = n
		default:
			return f, invalid
		}
	}
	_, err = decodeCursor(f)
	return f, err
}

func (s *service) serve(w http.ResponseWriter, r *http.Request) {
	if _, exists := s.projects[r.PathValue("project")]; !exists {
		reject(w, 404, "not_found")
		return
	}
	var filters Filters
	var id int64
	var err error
	if value := r.PathValue("id"); value != "" {
		if !positiveInteger.MatchString(value) || r.URL.RawQuery != "" {
			reject(w, 400, "invalid_request")
			return
		}
		id, err = strconv.ParseInt(value, 10, 64)
	} else {
		filters, err = parseFilters(r.URL.RawQuery)
	}
	if err != nil {
		reject(w, 400, "invalid_request")
		return
	}
	select {
	case s.slots <- struct{}{}:
		defer func() { <-s.slots }()
	default:
		w.Header().Set("Retry-After", "1")
		reject(w, 429, "research_busy")
		return
	}
	ctx, cancel := context.WithTimeout(r.Context(), s.timeout)
	defer cancel()
	var result any
	if id != 0 {
		result, err = s.repository.Get(ctx, id)
	} else {
		result, err = s.repository.List(ctx, filters)
	}
	if err == nil {
		reply(w, 200, result)
		return
	}
	var state interface{ SQLState() string }
	switch {
	case errors.Is(err, sql.ErrNoRows):
		reject(w, 404, "not_found")
	case errors.Is(ctx.Err(), context.DeadlineExceeded), errors.Is(err, context.DeadlineExceeded), errors.As(err, &state) && state.SQLState() == "57014":
		reject(w, 504, "research_timeout")
	default:
		reject(w, 503, "research_unavailable")
	}
}