package traefikoidc

import (
	"crypto"
	"crypto/ecdsa"
	"crypto/rsa"
	"crypto/x509"
	"encoding/base64"
	"encoding/json"
	"encoding/pem"
	"fmt"
	"math/big"
	"strings"
	"sync"
	"time"
)

var (
	replayCacheMu sync.Mutex
	replayCache   = make(map[string]time.Time)
)

// cleanupReplayCache iterates through the replay cache and removes entries
// whose expiration time is before the current time. This function should be
// called periodically to prevent the cache from growing indefinitely.
// It acquires a mutex to ensure thread safety during cleanup.
func cleanupReplayCache() {
	now := time.Now()
	for token, expiry := range replayCache {
		if expiry.Before(now) {
			delete(replayCache, token)
		}
	}
}

// ClockSkewToleranceFuture defines the tolerance for future-based claims like 'exp'.
// Allows for more leniency with expiration checks.
var ClockSkewToleranceFuture = 2 * time.Minute

// ClockSkewTolerancePast defines the tolerance for past-based claims like 'iat' and 'nbf'.
// A smaller tolerance is typically used here to prevent accepting tokens issued too far in the future.
var (
	ClockSkewTolerancePast = 10 * time.Second
	ClockSkewTolerance     = 2 * time.Minute
)

// JWT represents a JSON Web Token as defined in RFC 7519.
type JWT struct {
	Header    map[string]interface{}
	Claims    map[string]interface{}
	Signature []byte
	Token     string
}

// parseJWT decodes a raw JWT string into its constituent parts: header, claims, and signature.
// It splits the token string by '.', decodes each part using base64 URL decoding,
// and unmarshals the header and claims JSON into maps. The raw signature bytes are stored.
// It performs basic format validation (expecting 3 parts).
// Note: This function does *not* validate the signature or the claims.
//
// Parameters:
//   - tokenString: The raw JWT string.
//
// Returns:
//   - A pointer to a JWT struct containing the decoded parts.
//   - An error if the token format is invalid or decoding/unmarshaling fails.
func parseJWT(tokenString string) (*JWT, error) {
	parts := strings.Split(tokenString, ".")
	if len(parts) != 3 {
		return nil, fmt.Errorf("invalid JWT format: expected 3 parts, got %d", len(parts))
	}

	jwt := &JWT{
		Token: tokenString,
	}

	headerBytes, err := base64.RawURLEncoding.DecodeString(parts[0])
	if err != nil {
		return nil, fmt.Errorf("invalid JWT format: failed to decode header: %v", err)
	}
	if err := json.Unmarshal(headerBytes, &jwt.Header); err != nil {
		return nil, fmt.Errorf("invalid JWT format: failed to unmarshal header: %v", err)
	}

	claimsBytes, err := base64.RawURLEncoding.DecodeString(parts[1])
	if err != nil {
		return nil, fmt.Errorf("invalid JWT format: failed to decode claims: %v", err)
	}
	if err := json.Unmarshal(claimsBytes, &jwt.Claims); err != nil {
		return nil, fmt.Errorf("invalid JWT format: failed to unmarshal claims: %v", err)
	}

	signatureBytes, err := base64.RawURLEncoding.DecodeString(parts[2])
	if err != nil {
		return nil, fmt.Errorf("invalid JWT format: failed to decode signature: %v", err)
	}
	jwt.Signature = signatureBytes

	return jwt, nil
}

// Verify performs standard claim validation on the JWT according to RFC 7519.
// It checks the following:
// - Algorithm ('alg') is supported.
// - Issuer ('iss') matches the expected issuerURL.
// - Audience ('aud') contains the expected clientID.
// - Expiration time ('exp') is in the future (within tolerance).
// - Issued at time ('iat') is in the past (within tolerance).
// - Not before time ('nbf'), if present, is in the past (within tolerance).
// - Subject ('sub') claim exists and is not empty.
// - JWT ID ('jti'), if present, is checked against a replay cache to prevent token reuse.
//
// Parameters:
//   - issuerURL: The expected issuer URL (e.g., "https://accounts.google.com").
//   - clientID: The expected audience value (the client ID of this application).
//
// Returns:
//   - nil if all standard claims are valid.
//   - An error describing the first validation failure encountered.
func (j *JWT) Verify(issuerURL, clientID string) error {
	// Validate algorithm to prevent algorithm switching attacks
	alg, ok := j.Header["alg"].(string)
	if !ok {
		return fmt.Errorf("missing 'alg' header")
	}
	supportedAlgs := map[string]bool{
		"RS256": true, "RS384": true, "RS512": true,
		"PS256": true, "PS384": true, "PS512": true,
		"ES256": true, "ES384": true, "ES512": true,
	}
	if !supportedAlgs[alg] {
		return fmt.Errorf("unsupported algorithm: %s", alg)
	}

	claims := j.Claims

	iss, ok := claims["iss"].(string)
	if !ok {
		return fmt.Errorf("missing 'iss' claim")
	}
	if err := verifyIssuer(iss, issuerURL); err != nil {
		return err
	}

	aud, ok := claims["aud"]
	if !ok {
		return fmt.Errorf("missing 'aud' claim")
	}
	if err := verifyAudience(aud, clientID); err != nil {
		return err
	}

	exp, ok := claims["exp"].(float64)
	if !ok {
		return fmt.Errorf("missing or invalid 'exp' claim")
	}
	if err := verifyExpiration(exp); err != nil {
		return err
	}

	iat, ok := claims["iat"].(float64)
	if !ok {
		return fmt.Errorf("missing or invalid 'iat' claim")
	}
	if err := verifyIssuedAt(iat); err != nil {
		return err
	}

	if nbf, ok := claims["nbf"].(float64); ok {
		if err := verifyNotBefore(nbf); err != nil {
			return err
		}
	}

	// Implement replay protection by checking the jti (JWT ID)
	if jti, ok := claims["jti"].(string); ok {
		// Skip replay detection for tokens that are being verified from the cache
		if j.Token == "" {
			// This is a parsed JWT without the original token string,
			// which means it's likely from a cached token verification
			return nil
		}

		replayCacheMu.Lock()
		cleanupReplayCache()
		if _, exists := replayCache[jti]; exists {
			replayCacheMu.Unlock()
			return fmt.Errorf("token replay detected")
		}
		expFloat, ok := claims["exp"].(float64)
		var expTime time.Time
		if ok {
			expTime = time.Unix(int64(expFloat), 0)
		} else {
			expTime = time.Now().Add(10 * time.Minute)
		}
		replayCache[jti] = expTime
		replayCacheMu.Unlock()
	}

	sub, ok := claims["sub"].(string)
	if !ok || sub == "" {
		return fmt.Errorf("missing or empty 'sub' claim")
	}

	return nil
}

// verifyAudience checks if the expected audience is present in the token's 'aud' claim.
// The 'aud' claim can be a single string or an array of strings.
//
// Parameters:
//   - tokenAudience: The 'aud' claim value extracted from the token (can be string or []interface{}).
//   - expectedAudience: The audience value expected for this application (client ID).
//
// Returns:
//   - nil if the expected audience is found.
//   - An error if the claim type is invalid or the expected audience is not present.
func verifyAudience(tokenAudience interface{}, expectedAudience string) error {
	switch aud := tokenAudience.(type) {
	case string:
		if aud != expectedAudience {
			return fmt.Errorf("invalid audience")
		}
	case []interface{}:
		found := false
		for _, v := range aud {
			if str, ok := v.(string); ok && str == expectedAudience {
				found = true
				break
			}
		}
		if !found {
			return fmt.Errorf("invalid audience")
		}
	default:
		return fmt.Errorf("invalid 'aud' claim type")
	}
	return nil
}

// verifyIssuer checks if the token's 'iss' claim matches the expected issuer URL.
//
// Parameters:
//   - tokenIssuer: The 'iss' claim value from the token.
//   - expectedIssuer: The expected issuer URL configured for the OIDC provider.
//
// Returns:
//   - nil if the issuers match.
//   - An error if the issuers do not match.
func verifyIssuer(tokenIssuer, expectedIssuer string) error {
	if tokenIssuer != expectedIssuer {
		return fmt.Errorf("invalid issuer (token: %s, expected: %s)", tokenIssuer, expectedIssuer)
	}
	return nil
}

// verifyTimeConstraint checks time-based claims ('exp', 'iat', 'nbf') against the current time,
// allowing for configurable clock skew. It uses different tolerances for past and future checks.
//
// Parameters:
//   - unixTime: The timestamp value from the claim (as a float64 Unix time).
//   - claimName: The name of the claim being verified ("exp", "iat", "nbf").
//   - future: A boolean indicating the direction of the check (true for 'exp', false for 'iat'/'nbf').
//
// Returns:
//   - nil if the time constraint is met within the allowed tolerance.
//   - An error describing the failure (e.g., "token has expired", "token used before issued").
func verifyTimeConstraint(unixTime float64, claimName string, future bool) error {
	claimTime := time.Unix(int64(unixTime), 0)
	now := time.Now() // Use current time without truncation

	var err error
	if future { // 'exp' check
		// Token is expired if Now is after (ClaimTime + FutureTolerance)
		allowedExpiry := claimTime.Add(ClockSkewToleranceFuture)
		if now.After(allowedExpiry) {
			err = fmt.Errorf("token has expired (exp: %v, now: %v, allowed_until: %v)", claimTime.UTC(), now.UTC(), allowedExpiry.UTC())
		}
	} else { // 'iat' or 'nbf' check
		// Token is invalid if Now is before (ClaimTime - PastTolerance)
		allowedStart := claimTime.Add(-ClockSkewTolerancePast)
		if now.Before(allowedStart) {
			reason := "not yet valid"
			if claimName == "iat" {
				reason = "used before issued"
			}
			err = fmt.Errorf("token %s (%s: %v, now: %v, allowed_from: %v)", reason, claimName, claimTime.UTC(), now.UTC(), allowedStart.UTC())
		}
	}

	return err
}

// verifyExpiration checks the 'exp' (Expiration Time) claim.
// It calls verifyTimeConstraint with future=true.
func verifyExpiration(expiration float64) error {
	return verifyTimeConstraint(expiration, "exp", true)
}

// verifyIssuedAt checks the 'iat' (Issued At) claim.
// It calls verifyTimeConstraint with future=false.
func verifyIssuedAt(issuedAt float64) error {
	return verifyTimeConstraint(issuedAt, "iat", false)
}

// verifyNotBefore checks the 'nbf' (Not Before) claim.
// It calls verifyTimeConstraint with future=false.
func verifyNotBefore(notBefore float64) error {
	return verifyTimeConstraint(notBefore, "nbf", false)
}

// verifySignature validates the JWT's signature using the provided public key.
// It parses the public key from PEM format, selects the appropriate hashing algorithm
// based on the 'alg' parameter (SHA256/384/512), hashes the token's signing input
// (header + "." + payload), and then verifies the signature against the hash using
// the corresponding RSA (PKCS1v15 or PSS) or ECDSA verification method.
//
// Parameters:
//   - tokenString: The raw, complete JWT string.
//   - publicKeyPEM: The public key corresponding to the private key used for signing, in PEM format.
//   - alg: The algorithm specified in the JWT header (e.g., "RS256", "ES384").
//
// Returns:
//   - nil if the signature is valid.
//   - An error if the token format is invalid, decoding fails, key parsing fails,
//     the algorithm is unsupported, or the signature verification fails.
func verifySignature(tokenString string, publicKeyPEM []byte, alg string) error {
	parts := strings.Split(tokenString, ".")
	if len(parts) != 3 {
		return fmt.Errorf("invalid token format")
	}
	signedContent := parts[0] + "." + parts[1]
	signature, err := base64.RawURLEncoding.DecodeString(parts[2])
	if err != nil {
		return fmt.Errorf("failed to decode signature: %w", err)
	}
	block, _ := pem.Decode(publicKeyPEM)
	if block == nil {
		return fmt.Errorf("failed to parse PEM block containing the public key")
	}
	pubKey, err := x509.ParsePKIXPublicKey(block.Bytes)
	if err != nil {
		return fmt.Errorf("failed to parse public key: %w", err)
	}
	var hashFunc crypto.Hash
	switch alg {
	case "RS256", "PS256", "ES256":
		hashFunc = crypto.SHA256
	case "RS384", "PS384", "ES384":
		hashFunc = crypto.SHA384
	case "RS512", "PS512", "ES512":
		hashFunc = crypto.SHA512
	default:
		return fmt.Errorf("unsupported algorithm: %s", alg)
	}
	h := hashFunc.New()
	h.Write([]byte(signedContent))
	hashed := h.Sum(nil)
	switch pubKey := pubKey.(type) {
	case *rsa.PublicKey:
		if strings.HasPrefix(alg, "RS") {
			return rsa.VerifyPKCS1v15(pubKey, hashFunc, hashed, signature)
		} else if strings.HasPrefix(alg, "PS") {
			return rsa.VerifyPSS(pubKey, hashFunc, hashed, signature, nil)
		} else {
			return fmt.Errorf("unexpected key type for algorithm %s", alg)
		}
	case *ecdsa.PublicKey:
		if strings.HasPrefix(alg, "ES") {
			var r, s big.Int
			sigLen := len(signature)
			if sigLen%2 != 0 {
				return fmt.Errorf("invalid ECDSA signature length")
			}
			r.SetBytes(signature[:sigLen/2])
			s.SetBytes(signature[sigLen/2:])
			if ecdsa.Verify(pubKey, hashed, &r, &s) {
				return nil
			} else {
				return fmt.Errorf("invalid ECDSA signature")
			}
		} else {
			return fmt.Errorf("unexpected key type for algorithm %s", alg)
		}
	default:
		return fmt.Errorf("unsupported public key type: %T", pubKey)
	}
}