4 files changed, 79 insertions, 0 deletions
diff --git a/common/orelse.go b/common/orelse.go
new file mode 100644
index 0000000..a96bde7
--- /dev/null
+++ b/common/orelse.go
@@ -0,0 +1,20 @@
+package common
+
+type Opt[T any] struct {
+	thing T
+	err   error
+}
+
+func Optional[T any](thing T, err error) Opt[T] {
+	return Opt[T]{
+		thing: thing,
+		err:   err,
+	}
+}
+
+func (o Opt[T]) OrElse(thing T) T {
+	if o.err != nil {
+		return thing
+	}
+	return o.thing
+}
diff --git a/common/retry.go b/common/retry.go
new file mode 100644
index 0000000..6b97ff3
--- /dev/null
+++ b/common/retry.go
@@ -0,0 +1,35 @@
+package common
+
+import (
+	"log"
+	"time"
+)
+
+// AutoRetry retries the supplier automatically, with given time limit and interval.
+// If maximum retry time limit is reached and the supplier still fails,
+// the last error will be returned.
+// If logger is not nil, retry information will be printed to it.
+func AutoRetry[T any](
+	supplier func() (T, error),
+	maxRetryTimes int,
+	retryInterval time.Duration,
+	logger *log.Logger) (T, error) {
+	var err error
+	for i := 0; i < maxRetryTimes; i++ {
+		ret, err := supplier()
+		if err != nil {
+			if logger != nil {
+				logger.Printf("Try %v/%v (sleep %vs): %v\n",
+					i, maxRetryTimes, retryInterval, err)
+			}
+			time.Sleep(retryInterval)
+			continue
+		}
+		// success
+		return ret, nil
+	}
+	if logger != nil {
+		logger.Printf("Max retry times reached, but it still fails. Last error: %v", err)
+	}
+	return *new(T), err
+}
diff --git a/common/types.go b/common/types.go
new file mode 100644
index 0000000..81456b4
--- /dev/null
+++ b/common/types.go
@@ -0,0 +1,3 @@
+package common
+
+type RoomId uint64
diff --git a/common/urlparse.go b/common/urlparse.go
new file mode 100644
index 0000000..dc72cee
--- /dev/null
+++ b/common/urlparse.go
@@ -0,0 +1,21 @@
+package common
+
+import (
+	"errors"
+	"net/url"
+	"strings"
+)
+
+// GetFileExtensionFromUrl
+// copied from https://elisegev.medium.com/get-a-file-extension-from-a-url-in-golang-5061d4a298a
+func GetFileExtensionFromUrl(rawUrl string) (string, error) {
+	u, err := url.Parse(rawUrl)
+	if err != nil {
+		return "", err
+	}
+	pos := strings.LastIndex(u.Path, ".")
+	if pos == -1 {
+		return "", errors.New("couldn't find a period to indicate a file extension")
+	}
+	return u.Path[pos+1 : len(u.Path)], nil
+}