// Package auth validates the bearer tokens issued by the go-loose SSO service
// and identifies the caller.
package auth

import (
	"context"
	"errors"
	"fmt"
	"strings"

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

	"go-places-api/internal/domain"
)

// Errors returned by verifiers. They are deliberately coarse: a caller learns
// that a token is unacceptable, not why, so probing cannot distinguish an
// expired token from a forged one.
var (
	// ErrMissingToken means no credential was presented.
	ErrMissingToken = errors.New("auth: no token presented")
	// ErrInvalidToken means the token failed verification for any reason.
	ErrInvalidToken = errors.New("auth: invalid token")
)

// Claims are the fields the application reads from a go-loose token.
type Claims struct {
	jwt.RegisteredClaims
	// Username and Email are copied from the identity provider. They are
	// optional because a token may only assert the subject.
	Username string `json:"username,omitempty"`
	Email    string `json:"email,omitempty"`
}

// User is the identity resolved from a token.
type User struct {
	domain.User
}

// Verifier turns a bearer token into an identity.
type Verifier interface {
	// Verify returns the identity the token asserts. It returns
	// ErrMissingToken or ErrInvalidToken for unusable credentials.
	Verify(ctx context.Context, token string) (User, error)
	// Name identifies the verification strategy, for logging.
	Name() string
}

// JWTVerifier verifies HS256-signed tokens issued by go-loose.
//
// Only HS256 is accepted. The algorithm is pinned rather than read from the
// token header so that a token cannot select "none" or an asymmetric algorithm
// and slip past verification.
type JWTVerifier struct {
	secret   []byte
	audience string
	issuer   string
}

var _ Verifier = (*JWTVerifier)(nil)

// NewJWTVerifier returns a verifier for tokens signed with secret. audience may
// be empty to skip the audience check; issuer likewise.
func NewJWTVerifier(secret, audience, issuer string) *JWTVerifier {
	return &JWTVerifier{secret: []byte(secret), audience: audience, issuer: issuer}
}

// Name implements Verifier.
func (v *JWTVerifier) Name() string { return "jwt" }

// Verify parses and validates the token, then extracts the identity.
func (v *JWTVerifier) Verify(ctx context.Context, token string) (User, error) {
	if err := ctx.Err(); err != nil {
		return User{}, err
	}
	if token == "" {
		return User{}, ErrMissingToken
	}
	if len(v.secret) == 0 {
		return User{}, fmt.Errorf("%w: no signing secret configured", ErrInvalidToken)
	}

	opts := []jwt.ParserOption{
		jwt.WithValidMethods([]string{jwt.SigningMethodHS256.Alg()}),
		jwt.WithExpirationRequired(),
	}
	if v.audience != "" {
		opts = append(opts, jwt.WithAudience(v.audience))
	}
	if v.issuer != "" {
		opts = append(opts, jwt.WithIssuer(v.issuer))
	}

	var claims Claims
	parsed, err := jwt.ParseWithClaims(token, &claims, keyFunc(v.secret), opts...)
	if err != nil {
		// The underlying reason stays unexported so the caller cannot use the
		// error text to learn why a token was rejected.
		return User{}, ErrInvalidToken
	}
	if !parsed.Valid {
		return User{}, ErrInvalidToken
	}
	if claims.Subject == "" {
		// Without a subject there is nobody to attribute the request to.
		return User{}, ErrInvalidToken
	}

	return User{domain.User{
		ID:       claims.Subject,
		Username: firstNonEmpty(claims.Username, claims.Subject),
		Email:    claims.Email,
	}}, nil
}

// keyFunc supplies the verification key. The token's own "alg" header is
// deliberately ignored because the parser is already pinned to HS256.
func keyFunc(secret []byte) jwt.Keyfunc {
	return func(t *jwt.Token) (any, error) {
		return secret, nil
	}
}

// DevVerifier accepts a small set of fixed tokens for local development. It is
// only wired up when AUTH_MODE=dev, and refuses to start in production.
type DevVerifier struct {
	tokens map[string]domain.User
}

var _ Verifier = (*DevVerifier)(nil)

// DefaultDevUser is the identity assumed by the "valid_token" dev token.
var DefaultDevUser = domain.User{
	ID:       "dev-user",
	Username: "dev",
	Email:    "dev@localhost",
}

// NewDevVerifier returns a verifier accepting the given token-to-user mapping.
func NewDevVerifier(tokens map[string]domain.User) *DevVerifier {
	if tokens == nil {
		tokens = map[string]domain.User{"valid_token": DefaultDevUser}
	}
	return &DevVerifier{tokens: tokens}
}

// Name implements Verifier.
func (v *DevVerifier) Name() string { return "dev" }

// Verify looks the token up in the fixed table.
func (v *DevVerifier) Verify(ctx context.Context, token string) (User, error) {
	if err := ctx.Err(); err != nil {
		return User{}, err
	}
	if token == "" {
		return User{}, ErrMissingToken
	}
	u, ok := v.tokens[token]
	if !ok {
		return User{}, ErrInvalidToken
	}
	return User{u}, nil
}

// BearerToken extracts the credential from an Authorization header value.
// Both "Bearer <token>" and a bare token are accepted, since some clients send
// the latter.
func BearerToken(header string) (string, error) {
	if strings.TrimSpace(header) == "" {
		return "", ErrMissingToken
	}

	// The scheme is split before trimming the credential, so "Bearer " with
	// nothing after it is reported as a missing token rather than read as a
	// bare token whose value is the word "Bearer".
	scheme, rest, hasSpace := strings.Cut(header, " ")
	if strings.EqualFold(scheme, "bearer") {
		token := strings.TrimSpace(rest)
		if !hasSpace || token == "" {
			return "", ErrMissingToken
		}
		return token, nil
	}

	header = strings.TrimSpace(header)
	// A bare token is accepted, but reject anything shaped like another scheme.
	if i := strings.IndexByte(header, ' '); i > 0 {
		return "", ErrInvalidToken
	}
	return header, nil
}

func firstNonEmpty(vals ...string) string {
	for _, v := range vals {
		if v != "" {
			return v
		}
	}
	return ""
}