package mock

import (
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
	"net/http"
	"net/url"
	"strconv"
	"strings"
	"time"

	"git.avsdev.uk/Go/job-search/internal/mock/findatender/ocds"
)

// Paging and request parameters of the release package endpoint.
const (
	// defaultLimit is the page size when the request sets no limit.
	defaultLimit = 100
	// maxLimit is the largest page size a request may ask for.
	maxLimit = 100
	// timeLayout is the documented layout of updatedFrom and updatedTo; see
	// parseTime for the others accepted.
	timeLayout = "2006-01-02T15:04:05"
	// BasePath is the path the endpoint is served at.
	BasePath = "/api/1.0/ocdsReleasePackages"
)

// server serves release packages from a Store; see NewHandler.
type server struct {
	store   *Store
	baseURL string
	// location is the zone for updatedFrom/updatedTo without an offset; nil
	// means UTC.
	location *time.Location
	opts     options
}

// NewHandler serves release packages from store. baseURL is the public base
// URL used in each package's uri and links.next. loc is the time zone that
// updatedFrom and updatedTo are read in when they carry no offset, as the
// documented yyyy-MM-ddTHH:mm:ss format doesn't (Europe/London for FTS).
// Stored release dates are UTC; comparisons are on absolute instants. It
// logs to slog.Default() unless given WithLogger.
func NewHandler(
	store *Store, baseURL string, location *time.Location, opts ...Option,
) http.HandlerFunc {
	handler := &server{
		store:    store,
		baseURL:  baseURL,
		location: location,
		opts:     applyOptions(opts),
	}
	return handler.handle
}

// encodeCursor makes the opaque cursor for a next link that resumes at store
// position i.
func encodeCursor(i int) string {
	return base64.RawURLEncoding.EncodeToString([]byte("idx:" + strconv.Itoa(i)))
}

// decodeCursor returns the release index in a cursor made by encodeCursor.
// Anything else, including a negative index, is an error.
func decodeCursor(cursor string) (int, error) {
	decoded, err := base64.RawURLEncoding.DecodeString(cursor)
	if err != nil || !strings.HasPrefix(string(decoded), "idx:") {
		return 0, fmt.Errorf("invalid cursor")
	}

	index, err := strconv.Atoi(strings.TrimPrefix(string(decoded), "idx:"))
	if err != nil || index < 0 {
		return 0, fmt.Errorf("invalid cursor")
	}

	return index, nil
}

func badRequest(w http.ResponseWriter, msg string) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(http.StatusBadRequest)
	body := map[string]string{"error": msg}
	json.NewEncoder(w).Encode(body) //nolint:errcheck,gosec // the client may have gone; nothing to do
}

// parseTime reads a query timestamp. Values without an offset (the documented
// yyyy-MM-ddTHH:mm:ss, or a plain date) are local time in location; values with
// an offset or Z (RFC 3339) mean exactly what they say.
func parseTime(value string, location *time.Location) (time.Time, error) {
	if location == nil {
		location = time.UTC
	}

	if parsed, err := time.Parse(time.RFC3339, value); err == nil {
		return parsed, nil
	}

	for _, layout := range []string{timeLayout, "2006-01-02"} {
		parsed, err := time.ParseInLocation(layout, value, location)
		if err == nil {
			return parsed, nil
		}
	}

	return time.Time{}, fmt.Errorf("must be in format yyyy-MM-ddTHH:mm:ss")
}

// pageQuery is a parsed release package request.
type pageQuery struct {
	from, upper time.Time // releases dated in [from, upper]
	start       int       // index of the first release to return
	limit       int
}

// handle answers a GET for a page of releases as an OCDS release package,
// with a links.next when more releases match.
func (s *server) handle(w http.ResponseWriter, r *http.Request) {
	if r.Method != http.MethodGet {
		w.WriteHeader(http.StatusMethodNotAllowed)
		return
	}

	query := r.URL.Query()
	page, err := s.parseQuery(query)
	if err != nil {
		badRequest(w, err.Error())
		return
	}

	releases, next, more := s.store.Page(
		page.from, page.upper, page.start, page.limit,
	)
	releasePackage := ocds.ReleasePackage{
		URI:           s.baseURL + r.URL.RequestURI(),
		Version:       "1.1",
		Extensions:    ocds.DeclaredExtensions,
		PublishedDate: ptr(time.Now().UTC().Truncate(time.Second)),
		Publisher: &ocds.ReleasePackagePublisher{
			Name: "OCDS Mock",
			URI:  s.baseURL,
		},
		Releases: releases,
	}
	if more {
		releasePackage.Links = &ocds.ReleasePackageLinks{
			Next: s.nextLink(query, next),
		}
	}

	w.Header().Set("Content-Type", "application/json; charset=utf-8")
	w.Header().Set("Access-Control-Allow-Origin", "*")
	encoder := json.NewEncoder(w)
	encoder.SetIndent("", "  ")
	encoder.SetEscapeHTML(false)
	if err := encoder.Encode(releasePackage); err != nil {
		s.opts.logger("handler").Warn("writing response failed", "error", err)
	}
}

// parseQuery reads and checks the query parameters. Its errors are meant for
// the client.
func (s *server) parseQuery(query url.Values) (pageQuery, error) {
	page := pageQuery{
		limit: defaultLimit,
		upper: time.Now().UTC(),
	}

	if value := query.Get("limit"); value != "" {
		number, err := strconv.Atoi(value)
		if err != nil || number < 1 || number > maxLimit {
			return page, fmt.Errorf(
				"limit must be an integer between 1 and %d", maxLimit,
			)
		}
		page.limit = number
	}

	if value := query.Get("updatedFrom"); value != "" {
		from, err := parseTime(value, s.location)
		if err != nil {
			return page, fmt.Errorf("updatedFrom %w", err)
		}
		page.from = from
	}

	if value := query.Get("updatedTo"); value != "" {
		until, err := parseTime(value, s.location)
		if err != nil {
			return page, fmt.Errorf("updatedTo %w", err)
		}
		if query.Get("updatedFrom") != "" && until.Before(page.from) {
			return page, errors.New("updatedTo must not be before updatedFrom")
		}
		if until.Before(page.upper) {
			page.upper = until
		}
	}

	if cursor := query.Get("cursor"); cursor != "" {
		index, err := decodeCursor(cursor)
		if err != nil {
			return page, err
		}
		page.start = index
	}

	return page, nil
}

// nextLink is the URL of the page starting at release index next: the same
// query with a new cursor.
func (s *server) nextLink(query url.Values, next int) string {
	nextQuery := url.Values{}
	for key, values := range query {
		if key != "cursor" {
			nextQuery[key] = values
		}
	}
	nextQuery.Set("cursor", encodeCursor(next))
	return s.baseURL + BasePath + "?" + nextQuery.Encode()
}