From 59333ba1adf065efcca4c706071de0094f6ce5d9 Mon Sep 17 00:00:00 2001
From: Hajime Hoshi <hajimehoshi@gmail.com>
Date: Thu, 26 Sep 2019 23:59:00 +0900
Subject: [PATCH] jsutil: Avoid allocating ArrayBuffer unnecessarily

---
 internal/jsutil/go113_js.go | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/internal/jsutil/go113_js.go b/internal/jsutil/go113_js.go
index 40bb339f4..6d2c06745 100644
--- a/internal/jsutil/go113_js.go
+++ b/internal/jsutil/go113_js.go
@@ -96,44 +96,56 @@ func sliceToByteSlice(s interface{}) (bs []byte) {
 	return
 }
 
+var temporaryBuffer = js.Global().Get("ArrayBuffer").New(16)
+
+func getTemporaryUint8Array(size int) js.Value {
+	if l := temporaryBuffer.Get("byteLength").Int(); l < size {
+		for l < size {
+			l *= 2
+		}
+		temporaryBuffer = js.Global().Get("ArrayBuffer").New(l)
+	}
+	return js.Global().Get("Uint8Array").New(temporaryBuffer, 0, size)
+}
+
 func SliceToTypedArray(s interface{}) (js.Value, func()) {
 	switch s := s.(type) {
 	case []int8:
-		a := js.Global().Get("Uint8Array").New(len(s))
+		a := getTemporaryUint8Array(len(s))
 		js.CopyBytesToJS(a, sliceToByteSlice(s))
 		buf := a.Get("buffer")
 		return js.Global().Get("Int8Array").New(buf, a.Get("byteOffset"), a.Get("byteLength")), func() {}
 	case []int16:
-		a := js.Global().Get("Uint8Array").New(len(s) * 2)
+		a := getTemporaryUint8Array(len(s) * 2)
 		js.CopyBytesToJS(a, sliceToByteSlice(s))
 		buf := a.Get("buffer")
 		return js.Global().Get("Int16Array").New(buf, a.Get("byteOffset"), a.Get("byteLength").Int()/2), func() {}
 	case []int32:
-		a := js.Global().Get("Uint8Array").New(len(s) * 4)
+		a := getTemporaryUint8Array(len(s) * 4)
 		js.CopyBytesToJS(a, sliceToByteSlice(s))
 		buf := a.Get("buffer")
 		return js.Global().Get("Int32Array").New(buf, a.Get("byteOffset"), a.Get("byteLength").Int()/4), func() {}
 	case []uint8:
-		a := js.Global().Get("Uint8Array").New(len(s))
+		a := getTemporaryUint8Array(len(s))
 		js.CopyBytesToJS(a, s)
 		return a, func() {}
 	case []uint16:
-		a := js.Global().Get("Uint8Array").New(len(s) * 2)
+		a := getTemporaryUint8Array(len(s) * 2)
 		js.CopyBytesToJS(a, sliceToByteSlice(s))
 		buf := a.Get("buffer")
 		return js.Global().Get("Uint16Array").New(buf, a.Get("byteOffset"), a.Get("byteLength").Int()/2), func() {}
 	case []uint32:
-		a := js.Global().Get("Uint8Array").New(len(s) * 4)
+		a := getTemporaryUint8Array(len(s) * 4)
 		js.CopyBytesToJS(a, sliceToByteSlice(s))
 		buf := a.Get("buffer")
 		return js.Global().Get("Uint32Array").New(buf, a.Get("byteOffset"), a.Get("byteLength").Int()/4), func() {}
 	case []float32:
-		a := js.Global().Get("Uint8Array").New(len(s) * 4)
+		a := getTemporaryUint8Array(len(s) * 4)
 		js.CopyBytesToJS(a, sliceToByteSlice(s))
 		buf := a.Get("buffer")
 		return js.Global().Get("Float32Array").New(buf, a.Get("byteOffset"), a.Get("byteLength").Int()/4), func() {}
 	case []float64:
-		a := js.Global().Get("Uint8Array").New(len(s) * 8)
+		a := getTemporaryUint8Array(len(s) * 8)
 		js.CopyBytesToJS(a, sliceToByteSlice(s))
 		buf := a.Get("buffer")
 		return js.Global().Get("Float64Array").New(buf, a.Get("byteOffset"), a.Get("byteLength").Int()/8), func() {}