package mock

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

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

const (
	defaultLimit = 100
	maxLimit     = 100
	timeLayout   = "2006-01-02T15:04:05"
	BasePath     = "/api/1.0/ocdsReleasePackages"
)

type server struct {
	store   *Store
	baseURL string
	loc     *time.Location // zone for updatedFrom/updatedTo without an offset; nil means UTC
}

// 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.
func NewHandler(
	store *Store, baseURL string, loc *time.Location,
) http.HandlerFunc {
	return (&server{store: store, baseURL: baseURL, loc: loc}).handle
}

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(c string) (int, error) {
	b, err := base64.RawURLEncoding.DecodeString(c)
	if err != nil || !strings.HasPrefix(string(b), "idx:") {
		return 0, fmt.Errorf("invalid cursor")
	}

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

	return n, nil
}

func badRequest(w http.ResponseWriter, msg string) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(http.StatusBadRequest)
	json.NewEncoder(w).Encode(map[string]string{"error": msg})
}

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

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

	for _, layout := range []string{timeLayout, "2006-01-02"} {
		if t, err := time.ParseInLocation(layout, v, loc); err == nil {
			return t, 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
}

func (s *server) handle(w http.ResponseWriter, req *http.Request) {
	if req.Method != http.MethodGet {
		w.WriteHeader(http.StatusMethodNotAllowed)
		return
	}

	q := req.URL.Query()
	pq, err := s.parseQuery(q)
	if err != nil {
		badRequest(w, err.Error())
		return
	}

	releases, next, more := s.store.Page(pq.from, pq.upper, pq.start, pq.limit)
	pkg := ocds.ReleasePackage{
		URI:           s.baseURL + req.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 {
		pkg.Links = &ocds.ReleasePackageLinks{Next: s.nextLink(q, next)}
	}

	w.Header().Set("Content-Type", "application/json; charset=utf-8")
	w.Header().Set("Access-Control-Allow-Origin", "*")
	enc := json.NewEncoder(w)
	enc.SetIndent("", "  ")
	enc.SetEscapeHTML(false)
	if err := enc.Encode(pkg); err != nil {
		log.Printf("handler: writing response: %v", err)
	}
}

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

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

	if v := q.Get("updatedFrom"); v != "" {
		t, err := parseTime(v, s.loc)
		if err != nil {
			return pq, fmt.Errorf("updatedFrom %w", err)
		}
		pq.from = t
	}

	if v := q.Get("updatedTo"); v != "" {
		t, err := parseTime(v, s.loc)
		if err != nil {
			return pq, fmt.Errorf("updatedTo %w", err)
		}
		if q.Get("updatedFrom") != "" && t.Before(pq.from) {
			return pq, errors.New("updatedTo must not be before updatedFrom")
		}
		if t.Before(pq.upper) {
			pq.upper = t
		}
	}

	if c := q.Get("cursor"); c != "" {
		n, err := decodeCursor(c)
		if err != nil {
			return pq, err
		}
		pq.start = n
	}

	return pq, nil
}

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