package sipguardian

import (
	"bytes"
	"regexp"
	"strconv"
	"strings"
	"sync"
	"time"

	"go.uber.org/zap"
)

// ValidationConfig holds configuration for SIP message validation
type ValidationConfig struct {
	// Enabled determines if validation is active
	Enabled bool `json:"enabled,omitempty"`

	// Mode determines validation strictness: permissive, strict, paranoid
	Mode ValidationMode `json:"mode,omitempty"`

	// MaxMessageSize limits SIP message size (default: 65535)
	MaxMessageSize int `json:"max_message_size,omitempty"`

	// BanOnNullBytes causes immediate ban for null byte injection
	BanOnNullBytes bool `json:"ban_on_null_bytes,omitempty"`

	// BanOnBinaryInjection causes immediate ban for binary data injection
	BanOnBinaryInjection bool `json:"ban_on_binary_injection,omitempty"`

	// DisabledRules lists rules to skip
	DisabledRules []string `json:"disabled_rules,omitempty"`
}

// ValidationMode determines validation strictness
type ValidationMode string

const (
	// ValidationModePermissive logs violations but only blocks critical attacks
	ValidationModePermissive ValidationMode = "permissive"

	// ValidationModeStrict enforces RFC 3261 compliance
	ValidationModeStrict ValidationMode = "strict"

	// ValidationModeParanoid applies extra security heuristics
	ValidationModeParanoid ValidationMode = "paranoid"
)

// ViolationSeverity indicates how serious a violation is
type ViolationSeverity string

const (
	SeverityCritical ViolationSeverity = "critical" // Immediate ban
	SeverityHigh     ViolationSeverity = "high"     // Count toward ban
	SeverityMedium   ViolationSeverity = "medium"   // Reject only
	SeverityLow      ViolationSeverity = "low"      // Log only
)

// Violation represents a single validation rule violation
type Violation struct {
	Rule     string            `json:"rule"`
	Severity ViolationSeverity `json:"severity"`
	Message  string            `json:"message"`
	Details  string            `json:"details,omitempty"`
}

// ValidationResult contains the result of validating a SIP message
type ValidationResult struct {
	Valid      bool        `json:"valid"`
	Violations []Violation `json:"violations,omitempty"`
	ShouldBan  bool        `json:"should_ban"`
	BanReason  string      `json:"ban_reason,omitempty"`
}

// SIPValidator validates SIP messages against RFC 3261 and security rules
type SIPValidator struct {
	config       ValidationConfig
	disabledSet  map[string]bool
	logger       *zap.Logger
	mu           sync.RWMutex
	stats        validatorStats
}

type validatorStats struct {
	totalValidated    int64
	totalValid        int64
	totalInvalid      int64
	violationsByRule  map[string]int64
	mu                sync.Mutex
}

// DefaultValidationConfig returns sensible defaults
func DefaultValidationConfig() ValidationConfig {
	return ValidationConfig{
		Enabled:              true,
		Mode:                 ValidationModePermissive,
		MaxMessageSize:       65535,
		BanOnNullBytes:       true,
		BanOnBinaryInjection: true,
		DisabledRules:        []string{},
	}
}

// Global validator instance
var (
	globalValidator   *SIPValidator
	validatorMu       sync.RWMutex
)

// GetValidator returns the global validator instance
func GetValidator(logger *zap.Logger) *SIPValidator {
	validatorMu.Lock()
	defer validatorMu.Unlock()

	if globalValidator == nil {
		globalValidator = NewSIPValidator(logger, DefaultValidationConfig())
	}
	return globalValidator
}

// SetValidationConfig updates the global validator configuration
func SetValidationConfig(config ValidationConfig) {
	validatorMu.Lock()
	defer validatorMu.Unlock()

	if globalValidator == nil {
		globalValidator = &SIPValidator{
			config:      config,
			disabledSet: make(map[string]bool),
			logger:      zap.NewNop(),
			stats: validatorStats{
				violationsByRule: make(map[string]int64),
			},
		}
	} else {
		globalValidator.config = config
	}

	// Rebuild disabled rules set
	globalValidator.disabledSet = make(map[string]bool)
	for _, rule := range config.DisabledRules {
		globalValidator.disabledSet[rule] = true
	}
}

// NewSIPValidator creates a new validator
func NewSIPValidator(logger *zap.Logger, config ValidationConfig) *SIPValidator {
	disabledSet := make(map[string]bool)
	for _, rule := range config.DisabledRules {
		disabledSet[rule] = true
	}

	return &SIPValidator{
		config:      config,
		disabledSet: disabledSet,
		logger:      logger,
		stats: validatorStats{
			violationsByRule: make(map[string]int64),
		},
	}
}

// Validate checks a SIP message for RFC compliance and security issues
func (v *SIPValidator) Validate(data []byte) ValidationResult {
	v.mu.RLock()
	config := v.config
	disabled := v.disabledSet
	v.mu.RUnlock()

	if !config.Enabled {
		return ValidationResult{Valid: true}
	}

	result := ValidationResult{Valid: true}
	var violations []Violation

	// Critical checks first (can cause immediate ban)
	if !disabled["null_bytes"] {
		if v := v.checkNullBytes(data); v != nil {
			violations = append(violations, *v)
			if config.BanOnNullBytes {
				result.ShouldBan = true
				result.BanReason = "validation_null_bytes"
			}
		}
	}

	if !disabled["binary_injection"] {
		if v := v.checkBinaryInjection(data); v != nil {
			violations = append(violations, *v)
			if config.BanOnBinaryInjection {
				result.ShouldBan = true
				result.BanReason = "validation_binary_injection"
			}
		}
	}

	// Size check
	if !disabled["oversized_message"] {
		if v := v.checkMessageSize(data, config.MaxMessageSize); v != nil {
			violations = append(violations, *v)
		}
	}

	// Parse headers for further validation
	headers := v.parseHeaders(data)

	// Required header checks
	if !disabled["missing_via"] {
		if v := v.checkRequiredHeader(headers, "Via", SeverityHigh); v != nil {
			violations = append(violations, *v)
		}
	}

	if !disabled["missing_from"] {
		if v := v.checkRequiredHeader(headers, "From", SeverityHigh); v != nil {
			violations = append(violations, *v)
		}
	}

	if !disabled["missing_to"] {
		if v := v.checkRequiredHeader(headers, "To", SeverityHigh); v != nil {
			violations = append(violations, *v)
		}
	}

	if !disabled["missing_call_id"] {
		if v := v.checkRequiredHeader(headers, "Call-ID", SeverityHigh); v != nil {
			violations = append(violations, *v)
		}
	}

	if !disabled["missing_cseq"] {
		if v := v.checkRequiredHeader(headers, "CSeq", SeverityHigh); v != nil {
			violations = append(violations, *v)
		}
	}

	if !disabled["missing_max_forwards"] {
		if v := v.checkRequiredHeader(headers, "Max-Forwards", SeverityMedium); v != nil {
			violations = append(violations, *v)
		}
	}

	// CSeq validation
	if !disabled["cseq_out_of_range"] {
		if v := v.checkCSeq(headers); v != nil {
			violations = append(violations, *v)
		}
	}

	// Content-Length validation
	if !disabled["content_length_mismatch"] {
		if v := v.checkContentLength(headers, data); v != nil {
			violations = append(violations, *v)
		}
	}

	// Via branch validation (strict/paranoid mode)
	if config.Mode == ValidationModeStrict || config.Mode == ValidationModeParanoid {
		if !disabled["via_invalid_branch"] {
			if v := v.checkViaBranch(headers); v != nil {
				violations = append(violations, *v)
			}
		}
	}

	// Request-URI validation (strict/paranoid mode)
	if config.Mode == ValidationModeStrict || config.Mode == ValidationModeParanoid {
		if !disabled["invalid_request_uri"] {
			if v := v.checkRequestURI(data); v != nil {
				violations = append(violations, *v)
			}
		}
	}

	// Paranoid mode extra checks
	if config.Mode == ValidationModeParanoid {
		if !disabled["suspicious_headers"] {
			violations = append(violations, v.checkSuspiciousHeaders(headers)...)
		}
	}

	// Process violations
	if len(violations) > 0 {
		result.Valid = false
		result.Violations = violations

		// Record stats
		v.stats.mu.Lock()
		v.stats.totalValidated++
		v.stats.totalInvalid++
		for _, viol := range violations {
			v.stats.violationsByRule[viol.Rule]++
		}
		v.stats.mu.Unlock()

		// Determine if violations warrant a ban (in strict/paranoid mode)
		if !result.ShouldBan && (config.Mode == ValidationModeStrict || config.Mode == ValidationModeParanoid) {
			for _, viol := range violations {
				if viol.Severity == SeverityCritical || viol.Severity == SeverityHigh {
					result.ShouldBan = true
					result.BanReason = "validation_" + viol.Rule
					break
				}
			}
		}
	} else {
		v.stats.mu.Lock()
		v.stats.totalValidated++
		v.stats.totalValid++
		v.stats.mu.Unlock()
	}

	return result
}

// checkNullBytes detects null byte injection attacks
func (v *SIPValidator) checkNullBytes(data []byte) *Violation {
	if bytes.Contains(data, []byte{0x00}) {
		return &Violation{
			Rule:     "null_bytes",
			Severity: SeverityCritical,
			Message:  "Null byte injection detected",
			Details:  "SIP message contains null bytes which may indicate an injection attack",
		}
	}
	return nil
}

// checkBinaryInjection detects non-ASCII binary data in headers
func (v *SIPValidator) checkBinaryInjection(data []byte) *Violation {
	// Split headers from body
	parts := bytes.SplitN(data, []byte("\r\n\r\n"), 2)
	headerSection := parts[0]

	// Check for non-printable ASCII in headers (except CR, LF, TAB)
	for i, b := range headerSection {
		if b < 0x09 || (b > 0x0D && b < 0x20) || b > 0x7E {
			// Allow UTF-8 in display names, but flag truly binary data
			if b < 0x80 {
				return &Violation{
					Rule:     "binary_injection",
					Severity: SeverityCritical,
					Message:  "Binary data injection detected",
					Details:  "Non-printable character 0x" + strconv.FormatInt(int64(b), 16) + " at position " + strconv.Itoa(i),
				}
			}
		}
	}
	return nil
}

// checkMessageSize validates message doesn't exceed limits
func (v *SIPValidator) checkMessageSize(data []byte, maxSize int) *Violation {
	if len(data) > maxSize {
		return &Violation{
			Rule:     "oversized_message",
			Severity: SeverityHigh,
			Message:  "Message exceeds maximum size",
			Details:  "Size " + strconv.Itoa(len(data)) + " exceeds limit " + strconv.Itoa(maxSize),
		}
	}
	return nil
}

// parseHeaders extracts headers from SIP message
func (v *SIPValidator) parseHeaders(data []byte) map[string][]string {
	headers := make(map[string][]string)

	// Split headers from body
	parts := bytes.SplitN(data, []byte("\r\n\r\n"), 2)
	headerSection := string(parts[0])

	lines := strings.Split(headerSection, "\r\n")
	// Skip request/status line
	for i := 1; i < len(lines); i++ {
		line := lines[i]
		if line == "" {
			continue
		}

		// Handle header continuation (line starting with whitespace)
		if len(line) > 0 && (line[0] == ' ' || line[0] == '\t') {
			// Append to previous header
			continue
		}

		// Parse header name: value
		colonIdx := strings.Index(line, ":")
		if colonIdx > 0 {
			name := strings.TrimSpace(line[:colonIdx])
			value := strings.TrimSpace(line[colonIdx+1:])

			// Normalize header name (compact forms)
			name = normalizeHeaderName(name)
			headers[name] = append(headers[name], value)
		}
	}

	return headers
}

// normalizeHeaderName handles SIP compact header forms
func normalizeHeaderName(name string) string {
	// Map compact forms to full names
	compactForms := map[string]string{
		"i": "Call-ID",
		"m": "Contact",
		"e": "Content-Encoding",
		"l": "Content-Length",
		"c": "Content-Type",
		"f": "From",
		"s": "Subject",
		"k": "Supported",
		"t": "To",
		"v": "Via",
	}

	lower := strings.ToLower(name)
	if full, ok := compactForms[lower]; ok {
		return full
	}

	// Normalize case for common headers
	switch lower {
	case "via":
		return "Via"
	case "from":
		return "From"
	case "to":
		return "To"
	case "call-id":
		return "Call-ID"
	case "cseq":
		return "CSeq"
	case "max-forwards":
		return "Max-Forwards"
	case "content-length":
		return "Content-Length"
	case "contact":
		return "Contact"
	default:
		return name
	}
}

// checkRequiredHeader validates a required header is present
func (v *SIPValidator) checkRequiredHeader(headers map[string][]string, name string, severity ViolationSeverity) *Violation {
	if values, ok := headers[name]; !ok || len(values) == 0 || values[0] == "" {
		return &Violation{
			Rule:     "missing_" + strings.ToLower(strings.ReplaceAll(name, "-", "_")),
			Severity: severity,
			Message:  "Required header missing: " + name,
		}
	}
	return nil
}

// checkCSeq validates CSeq header format and range
func (v *SIPValidator) checkCSeq(headers map[string][]string) *Violation {
	values, ok := headers["CSeq"]
	if !ok || len(values) == 0 {
		return nil // Missing CSeq handled by required header check
	}

	cseq := values[0]
	parts := strings.Fields(cseq)
	if len(parts) < 2 {
		return &Violation{
			Rule:     "cseq_invalid_format",
			Severity: SeverityHigh,
			Message:  "Invalid CSeq format",
			Details:  "Expected 'sequence method', got: " + cseq,
		}
	}

	seq, err := strconv.ParseInt(parts[0], 10, 64)
	if err != nil {
		return &Violation{
			Rule:     "cseq_invalid_sequence",
			Severity: SeverityHigh,
			Message:  "CSeq sequence number invalid",
			Details:  "Could not parse: " + parts[0],
		}
	}

	// RFC 3261: CSeq must be 0 to 2^31-1
	if seq < 0 || seq > 2147483647 {
		return &Violation{
			Rule:     "cseq_out_of_range",
			Severity: SeverityMedium,
			Message:  "CSeq sequence number out of range",
			Details:  "Value " + parts[0] + " outside valid range 0-2147483647",
		}
	}

	return nil
}

// checkContentLength validates Content-Length matches actual body
func (v *SIPValidator) checkContentLength(headers map[string][]string, data []byte) *Violation {
	values, ok := headers["Content-Length"]
	if !ok || len(values) == 0 {
		return nil // Optional header
	}

	claimLength, err := strconv.Atoi(values[0])
	if err != nil {
		return &Violation{
			Rule:     "content_length_invalid",
			Severity: SeverityHigh,
			Message:  "Invalid Content-Length value",
			Details:  "Could not parse: " + values[0],
		}
	}

	// Find actual body
	parts := bytes.SplitN(data, []byte("\r\n\r\n"), 2)
	actualLength := 0
	if len(parts) > 1 {
		actualLength = len(parts[1])
	}

	// Allow some tolerance for line ending differences
	if claimLength != actualLength {
		// Check if difference is just CRLF vs LF
		if abs(claimLength-actualLength) > 2 {
			return &Violation{
				Rule:     "content_length_mismatch",
				Severity: SeverityHigh,
				Message:  "Content-Length doesn't match body size",
				Details:  "Claimed " + values[0] + ", actual " + strconv.Itoa(actualLength),
			}
		}
	}

	return nil
}

// checkViaBranch validates Via branch parameter format
func (v *SIPValidator) checkViaBranch(headers map[string][]string) *Violation {
	values, ok := headers["Via"]
	if !ok || len(values) == 0 {
		return nil
	}

	via := values[0]
	// RFC 3261: branch must start with "z9hG4bK" for 3261-compliant UAs
	branchPattern := regexp.MustCompile(`branch=([^;,\s]+)`)
	matches := branchPattern.FindStringSubmatch(via)

	if len(matches) > 1 {
		branch := matches[1]
		if !strings.HasPrefix(branch, "z9hG4bK") {
			return &Violation{
				Rule:     "via_invalid_branch",
				Severity: SeverityLow,
				Message:  "Via branch doesn't follow RFC 3261 format",
				Details:  "Branch should start with 'z9hG4bK', got: " + branch,
			}
		}
	}

	return nil
}

// checkRequestURI validates Request-URI format
func (v *SIPValidator) checkRequestURI(data []byte) *Violation {
	// Get first line
	idx := bytes.Index(data, []byte("\r\n"))
	if idx < 0 {
		return &Violation{
			Rule:     "invalid_request_line",
			Severity: SeverityHigh,
			Message:  "Could not parse request line",
		}
	}

	requestLine := string(data[:idx])
	parts := strings.Fields(requestLine)

	if len(parts) < 3 {
		return &Violation{
			Rule:     "invalid_request_line",
			Severity: SeverityHigh,
			Message:  "Malformed request line",
			Details:  requestLine,
		}
	}

	// Check if it's a response (starts with SIP/2.0)
	if strings.HasPrefix(parts[0], "SIP/") {
		return nil // Skip URI validation for responses
	}

	uri := parts[1]

	// Basic SIP URI validation
	if !strings.HasPrefix(strings.ToLower(uri), "sip:") &&
	   !strings.HasPrefix(strings.ToLower(uri), "sips:") &&
	   !strings.HasPrefix(strings.ToLower(uri), "tel:") {
		return &Violation{
			Rule:     "invalid_request_uri",
			Severity: SeverityHigh,
			Message:  "Invalid Request-URI scheme",
			Details:  "Expected sip:, sips:, or tel: URI, got: " + uri,
		}
	}

	return nil
}

// checkSuspiciousHeaders detects potentially malicious header patterns
func (v *SIPValidator) checkSuspiciousHeaders(headers map[string][]string) []Violation {
	var violations []Violation

	// Check for extremely long header values
	for name, values := range headers {
		for _, val := range values {
			if len(val) > 1024 {
				violations = append(violations, Violation{
					Rule:     "suspicious_long_header",
					Severity: SeverityMedium,
					Message:  "Unusually long header value",
					Details:  name + " has " + strconv.Itoa(len(val)) + " bytes",
				})
			}
		}
	}

	// Check for excessive number of Via headers (potential loop)
	if vias, ok := headers["Via"]; ok && len(vias) > 70 {
		violations = append(violations, Violation{
			Rule:     "suspicious_via_count",
			Severity: SeverityMedium,
			Message:  "Excessive Via headers",
			Details:  strconv.Itoa(len(vias)) + " Via headers (potential loop)",
		})
	}

	// Check for SQL injection patterns in headers
	sqlPatterns := []string{
		"' OR '",
		"' OR 1=1",
		"'; DROP",
		"UNION SELECT",
		"--",
	}
	for name, values := range headers {
		for _, val := range values {
			upperVal := strings.ToUpper(val)
			for _, pattern := range sqlPatterns {
				if strings.Contains(upperVal, pattern) {
					violations = append(violations, Violation{
						Rule:     "suspicious_sql_injection",
						Severity: SeverityHigh,
						Message:  "Potential SQL injection in header",
						Details:  name + " contains suspicious pattern",
					})
					break
				}
			}
		}
	}

	return violations
}

// GetStats returns validator statistics
func (v *SIPValidator) GetStats() map[string]interface{} {
	v.stats.mu.Lock()
	defer v.stats.mu.Unlock()

	violations := make(map[string]int64)
	for rule, count := range v.stats.violationsByRule {
		violations[rule] = count
	}

	return map[string]interface{}{
		"total_validated":    v.stats.totalValidated,
		"total_valid":        v.stats.totalValid,
		"total_invalid":      v.stats.totalInvalid,
		"violations_by_rule": violations,
		"config_mode":        v.config.Mode,
		"config_enabled":     v.config.Enabled,
	}
}

// IsEnabled returns whether validation is enabled
func (v *SIPValidator) IsEnabled() bool {
	v.mu.RLock()
	defer v.mu.RUnlock()
	return v.config.Enabled
}

// abs returns absolute value of an int
func abs(n int) int {
	if n < 0 {
		return -n
	}
	return n
}

// Cleanup resets old statistics (called periodically)
func (v *SIPValidator) Cleanup(maxAge time.Duration) {
	// For now, validation stats don't need time-based cleanup
	// Could add per-IP validation tracking in the future
}