diff --git a/README.md b/README.md
index cedf957..9b78389 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1 @@
-# common
-code that I re-use among many Go projects
+A bunch of Go packages that I use in multiple projects.
diff --git a/filerotate/filerotate.go b/filerotate/filerotate.go
new file mode 100644
index 0000000..32ed73a
--- /dev/null
+++ b/filerotate/filerotate.go
@@ -0,0 +1,197 @@
+package filerotate
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"sync"
+	"time"
+)
+
+type Config struct {
+	DidClose           func(path string, didRotate bool)
+	PathIfShouldRotate func(creationTime time.Time, now time.Time) string
+}
+
+type File struct {
+	sync.Mutex
+
+	// Path is the path of the current file
+	Path string
+
+	creationTime time.Time
+
+	//Location *time.Location
+
+	config Config
+	file   *os.File
+
+	// position in the file of last Write or Write2, exposed for tests
+	lastWritePos int64
+}
+
+func IsSameDay(t1, t2 time.Time) bool {
+	return t1.YearDay() == t2.YearDay()
+}
+
+func IsSameHour(t1, t2 time.Time) bool {
+	return t1.YearDay() == t2.YearDay() && t1.Hour() == t2.Hour()
+}
+
+func New(config *Config) (*File, error) {
+	if nil == config {
+		return nil, fmt.Errorf("must provide config")
+	}
+	if config.PathIfShouldRotate == nil {
+		return nil, fmt.Errorf("must provide config.ShouldRotate")
+	}
+	file := &File{
+		config: *config,
+	}
+	err := file.reopenIfNeeded()
+	if err != nil {
+		return nil, err
+	}
+	return file, nil
+}
+
+func NewDaily(dir string, didClose func(path string, didRotate bool)) (*File, error) {
+	daily := func(creationTime time.Time, now time.Time) string {
+		if IsSameDay(creationTime, now) {
+			return ""
+		}
+		name := now.Format("2006-01-02") + ".txt"
+		return filepath.Join(dir, name)
+	}
+
+	config := Config{
+		DidClose:           didClose,
+		PathIfShouldRotate: daily,
+	}
+	return New(&config)
+}
+
+func NewHourly(dir string, didClose func(path string, didRotate bool)) (*File, error) {
+	hourly := func(creationTime time.Time, now time.Time) string {
+		if IsSameHour(creationTime, now) {
+			return ""
+		}
+		name := now.Format("2006-01-02_15") + ".txt"
+		return filepath.Join(dir, name)
+	}
+	config := Config{
+		DidClose:           didClose,
+		PathIfShouldRotate: hourly,
+	}
+	return New(&config)
+}
+
+func (f *File) close(didRotate bool) error {
+	if f.file == nil {
+		return nil
+	}
+	err := f.file.Close()
+	f.file = nil
+	if err == nil && f.config.DidClose != nil {
+		f.config.DidClose(f.Path, didRotate)
+	}
+	return err
+}
+
+/*
+func nowInMaybeLocation(loc *time.Location) time.Time {
+	now := time.Now()
+	if loc != nil {
+		now = now.In(loc)
+	}
+	return now
+}
+*/
+
+func (f *File) open(path string) error {
+	f.Path = path
+	f.creationTime = time.Now()
+	// we can't assume that the dir for the file already exists
+	dir := filepath.Dir(f.Path)
+	err := os.MkdirAll(dir, 0755)
+	if err != nil {
+		return err
+	}
+
+	// would be easier to open with os.O_APPEND but Seek() doesn't work in that case
+	flag := os.O_CREATE | os.O_WRONLY
+	f.file, err = os.OpenFile(f.Path, flag, 0644)
+	if err != nil {
+		return err
+	}
+	_, err = f.file.Seek(0, io.SeekEnd)
+	return err
+}
+
+func (f *File) reopenIfNeeded() error {
+	now := time.Now()
+	newPath := f.config.PathIfShouldRotate(f.creationTime, now)
+	if newPath == "" {
+		return nil
+	}
+	err := f.close(true)
+	if err != nil {
+		return err
+	}
+	return f.open(newPath)
+}
+
+func (f *File) write(d []byte, flush bool) (int64, int, error) {
+	err := f.reopenIfNeeded()
+	if err != nil {
+		return 0, 0, err
+	}
+	f.lastWritePos, err = f.file.Seek(0, io.SeekCurrent)
+	if err != nil {
+		return 0, 0, err
+	}
+	n, err := f.file.Write(d)
+	if err != nil {
+		return 0, n, err
+	}
+	if flush {
+		err = f.file.Sync()
+	}
+	return f.lastWritePos, n, err
+}
+
+// Write writes data to a file
+func (f *File) Write(d []byte) (int, error) {
+	f.Lock()
+	defer f.Unlock()
+
+	_, n, err := f.write(d, false)
+	return n, err
+}
+
+// Write2 writes data to a file, optionally flushes. To enable users to later
+// seek to where the data was written, it returns name of the file where data
+// was written, offset at which the data was written, number of bytes and error
+func (f *File) Write2(d []byte, flush bool) (string, int64, int, error) {
+	f.Lock()
+	defer f.Unlock()
+
+	writtenAtPos, n, err := f.write(d, flush)
+	return f.Path, writtenAtPos, n, err
+}
+
+func (f *File) Close() error {
+	f.Lock()
+	defer f.Unlock()
+
+	return f.close(false)
+}
+
+// Flush flushes the file
+func (f *File) Flush() error {
+	f.Lock()
+	defer f.Unlock()
+
+	return f.file.Sync()
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..56592b7
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module github.com/kjk/common
+
+go 1.17