package main

import (
	"bytes"
	"fmt"
	"log"
)

func main() {
	var (
		buf    bytes.Buffer
		logger = log.New(&buf, "[Prefix]", log.Lshortfile)
	)

	logger.Print("Hello!")

	// [Prefix]main.go:15: Hello!
	fmt.Print(&buf)
}

const (
	Ldate = 1 << iota
	Ltime
	Lmicroseconds
	Llongfile
	Lshortfile
	LUTC
	Lmsgprefix
	LstdFlags = Ldate | Ltime
)

if flag&Lmsgprefix == 0 {
	*buf = append(*buf, prefix...)
}

// Cheap integer to fixed-width decimal ASCII. Give a negative width to avoid zero-padding.
func itoa(buf *[]byte, i int, wid int) {
	// Assemble decimal in reverse order.
	var b [20]byte
	bp := len(b) - 1
	for i >= 10 || wid > 1 {
		wid--
		q := i / 10
		b[bp] = byte('0' + i - q*10)
		bp--
		i = q
	}
	// i < 10
	b[bp] = byte('0' + i)
	*buf = append(*buf, b[bp:]...)
}

type Logger struct {
	outMu sync.Mutex
	out   io.Writer // destination for output

	prefix    atomic.Pointer[string] // prefix on each line to identify the logger (but see Lmsgprefix)
	flag      atomic.Int32           // properties
	isDiscard atomic.Bool
}

func TestAtomic(t *testing.T) {
	var flag atomic.Int32
	val := int32(1024)
	flag.Store(val)
	assert.Equal(t, val, flag.Load())

	var prefix atomic.Pointer[string]
	str := "p"
	prefix.Store(&str)
	assert.Equal(t, str, *prefix.Load())
}

var bufferPool = sync.Pool{New: func() any { return new([]byte) }}

func getBuffer() *[]byte {
	p := bufferPool.Get().(*[]byte)
	*p = (*p)[:0]
	return p
}

func putBuffer(p *[]byte) {
	if cap(*p) > 64<<10 {
		*p = nil
	}
	bufferPool.Put(p)
}

// Caller reports file and line number information about function invocations on
// the calling goroutine's stack. The argument skip is the number of stack frames
// to ascend, with 0 identifying the caller of Caller.  (For historical reasons the
// meaning of skip differs between Caller and [Callers].) The return values report the
// program counter, file name, and line number within the file of the corresponding
// call. The boolean ok is false if it was not possible to recover the information.
func Caller(skip int) (pc uintptr, file string, line int, ok bool) {
	rpc := make([]uintptr, 1)
	n := callers(skip+1, rpc[:])
	if n < 1 {
		return
	}
	frame, _ := CallersFrames(rpc).Next()
	return frame.PC, frame.File, frame.Line, frame.PC != 0
}

func TestCaller(t *testing.T) {
	// Skip 0 means we're getting the Caller inside exampleFunction itself
	pc, file, line, ok := runtime.Caller(0)
	if !ok {
		fmt.Println("runtime.Caller failed")
		return
	}

	// Retrieve the function details using the program counter
	fn := runtime.FuncForPC(pc)
	functionName := fn.Name()

	t.Logf("Called from function: %s\n", functionName)
	t.Logf("In file: %s\n", file)
	t.Logf("At line: %d\n", line)
}

func (l *Logger) Printf(format string, v ...any) {
	l.output(0, 2, func(b []byte) []byte {
		return fmt.Appendf(b, format, v...)
	})
}

package log

import (
	"fmt"
	"io"
	"runtime"
	"sync"
	"sync/atomic"
	"time"
)

// These flags define which text to prefix to each log entry generated by the [Logger].
// Bits are or'ed together to control what's printed.
// With the exception of the Lmsgprefix flag, there is no
// control over the order they appear (the order listed here)
// or the format they present (as described in the comments).
// The prefix is followed by a colon only when Llongfile or Lshortfile
// is specified.
// For example, flags Ldate | Ltime (or LstdFlags) produce,
//
//	2009/01/23 01:23:23 message
//
// while flags Ldate | Ltime | Lmicroseconds | Llongfile produce,
//
//	2009/01/23 01:23:23.123123 /a/b/c/d.go:23: message
const (
	Ldate = 1 << iota
	Ltime
	Lmicroseconds
	Llongfile
	Lshortfile
	LUTC
	Lmsgprefix
	LstdFlags = Ldate | Ltime
)

// A Logger represents an active logging object that generates lines of
// output to an [io.Writer]. Each logging operation makes a single call to
// the Writer's Write method. A Logger can be used simultaneously from
// multiple goroutines; it guarantees to serialize access to the Writer.
type Logger struct {
	outMu sync.Mutex
	out   io.Writer // destination for output

	prefix    atomic.Pointer[string] // prefix on each line to identify the logger (but see Lmsgprefix)
	flag      atomic.Int32           // properties
	isDiscard atomic.Bool
}

// New creates a new [Logger]. The out variable sets the
// destination to which log data will be written.
// The prefix appears at the beginning of each generated log line, or
// after the log header if the [Lmsgprefix] flag is provided.
// The flag argument defines the logging properties.
func New(out io.Writer, prefix string, flag int) *Logger {
	l := new(Logger)
	l.SetOutput(out)
	l.SetPrefix(prefix)
	l.SetFlags(flag)
	return l
}

// SetOutput sets the output destination for the logger.
func (l *Logger) SetOutput(w io.Writer) {
	l.outMu.Lock()
	defer l.outMu.Unlock()
	l.out = w
	l.isDiscard.Store(w == io.Discard)
}

// SetPrefix sets the output prefix for the logger.
func (l *Logger) SetPrefix(prefix string) {
	l.prefix.Store(&prefix)
}

// SetFlags sets the output flags for the logger.
// The flag bits are [Ldate], [Ltime], and so on.
func (l *Logger) SetFlags(flag int) {
	l.flag.Store(int32(flag))
}

// Writer returns the output destination for the logger.
func (l *Logger) Writer() io.Writer {
	l.outMu.Lock()
	defer l.outMu.Unlock()
	return l.out
}

// Prefix returns the output prefix for the logger.
func (l *Logger) Prefix() string {
	if p := l.prefix.Load(); p != nil {
		return *p
	}
	return ""
}

// Flags returns the output flags for the logger.
// The flag bits are [Ldate], [Ltime], and so on.
func (l *Logger) Flags() int {
	return int(l.flag.Load())
}

// Cheap integer to fixed-width decimal ASCII. Give a negative width to avoid zero-padding.
func itoa(buf *[]byte, i int, wid int) {
	// Assemble decimal in reverse order.
	var b [20]byte
	bp := len(b) - 1
	for i >= 10 || wid > 1 {
		wid--
		q := i / 10
		b[bp] = byte('0' + i - q*10)
		bp--
		i = q
	}
	// i < 10
	b[bp] = byte('0' + i)
	*buf = append(*buf, b[bp:]...)
}

// formatHeader writes log header to buf in following order:
//   - l.prefix (if it's not blank and Lmsgprefix is unset),
//   - date and/or time (if corresponding flags are provided),
//   - file and line number (if corresponding flags are provided),
//   - l.prefix (if it's not blank and Lmsgprefix is set).
func formatHeader(buf *[]byte, t time.Time, prefix string, flag int, file string, line int) {
	if flag&Lmsgprefix == 0 {
		*buf = append(*buf, prefix...)
	}
	if flag&(Ldate|Ltime|Lmicroseconds) != 0 {
		if flag&LUTC != 0 {
			t = t.UTC()
		}
		if flag&Ldate != 0 {
			year, month, day := t.Date()
			itoa(buf, year, 4)
			*buf = append(*buf, '/')
			itoa(buf, int(month), 2)
			*buf = append(*buf, '/')
			itoa(buf, day, 2)
			*buf = append(*buf, ' ')
		}
		if flag&(Ltime|Lmicroseconds) != 0 {
			hour, min, sec := t.Clock()
			itoa(buf, hour, 2)
			*buf = append(*buf, ':')
			itoa(buf, min, 2)
			*buf = append(*buf, ':')
			itoa(buf, sec, 2)
			if flag&Lmicroseconds != 0 {
				*buf = append(*buf, '.')
				itoa(buf, t.Nanosecond()/1e3, 6)
			}
			*buf = append(*buf, ' ')
		}
	}
	if flag&(Lshortfile|Llongfile) != 0 {
		if flag&Lshortfile != 0 {
			short := file
			for i := len(file) - 1; i > 0; i-- {
				if file[i] == '/' {
					short = file[i+1:]
					break
				}
			}
			file = short
		}
		*buf = append(*buf, file...)
		*buf = append(*buf, ':')
		itoa(buf, line, -1)
		*buf = append(*buf, ": "...)
	}
	if flag&Lmsgprefix != 0 {
		*buf = append(*buf, prefix...)
	}
}

var bufferPool = sync.Pool{New: func() any { return new([]byte) }}

func getBuffer() *[]byte {
	p := bufferPool.Get().(*[]byte)
	*p = (*p)[:0]
	return p
}

func putBuffer(p *[]byte) {
	if cap(*p) > 64<<10 {
		*p = nil
	}
	bufferPool.Put(p)
}

// output can take either a calldepth or a pc to get source line information.
// It uses the pc if it is non-zero.
func (l *Logger) output(pc uintptr, calldepth int, appendOutput func([]byte) []byte) error {
	if l.isDiscard.Load() {
		return nil
	}

	now := time.Now() // get this early.

	// Load prefix and flag once so that their value is consistent within
	// this call regardless of any concurrent changes to their value.
	prefix := l.Prefix()
	flag := l.Flags()

	var file string
	var line int
	if flag&(Lshortfile|Llongfile) != 0 {
		if pc == 0 {
			var ok bool
			_, file, line, ok = runtime.Caller(calldepth)
			if !ok {
				file = "???"
				line = 0
			}
		} else {
			fs := runtime.CallersFrames([]uintptr{pc})
			f, _ := fs.Next()
			file = f.File
			if file == "" {
				file = "???"
			}
			line = f.Line
		}
	}

	buf := getBuffer()
	defer putBuffer(buf)
	formatHeader(buf, now, prefix, flag, file, line)
	*buf = appendOutput(*buf)
	if len(*buf) == 0 || (*buf)[len(*buf)-1] != '\n' {
		*buf = append(*buf, '\n')
	}

	l.outMu.Lock()
	defer l.outMu.Unlock()
	_, err := l.out.Write(*buf)
	return err
}

// Printf calls l.Output to print to the logger.
// Arguments are handled in the manner of [fmt.Printf].
func (l *Logger) Printf(format string, v ...any) {
	l.output(0, 2, func(b []byte) []byte {
		return fmt.Appendf(b, format, v...)
	})
}