reflect: rtype.MethodByName using binary search

Change-Id: If36e9fd7d6b1993ca2d0d382e7fa52212170c798
Reviewed-on: https://go-review.googlesource.com/c/go/+/425481
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/reflect/all_test.go

@@ -2384,6 +2384,16 @@ func TestMethod(t *testing.T) {
 		t.Errorf("NoArgs returned %d values; want 0", n)
 	}
 
+	_, ok = TypeOf(&p).MethodByName("AA")
+	if ok {
+		t.Errorf(`MethodByName("AA") should have failed`)
+	}
+
+	_, ok = TypeOf(&p).MethodByName("ZZ")
+	if ok {
+		t.Errorf(`MethodByName("ZZ") should have failed`)
+	}
+
 	// Curried method of value.
 	tfunc := TypeOf((func(int) int)(nil))
 	v := ValueOf(p).Method(1)

src/reflect/type.go

@@ -892,12 +892,26 @@ func (t *rtype) MethodByName(name string) (m Method, ok bool) {
 	if ut == nil {
 		return Method{}, false
 	}
-	// TODO(mdempsky): Binary search.
-	for i, p := range ut.exportedMethods() {
-		if t.nameOff(p.name).name() == name {
-			return t.Method(i), true
+
+	methods := ut.exportedMethods()
+
+	// We are looking for the first index i where the string becomes >= s.
+	// This is a copy of sort.Search, with f(h) replaced by (t.nameOff(methods[h].name).name() >= name).
+	i, j := 0, len(methods)
+	for i < j {
+		h := int(uint(i+j) >> 1) // avoid overflow when computing h
+		// i ≤ h < j
+		if !(t.nameOff(methods[h].name).name() >= name) {
+			i = h + 1 // preserves f(i-1) == false
+		} else {
+			j = h // preserves f(j) == true
 		}
 	}
+	// i == j, f(i-1) == false, and f(j) (= f(i)) == true  =>  answer is i.
+	if i < len(methods) && name == t.nameOff(methods[i].name).name() {
+		return t.Method(i), true
+	}
+
 	return Method{}, false
 }