package usertoken

import (
	"context"
	"crypto/rsa"
	"errors"
	"fmt"
	"strings"
	"time"

	"github.com/golang-jwt/jwt/v5"
)

const defaultSkew = time.Minute

type tokenVerifier interface {
	verify(ctx context.Context, raw string) (Caller, error)
}

type staticVerifier struct {
	key  *rsa.PublicKey
	iss  string
	aud  string
	skew time.Duration
}

func newStaticVerifier(key *rsa.PublicKey, iss, aud string, skew time.Duration) (tokenVerifier, error) {
	if key == nil {
		return nil, fmt.Errorf("public key is nil")
	}
	iss = strings.TrimSpace(iss)
	aud = strings.TrimSpace(aud)
	if iss == "" || aud == "" {
		return nil, fmt.Errorf("issuer and audience are required")
	}
	if skew <= 0 {
		skew = defaultSkew
	}
	return &staticVerifier{key: key, iss: iss, aud: aud, skew: skew}, nil
}

func (v *staticVerifier) verify(_ context.Context, raw string) (Caller, error) {
	if strings.TrimSpace(raw) == "" {
		return Caller{}, unauthorized("empty token")
	}
	parser := jwt.NewParser(
		jwt.WithValidMethods([]string{jwt.SigningMethodRS256.Alg()}),
		jwt.WithIssuer(v.iss),
		jwt.WithAudience(v.aud),
		jwt.WithLeeway(v.skew),
		jwt.WithExpirationRequired(),
		jwt.WithIssuedAt(),
	)
	var claims tokenClaims
	tok, err := parser.ParseWithClaims(raw, &claims, func(t *jwt.Token) (any, error) {
		if t.Method == nil || t.Method.Alg() != jwt.SigningMethodRS256.Alg() {
			return nil, unauthorized("unexpected alg")
		}
		return v.key, nil
	})
	if err != nil || tok == nil || !tok.Valid {
		if err == nil {
			err = errors.New("invalid token")
		}
		return Caller{}, fmt.Errorf("verify: %w", errors.Join(err, ErrUnauthorized))
	}
	return callerFromClaims(claims.Subject, claims.PreferredUsername, claims.Groups)
}

func callerFromClaims(subject, username string, groups *[]string) (Caller, error) {
	subject = strings.TrimSpace(subject)
	if subject == "" {
		return Caller{}, unauthorized("empty sub")
	}
	username = strings.TrimSpace(username)
	if username == "" {
		username = subject
	}
	c := Caller{Subject: subject, Username: username}
	if groups != nil {
		c.Groups = append([]string{}, (*groups)...)
	}
	return c, nil
}