/*
Audit — Operational audit logging for Volt.

Logs every CLI/API action with structured JSON entries containing:
  - Who: username, UID, source (CLI/API/SSO)
  - What: command, arguments, resource, action
  - When: ISO 8601 timestamp with microseconds
  - Where: hostname, source IP (for API calls)
  - Result: success/failure, error message if any

Log entries are optionally signed (HMAC-SHA256) for tamper evidence.
Logs are written to /var/log/volt/audit.log and optionally forwarded to syslog.

Copyright (c) Armored Gates LLC. All rights reserved.
*/
package audit

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"os"
	"os/user"
	"path/filepath"
	"strings"
	"sync"
	"time"
)

// ── Constants ────────────────────────────────────────────────────────────────

const (
	// DefaultAuditLog is the default audit log file path.
	DefaultAuditLog = "/var/log/volt/audit.log"

	// DefaultAuditDir is the default audit log directory.
	DefaultAuditDir = "/var/log/volt"

	// MaxLogSize is the max size of a single log file before rotation (50MB).
	MaxLogSize = 50 * 1024 * 1024

	// MaxLogFiles is the max number of rotated log files to keep.
	MaxLogFiles = 10
)

// ── Audit Entry ──────────────────────────────────────────────────────────────

// Entry represents a single audit log entry.
type Entry struct {
	Timestamp   string `json:"timestamp"`              // ISO 8601
	ID          string `json:"id"`                     // Unique event ID
	User        string `json:"user"`                   // Username
	UID         int    `json:"uid"`                    // User ID
	Source      string `json:"source"`                 // "cli", "api", "sso"
	Action      string `json:"action"`                 // e.g., "container.create"
	Resource    string `json:"resource,omitempty"`      // e.g., "web-app"
	Command     string `json:"command"`                // Full command string
	Args        []string `json:"args,omitempty"`        // Command arguments
	Result      string `json:"result"`                 // "success" or "failure"
	Error       string `json:"error,omitempty"`         // Error message if failure
	Hostname    string `json:"hostname"`               // Node hostname
	SourceIP    string `json:"source_ip,omitempty"`     // For API calls
	SessionID   string `json:"session_id,omitempty"`    // CLI session ID
	Duration    string `json:"duration,omitempty"`      // Command execution time
	Signature   string `json:"signature,omitempty"`     // HMAC-SHA256 for tamper evidence
}

// ── Logger ───────────────────────────────────────────────────────────────────

// Logger handles audit log writing.
type Logger struct {
	logPath    string
	hmacKey    []byte // nil = no signing
	mu         sync.Mutex
	file       *os.File
	syslogFwd  bool
}

// NewLogger creates an audit logger.
func NewLogger(logPath string) *Logger {
	if logPath == "" {
		logPath = DefaultAuditLog
	}
	return &Logger{
		logPath: logPath,
	}
}

// SetHMACKey enables tamper-evident signing with the given key.
func (l *Logger) SetHMACKey(key []byte) {
	l.hmacKey = key
}

// EnableSyslog enables forwarding audit entries to syslog.
func (l *Logger) EnableSyslog(enabled bool) {
	l.syslogFwd = enabled
}

// Log writes an audit entry to the log file.
func (l *Logger) Log(entry Entry) error {
	l.mu.Lock()
	defer l.mu.Unlock()

	// Fill in defaults
	if entry.Timestamp == "" {
		entry.Timestamp = time.Now().UTC().Format(time.RFC3339Nano)
	}
	if entry.ID == "" {
		entry.ID = generateEventID()
	}
	if entry.Hostname == "" {
		entry.Hostname, _ = os.Hostname()
	}
	if entry.User == "" {
		if u, err := user.Current(); err == nil {
			entry.User = u.Username
			// UID parsing handled by the caller
		}
	}
	if entry.UID == 0 {
		entry.UID = os.Getuid()
	}
	if entry.Source == "" {
		entry.Source = "cli"
	}

	// Sign the entry if HMAC key is set
	if l.hmacKey != nil {
		entry.Signature = l.signEntry(entry)
	}

	// Serialize to JSON
	data, err := json.Marshal(entry)
	if err != nil {
		return fmt.Errorf("audit: marshal entry: %w", err)
	}

	// Ensure log directory exists
	dir := filepath.Dir(l.logPath)
	if err := os.MkdirAll(dir, 0750); err != nil {
		return fmt.Errorf("audit: create dir: %w", err)
	}

	// Check rotation
	if err := l.rotateIfNeeded(); err != nil {
		// Log rotation failure shouldn't block audit logging
		fmt.Fprintf(os.Stderr, "audit: rotation warning: %v\n", err)
	}

	// Open/reopen file
	if l.file == nil {
		f, err := os.OpenFile(l.logPath, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0640)
		if err != nil {
			return fmt.Errorf("audit: open log: %w", err)
		}
		l.file = f
	}

	// Write entry (one JSON object per line)
	if _, err := l.file.Write(append(data, '\n')); err != nil {
		return fmt.Errorf("audit: write entry: %w", err)
	}

	// Syslog forwarding
	if l.syslogFwd {
		l.forwardToSyslog(entry)
	}

	return nil
}

// Close closes the audit log file.
func (l *Logger) Close() error {
	l.mu.Lock()
	defer l.mu.Unlock()
	if l.file != nil {
		err := l.file.Close()
		l.file = nil
		return err
	}
	return nil
}

// LogCommand is a convenience method for logging CLI commands.
func (l *Logger) LogCommand(action, resource, command string, args []string, err error) error {
	entry := Entry{
		Action:   action,
		Resource: resource,
		Command:  command,
		Args:     args,
		Result:   "success",
	}
	if err != nil {
		entry.Result = "failure"
		entry.Error = err.Error()
	}
	return l.Log(entry)
}

// ── Search ───────────────────────────────────────────────────────────────────

// SearchOptions configures audit log search.
type SearchOptions struct {
	User     string
	Action   string
	Resource string
	Result   string
	Since    time.Time
	Until    time.Time
	Limit    int
}

// Search reads and filters audit log entries.
func Search(logPath string, opts SearchOptions) ([]Entry, error) {
	if logPath == "" {
		logPath = DefaultAuditLog
	}

	data, err := os.ReadFile(logPath)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, nil
		}
		return nil, fmt.Errorf("audit: read log: %w", err)
	}

	lines := strings.Split(strings.TrimSpace(string(data)), "\n")
	var results []Entry

	for _, line := range lines {
		if line == "" {
			continue
		}

		var entry Entry
		if err := json.Unmarshal([]byte(line), &entry); err != nil {
			continue // Skip malformed entries
		}

		// Apply filters
		if opts.User != "" && entry.User != opts.User {
			continue
		}
		if opts.Action != "" && !matchAction(entry.Action, opts.Action) {
			continue
		}
		if opts.Resource != "" && entry.Resource != opts.Resource {
			continue
		}
		if opts.Result != "" && entry.Result != opts.Result {
			continue
		}
		if !opts.Since.IsZero() {
			entryTime, err := time.Parse(time.RFC3339Nano, entry.Timestamp)
			if err != nil || entryTime.Before(opts.Since) {
				continue
			}
		}
		if !opts.Until.IsZero() {
			entryTime, err := time.Parse(time.RFC3339Nano, entry.Timestamp)
			if err != nil || entryTime.After(opts.Until) {
				continue
			}
		}

		results = append(results, entry)

		if opts.Limit > 0 && len(results) >= opts.Limit {
			break
		}
	}

	return results, nil
}

// matchAction checks if an action matches a filter pattern.
// Supports prefix matching: "container" matches "container.create", "container.delete", etc.
func matchAction(action, filter string) bool {
	if action == filter {
		return true
	}
	return strings.HasPrefix(action, filter+".")
}

// Verify checks the HMAC signatures of audit log entries.
func Verify(logPath string, hmacKey []byte) (total, valid, invalid, unsigned int, err error) {
	if logPath == "" {
		logPath = DefaultAuditLog
	}

	data, err := os.ReadFile(logPath)
	if err != nil {
		return 0, 0, 0, 0, fmt.Errorf("audit: read log: %w", err)
	}

	lines := strings.Split(strings.TrimSpace(string(data)), "\n")
	l := &Logger{hmacKey: hmacKey}

	for _, line := range lines {
		if line == "" {
			continue
		}

		var entry Entry
		if err := json.Unmarshal([]byte(line), &entry); err != nil {
			continue
		}

		total++

		if entry.Signature == "" {
			unsigned++
			continue
		}

		// Recompute signature and compare
		savedSig := entry.Signature
		entry.Signature = ""
		expected := l.signEntry(entry)

		if savedSig == expected {
			valid++
		} else {
			invalid++
		}
	}

	return total, valid, invalid, unsigned, nil
}

// ── Internal ─────────────────────────────────────────────────────────────────

// signEntry computes HMAC-SHA256 over the entry's key fields.
func (l *Logger) signEntry(entry Entry) string {
	// Build canonical string from entry fields (excluding signature)
	canonical := fmt.Sprintf("%s|%s|%s|%d|%s|%s|%s|%s|%s",
		entry.Timestamp,
		entry.ID,
		entry.User,
		entry.UID,
		entry.Source,
		entry.Action,
		entry.Resource,
		entry.Command,
		entry.Result,
	)

	mac := hmac.New(sha256.New, l.hmacKey)
	mac.Write([]byte(canonical))
	return hex.EncodeToString(mac.Sum(nil))
}

// rotateIfNeeded checks if the current log file exceeds MaxLogSize and rotates.
func (l *Logger) rotateIfNeeded() error {
	info, err := os.Stat(l.logPath)
	if err != nil {
		return nil // File doesn't exist yet, no rotation needed
	}

	if info.Size() < MaxLogSize {
		return nil
	}

	// Close current file
	if l.file != nil {
		l.file.Close()
		l.file = nil
	}

	// Rotate: audit.log → audit.log.1, audit.log.1 → audit.log.2, etc.
	for i := MaxLogFiles - 1; i >= 1; i-- {
		old := fmt.Sprintf("%s.%d", l.logPath, i)
		new := fmt.Sprintf("%s.%d", l.logPath, i+1)
		os.Rename(old, new)
	}
	os.Rename(l.logPath, l.logPath+".1")

	// Remove oldest if over limit
	oldest := fmt.Sprintf("%s.%d", l.logPath, MaxLogFiles+1)
	os.Remove(oldest)

	return nil
}

// forwardToSyslog sends an audit entry to the system logger.
func (l *Logger) forwardToSyslog(entry Entry) {
	msg := fmt.Sprintf("volt-audit: user=%s action=%s resource=%s result=%s",
		entry.User, entry.Action, entry.Resource, entry.Result)
	if entry.Error != "" {
		msg += " error=" + entry.Error
	}
	// Use logger command for syslog forwarding (no direct syslog dependency)
	// This is fire-and-forget — we don't want syslog failures to block audit
	cmd := fmt.Sprintf("logger -t volt-audit -p auth.info '%s'", msg)
	_ = os.WriteFile("/dev/null", []byte(cmd), 0) // placeholder; real impl would exec
}

// generateEventID creates a unique event ID based on timestamp.
func generateEventID() string {
	return fmt.Sprintf("evt-%d", time.Now().UnixNano()/int64(time.Microsecond))
}

// ── Global Logger ────────────────────────────────────────────────────────────

var (
	globalLogger     *Logger
	globalLoggerOnce sync.Once
)

// DefaultLogger returns the global audit logger (singleton).
func DefaultLogger() *Logger {
	globalLoggerOnce.Do(func() {
		globalLogger = NewLogger("")
	})
	return globalLogger
}

// LogAction is a convenience function using the global logger.
func LogAction(action, resource string, cmdArgs []string, err error) {
	command := "volt"
	if len(cmdArgs) > 0 {
		command = "volt " + strings.Join(cmdArgs, " ")
	}
	_ = DefaultLogger().LogCommand(action, resource, command, cmdArgs, err)
}