Golang logger with custom colour support

logger.go

package logger

import (
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"runtime"
	"strings"
	"sync"
	"time"
)

var mutex sync.Mutex // ensures atomic writes; protects the following fields
var f *os.File       // handle to open log file
var callerColours map[string]string

// SetCallerLogColours maps code filenames (e.g. `main` for `main.go`, `sort` for `sort.go`)
// to a custom colour code when using INFO log statements from that go file.
func SetCallerLogColours(colours map[string]string) {
	callerColours = colours
}

// Errorf prints a nicely formatted error to stderr
func Errorf(format string, args ...interface{}) {
	logLine := formatLogLine(format, args...)
	write(brightred("ERR " + " " + logLine))
}

// Warnf prints a nicely formatted warning to stderr
func Warnf(format string, args ...interface{}) {
	logLine := formatLogLine(format, args...)
	write(yellow("WARN" + " " + logLine))
}

// Infof prints a nicely formatted info line to stderr
func Infof(format string, args ...interface{}) {
	logLine := formatLogLine(format, args...)

	_, fn, _, _ := runtime.Caller(1)
	fn = basename(fn)

	if colour, ok := callerColours[fn]; ok {
		write(customColour("INFO "+logLine, colour))
	} else {
		write(blue("INFO " + logLine))
	}
}

// Debugf prints a nicely formatted debug line to stderr
func Debugf(format string, args ...interface{}) {
	logLine := formatLogLine(format, args...)
	write("----" + " " + logLine)
}

func init() {
	isodate := time.Now().Format("2006-01-02")

	for i := 0; i < 100; i++ {
		filename := fmt.Sprintf("log/%s_%.2d.log", isodate, i)

		if !fileExists(filename) {
			var err error
			if f, err = os.Create(filename); err != nil {
				panic(err)
			}

			break
		}
	}

	if f == nil {
		panic("couldn't find an unused log filename")
	}
}

func fileExists(filename string) bool {
	if _, err := os.Stat(filename); err == nil {
		// `filename` exists

		return true

	} else if errors.Is(err, os.ErrNotExist) {
		return false
		// `filename` does *not* exist

	} else {
		// Schrodinger: file may or may not exist. See err for details.
		// Therefore, do *NOT* use !os.IsNotExist(err) to test for file existence
		panic(err)
	}
}

func write(msg string) {
	mutex.Lock()
	defer mutex.Unlock()

	msg = fmt.Sprintf("%s %s", time.Now().Format("15:04:05.000"), msg)
	msg = strings.TrimSuffix(msg, "\n") + "\n"

	os.Stderr.WriteString(msg)
	f.WriteString(msg)
}

func formatLogLine(format string, args ...interface{}) string {
	_, filename, lineNumber, _ := runtime.Caller(2)
	msg := fmt.Sprintf(format, args...)
	return fmt.Sprintf("%s:%-3d %s", formatFilename(filename), lineNumber, msg)
}

func formatFilename(filename string) string {
	bn := basename(filename)
	if len(bn) > 6 {
		bn = bn[:5] + "…"
	}
	return fmt.Sprintf("%6s", bn)
}

func basename(filename string) string {
	return strings.TrimSuffix(filepath.Base(filename), filepath.Ext(filename))
}

func customColour(message, terminalCode string) string {
	return terminalCode + message + reset
}

func red(message string) string {
	return FgRed + message + reset
}

func brightred(message string) string {
	return FgBright + FgRed + message + reset
}

func yellow(message string) string {
	return FgYellow + message + reset
}

func cyan(message string) string {
	return FgCyan + message + reset
}

func brightcyan(message string) string {
	return FgBright + FgCyan + message + reset
}

func magenta(message string) string {
	return FgMagenta + message + reset
}

func brightmagenta(message string) string {
	return FgBright + FgMagenta + message + reset
}

func blue(message string) string {
	return FgBlue + message + reset
}

func brightblue(message string) string {
	return FgBright + FgBlue + message + reset
}

const (
	reset = "\x1b[0m"

	// FgBright makes a foreground colour bold/bright
	FgBright = "\x1b[1m"

	// FgRed sets the foreground text colour to red
	FgRed = "\x1b[31m"

	// FgYellow sets the foreground text colour to red
	FgYellow = "\x1b[33m"

	// FgBlue sets the foreground text colour to red
	FgBlue = "\x1b[34m"

	// FgMagenta sets the foreground text colour to red
	FgMagenta = "\x1b[35m"

	// FgCyan sets the foreground text colour to red
	FgCyan = "\x1b[36m"
)