From 6f4d89045a2129f1aef73fd86d60c4d3d673e036 Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Tue, 21 Feb 2023 20:24:07 +0530
Subject: [PATCH] A nicer implementation of sync.Once

Doesnt require storing the result of the function in a dedicated global
variable with a dedicated getter function
---
 tools/utils/once.go      | 35 +++++++++++++++++++++++++++++++++++
 tools/utils/once_test.go | 24 ++++++++++++++++++++++++
 2 files changed, 59 insertions(+)
 create mode 100644 tools/utils/once.go
 create mode 100644 tools/utils/once_test.go

diff --git a/tools/utils/once.go b/tools/utils/once.go
new file mode 100644
index 000000000..03734448d
--- /dev/null
+++ b/tools/utils/once.go
@@ -0,0 +1,35 @@
+// License: GPLv3 Copyright: 2023, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package utils
+
+import (
+	"fmt"
+	"sync"
+	"sync/atomic"
+)
+
+var _ = fmt.Print
+
+type Once[T any] struct {
+	done       uint32
+	mutex      sync.Mutex
+	cached_val T
+
+	Run func() T
+}
+
+func (self *Once[T]) Get() T {
+	if atomic.LoadUint32(&self.done) == 0 {
+		self.do_slow()
+	}
+	return self.cached_val
+}
+
+func (self *Once[T]) do_slow() {
+	self.mutex.Lock()
+	defer self.mutex.Unlock()
+	if atomic.LoadUint32(&self.done) == 0 {
+		defer atomic.StoreUint32(&self.done, 1)
+		self.cached_val = self.Run()
+	}
+}
diff --git a/tools/utils/once_test.go b/tools/utils/once_test.go
new file mode 100644
index 000000000..c748dd16a
--- /dev/null
+++ b/tools/utils/once_test.go
@@ -0,0 +1,24 @@
+// License: GPLv3 Copyright: 2023, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package utils
+
+import (
+	"fmt"
+	"testing"
+)
+
+var _ = fmt.Print
+
+func TestOnce(t *testing.T) {
+	num := 0
+	var G = (&Once[string]{Run: func() string {
+		num++
+		return fmt.Sprintf("%d", num)
+	}}).Get
+	G()
+	G()
+	G()
+	if num != 1 {
+		t.Fatalf("num unexpectedly: %d", num)
+	}
+}