backend-service/internal/logger/logger.go

package logger

import (
	"context"
	"encoding/json"
	"fmt"
	"log"
	"os"
	"runtime"
	"time"
)

// LogLevel represents the severity level of a log entry
type LogLevel string

const (
	DEBUG LogLevel = "DEBUG"
	INFO  LogLevel = "INFO"
	WARN  LogLevel = "WARN"
	ERROR LogLevel = "ERROR"
	FATAL LogLevel = "FATAL"
)

// LogEntry represents a structured log entry
type LogEntry struct {
	Timestamp string                 `json:"timestamp"`
	Level     LogLevel               `json:"level"`
	Message   string                 `json:"message"`
	Service   string                 `json:"service"`
	TraceID   string                 `json:"trace_id,omitempty"`
	UserID    string                 `json:"user_id,omitempty"`
	Fields    map[string]interface{} `json:"fields,omitempty"`
	Error     *ErrorDetails          `json:"error,omitempty"`
	Source    *SourceLocation        `json:"source,omitempty"`
}

// ErrorDetails contains error-specific information
type ErrorDetails struct {
	Type       string `json:"type"`
	Message    string `json:"message"`
	StackTrace string `json:"stack_trace,omitempty"`
}

// SourceLocation contains source code location information
type SourceLocation struct {
	File     string `json:"file"`
	Line     int    `json:"line"`
	Function string `json:"function"`
}

// Logger provides structured logging capabilities
type Logger struct {
	service string
	level   LogLevel
}

// New creates a new logger instance
func New(service string) *Logger {
	return &Logger{
		service: service,
		level:   INFO, // Default level
	}
}

// SetLevel sets the minimum log level
func (l *Logger) SetLevel(level LogLevel) {
	l.level = level
}

// GetLevel returns the current log level
func (l *Logger) GetLevel() LogLevel {
	return l.level
}

// IsLevelEnabled checks if a log level is enabled
func (l *Logger) IsLevelEnabled(level LogLevel) bool {
	return l.shouldLog(level)
}

// Debug logs a debug message
func (l *Logger) Debug(message string, fields ...map[string]interface{}) {
	if l.shouldLog(DEBUG) {
		l.log(DEBUG, message, nil, fields...)
	}
}

// Info logs an info message
func (l *Logger) Info(message string, fields ...map[string]interface{}) {
	if l.shouldLog(INFO) {
		l.log(INFO, message, nil, fields...)
	}
}

// Warn logs a warning message
func (l *Logger) Warn(message string, fields ...map[string]interface{}) {
	if l.shouldLog(WARN) {
		l.log(WARN, message, nil, fields...)
	}
}

// Error logs an error message
func (l *Logger) Error(message string, err error, fields ...map[string]interface{}) {
	if l.shouldLog(ERROR) {
		l.log(ERROR, message, err, fields...)
	}
}

// Fatal logs a fatal message and exits
func (l *Logger) Fatal(message string, err error, fields ...map[string]interface{}) {
	l.log(FATAL, message, err, fields...)
	os.Exit(1)
}

// WithContext creates a logger with context information
func (l *Logger) WithContext(ctx context.Context) *ContextLogger {
	return &ContextLogger{
		logger: l,
		ctx:    ctx,
	}
}

// WithFields creates a logger with predefined fields
func (l *Logger) WithFields(fields map[string]interface{}) *FieldLogger {
	return &FieldLogger{
		logger: l,
		fields: fields,
	}
}

// log performs the actual logging
func (l *Logger) log(level LogLevel, message string, err error, fields ...map[string]interface{}) {
	entry := LogEntry{
		Timestamp: time.Now().UTC().Format(time.RFC3339),
		Level:     level,
		Message:   message,
		Service:   l.service,
	}

	// Add fields if provided
	if len(fields) > 0 && fields[0] != nil {
		entry.Fields = fields[0]
	}

	// Add error details if provided
	if err != nil {
		entry.Error = &ErrorDetails{
			Type:    fmt.Sprintf("%T", err),
			Message: err.Error(),
		}

		// Add stack trace for errors and fatal logs
		if level == ERROR || level == FATAL {
			entry.Error.StackTrace = getStackTrace()
		}
	}

	// Add source location for errors and fatal logs
	if level == ERROR || level == FATAL {
		entry.Source = getSourceLocation(3) // Skip 3 frames: log, Error/Fatal, caller
	}

	// Output the log entry
	l.output(entry)
}

// shouldLog checks if the message should be logged based on level
func (l *Logger) shouldLog(level LogLevel) bool {
	levels := map[LogLevel]int{
		DEBUG: 0,
		INFO:  1,
		WARN:  2,
		ERROR: 3,
		FATAL: 4,
	}

	return levels[level] >= levels[l.level]
}

// output writes the log entry to the output
func (l *Logger) output(entry LogEntry) {
	jsonBytes, err := json.Marshal(entry)
	if err != nil {
		// Fallback to standard logging if JSON marshaling fails
		log.Printf("LOGGER_ERROR: Failed to marshal log entry: %v", err)
		log.Printf("%s [%s] %s: %s", entry.Timestamp, entry.Level, entry.Service, entry.Message)
		return
	}

	fmt.Println(string(jsonBytes))
}

// getStackTrace returns the current stack trace
func getStackTrace() string {
	buf := make([]byte, 4096)
	n := runtime.Stack(buf, false)
	return string(buf[:n])
}

// getSourceLocation returns the source location information
func getSourceLocation(skip int) *SourceLocation {
	pc, file, line, ok := runtime.Caller(skip)
	if !ok {
		return nil
	}

	fn := runtime.FuncForPC(pc)
	if fn == nil {
		return nil
	}

	return &SourceLocation{
		File:     file,
		Line:     line,
		Function: fn.Name(),
	}
}

// ContextLogger wraps a logger with context information
type ContextLogger struct {
	logger *Logger
	ctx    context.Context
}

// Debug logs a debug message with context
func (cl *ContextLogger) Debug(message string, fields ...map[string]interface{}) {
	cl.logWithContext(DEBUG, message, nil, fields...)
}

// Info logs an info message with context
func (cl *ContextLogger) Info(message string, fields ...map[string]interface{}) {
	cl.logWithContext(INFO, message, nil, fields...)
}

// Warn logs a warning message with context
func (cl *ContextLogger) Warn(message string, fields ...map[string]interface{}) {
	cl.logWithContext(WARN, message, nil, fields...)
}

// Error logs an error message with context
func (cl *ContextLogger) Error(message string, err error, fields ...map[string]interface{}) {
	cl.logWithContext(ERROR, message, err, fields...)
}

// logWithContext logs with context information
func (cl *ContextLogger) logWithContext(level LogLevel, message string, err error, fields ...map[string]interface{}) {
	// Extract context information
	contextFields := make(map[string]interface{})

	// Add trace ID if available
	if traceID := cl.ctx.Value("trace_id"); traceID != nil {
		contextFields["trace_id"] = traceID
	}

	// Add user ID if available
	if userID := cl.ctx.Value("user_id"); userID != nil {
		contextFields["user_id"] = userID
	}

	// Merge with provided fields
	if len(fields) > 0 && fields[0] != nil {
		for k, v := range fields[0] {
			contextFields[k] = v
		}
	}

	cl.logger.log(level, message, err, contextFields)
}

// FieldLogger wraps a logger with predefined fields
type FieldLogger struct {
	logger *Logger
	fields map[string]interface{}
}

// Debug logs a debug message with predefined fields
func (fl *FieldLogger) Debug(message string, additionalFields ...map[string]interface{}) {
	fl.logWithFields(DEBUG, message, nil, additionalFields...)
}

// Info logs an info message with predefined fields
func (fl *FieldLogger) Info(message string, additionalFields ...map[string]interface{}) {
	fl.logWithFields(INFO, message, nil, additionalFields...)
}

// Warn logs a warning message with predefined fields
func (fl *FieldLogger) Warn(message string, additionalFields ...map[string]interface{}) {
	fl.logWithFields(WARN, message, nil, additionalFields...)
}

// Error logs an error message with predefined fields
func (fl *FieldLogger) Error(message string, err error, additionalFields ...map[string]interface{}) {
	fl.logWithFields(ERROR, message, err, additionalFields...)
}

// logWithFields logs with predefined fields
func (fl *FieldLogger) logWithFields(level LogLevel, message string, err error, additionalFields ...map[string]interface{}) {
	// Merge predefined fields with additional fields
	mergedFields := make(map[string]interface{})

	// Add predefined fields
	for k, v := range fl.fields {
		mergedFields[k] = v
	}

	// Add additional fields
	if len(additionalFields) > 0 && additionalFields[0] != nil {
		for k, v := range additionalFields[0] {
			mergedFields[k] = v
		}
	}

	fl.logger.log(level, message, err, mergedFields)
}

// Global logger instance
var globalLogger = New("app")

// SetGlobalLevel sets the global logger level
func SetGlobalLevel(level LogLevel) {
	globalLogger.SetLevel(level)
}

// Debug logs a debug message using the global logger
func Debug(message string, fields ...map[string]interface{}) {
	globalLogger.Debug(message, fields...)
}

// Info logs an info message using the global logger
func Info(message string, fields ...map[string]interface{}) {
	globalLogger.Info(message, fields...)
}

// Warn logs a warning message using the global logger
func Warn(message string, fields ...map[string]interface{}) {
	globalLogger.Warn(message, fields...)
}

// Error logs an error message using the global logger
func Error(message string, err error, fields ...map[string]interface{}) {
	globalLogger.Error(message, err, fields...)
}

// Fatal logs a fatal message using the global logger and exits
func Fatal(message string, err error, fields ...map[string]interface{}) {
	globalLogger.Fatal(message, err, fields...)
}