package research

import (
	"bytes"
	"crypto/sha256"
	"encoding/base64"
	"encoding/hex"
	"encoding/json"
	"errors"
	"io"
	"strconv"
	"strings"
)

// Cursors locate a public corpus row, not an account or authentication token.
// Binding to normalized filters prevents accidentally paging a different query.
type position struct {
	Version int    `json:"v"`
	ID      int64  `json:"id"`
	Key     string `json:"key"`
	Before  bool   `json:"before"`
	Filter  string `json:"filter"`
}

var errCursor = errors.New("invalid cursor")

func filterKey(f Filters) string {
	sort, search := f.Sort, f.SearchIn
	if sort == "" {
		sort = "recent"
	}
	if search == "" {
		search = "text"
	}
	data, _ := json.Marshal([]string{strings.TrimSpace(f.Query), f.Topic, sort, search, strconv.Itoa(f.PageSize)})
	sum := sha256.Sum256(data)
	return hex.EncodeToString(sum[:16])
}
func encodeCursor(f Filters, p position, before bool) string {
	p.Version, p.Before, p.Filter = 1, before, filterKey(f)
	if f.Sort != "title" {
		p.Key = ""
	}
	data, _ := json.Marshal(p)
	return base64.RawURLEncoding.EncodeToString(data)
}
func decodeCursor(f Filters) (position, error) {
	var p position
	if f.Cursor == "" {
		return p, nil
	}
	if len(f.Cursor) > 32768 {
		return p, errCursor
	}
	data, err := base64.RawURLEncoding.Strict().DecodeString(f.Cursor)
	if err != nil {
		return p, errCursor
	}
	decoder := json.NewDecoder(bytes.NewReader(data))
	decoder.DisallowUnknownFields()
	if decoder.Decode(&p) != nil || decoder.Decode(new(any)) != io.EOF || p.Version != 1 || p.ID <= 0 || p.Filter != filterKey(f) || strings.ContainsRune(p.Key, 0) || (f.Sort != "title" && p.Key != "") {
		return position{}, errCursor
	}
	return p, nil
}