From 960a0b79bf3a4d4f5839159fc87b391ac1302ade Mon Sep 17 00:00:00 2001
From: Damian Gryski <damian@gryski.com>
Date: Tue, 28 Feb 2023 09:54:39 -0800
Subject: [PATCH] reflect: add SetLen()

---
 src/reflect/value.go      | 10 +++++++++-
 src/reflect/value_test.go | 17 +++++++++++++++--
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/src/reflect/value.go b/src/reflect/value.go
index e6c039f0..844c2e91 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -952,7 +952,15 @@ func (v Value) SetCap(n int) {
 }
 
 func (v Value) SetLen(n int) {
-	panic("unimplemented: (reflect.Value).SetLen()")
+	if v.typecode.Kind() != Slice {
+		panic(&ValueError{"reflect.Value.SetLen", v.Kind()})
+	}
+
+	hdr := (*sliceHeader)(v.value)
+	if int(uintptr(n)) != n || uintptr(n) > hdr.cap {
+		panic("reflect.Value.SetLen: slice length out of range")
+	}
+	hdr.len = uintptr(n)
 }
 
 func (v Value) checkAddressable() {
diff --git a/src/reflect/value_test.go b/src/reflect/value_test.go
index a50edd06..53dbc9cb 100644
--- a/src/reflect/value_test.go
+++ b/src/reflect/value_test.go
@@ -137,7 +137,7 @@ func TestSlice(t *testing.T) {
 	s28ref := refs.Slice(2, 8)
 
 	if len(s28) != s28ref.Len() || cap(s28) != s28ref.Cap() {
-		t.Errorf("len(s28)=%d s28ref.Len()=%d cap(s28)=%d s28ref.Cap()=%d\n", len(s28), s28ref.Len(), cap(s28), s28ref.Cap())
+		t.Errorf("Slice: len(s28)=%d s28ref.Len()=%d cap(s28)=%d s28ref.Cap()=%d\n", len(s28), s28ref.Len(), cap(s28), s28ref.Cap())
 	}
 
 	for i, got := range s28 {
@@ -151,7 +151,7 @@ func TestSlice(t *testing.T) {
 	s268ref := refs.Slice3(2, 6, 8)
 
 	if len(s268) != s268ref.Len() || cap(s268) != s268ref.Cap() {
-		t.Errorf("len(s268)=%d s268ref.Len()=%d cap(s268)=%d s268ref.Cap()=%d\n", len(s268), s268ref.Len(), cap(s268), s268ref.Cap())
+		t.Errorf("Slice3: len(s268)=%d s268ref.Len()=%d cap(s268)=%d s268ref.Cap()=%d\n", len(s268), s268ref.Len(), cap(s268), s268ref.Cap())
 	}
 
 	for i, got := range s268 {
@@ -161,6 +161,19 @@ func TestSlice(t *testing.T) {
 		}
 	}
 
+	// should be equivalent to s28 now, except for the capacity which doesn't change
+	s268ref.SetLen(6)
+	if len(s28) != s268ref.Len() || cap(s268) != s268ref.Cap() {
+		t.Errorf("SetLen: len(s268)=%d s268ref.Len()=%d cap(s268)=%d s268ref.Cap()=%d\n", len(s28), s268ref.Len(), cap(s268), s268ref.Cap())
+	}
+
+	for i, got := range s28 {
+		want := int(s268ref.Index(i).Int())
+		if got != want {
+			t.Errorf("s28[%d]=%d, want %d", i, got, want)
+		}
+	}
+
 	refs = MakeSlice(TypeOf(s), 5, 10)
 	s = refs.Interface().([]int)