package relay

import (
	"encoding/base64"
	"encoding/json"
	"fmt"
)

// RegionMaster is one region's active issuer master (spec §4): the master
// key bytes and the kid it's issued under.
type RegionMaster struct {
	Region string
	Kid    string
	Master []byte
}

// RegionURLs returns the STUN/TURN URLs for a region, built from the
// platform domain.
type RegionURLs func(region string) []string

// ConfigEntry is one region's worth of the CARLOS_RELAY value (spec §2):
// the URLs to try, the derived key, and the username suffix a client
// appends its own expiry to.
type ConfigEntry struct {
	Region         string   `json:"region"`
	URLs           []string `json:"urls"`
	Key            string   `json:"key"`
	UsernameSuffix string   `json:"username_suffix"`
}

// BuildConfig renders CARLOS_RELAY's value for r: base64(std, padded) of
// the JSON array from spec §2, one entry per region in r.Regions order.
// masters must cover every region in r.Regions, or BuildConfig refuses
// rather than issue a config a region can't actually serve.
func BuildConfig(acct, app string, r *Record, masters []RegionMaster, urls RegionURLs) (string, map[string]string, error) {
	byRegion := map[string]RegionMaster{}
	for _, m := range masters {
		byRegion[m.Region] = m
	}
	entries := make([]ConfigEntry, 0, len(r.Regions))
	suffixes := map[string]string{}
	for _, region := range r.Regions {
		m, ok := byRegion[region]
		if !ok {
			return "", nil, fmt.Errorf("relay: no issuer master for region %s", region)
		}
		key := DeriveKey(m.Master, acct, app, r.Incarnation)
		suffix := Suffix(acct, app, r.Incarnation, m.Kid)
		suffixes[region] = suffix
		entries = append(entries, ConfigEntry{Region: region, URLs: urls(region), Key: base64.StdEncoding.EncodeToString(key), UsernameSuffix: suffix})
	}
	js, err := json.Marshal(entries)
	if err != nil {
		return "", nil, err
	}
	return base64.StdEncoding.EncodeToString(js), suffixes, nil
}

// ParseConfig decodes a CARLOS_RELAY value back into its entries, for the
// config-ack (Task 12) and tests.
func ParseConfig(value string) ([]ConfigEntry, error) {
	js, err := base64.StdEncoding.DecodeString(value)
	if err != nil {
		return nil, fmt.Errorf("relay: CARLOS_RELAY is not base64: %w", err)
	}
	var out []ConfigEntry
	if err := json.Unmarshal(js, &out); err != nil {
		return nil, fmt.Errorf("relay: CARLOS_RELAY is not a JSON array: %w", err)
	}
	return out, nil
}