From a7e3cf08266d939e3b664c08ac97972ba53bde18 Mon Sep 17 00:00:00 2001
From: Damian Gryski <damian@gryski.com>
Date: Sat, 25 Feb 2023 17:45:29 -0800
Subject: [PATCH] reflect: add Slice3()

---
 src/reflect/value.go      | 27 +++++++++++++++++++++++++++
 src/reflect/value_test.go | 14 ++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/src/reflect/value.go b/src/reflect/value.go
index 3216ada5..70a9ed28 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -431,6 +431,33 @@ func (v Value) Slice(i, j int) Value {
 }
 
 func (v Value) Slice3(i, j, k int) Value {
+	switch v.Kind() {
+	case Slice:
+		hdr := *(*sliceHeader)(v.value)
+		i, j, k := uintptr(i), uintptr(j), uintptr(k)
+
+		if j < i || k < j || hdr.len < k {
+			slicePanic()
+		}
+
+		elemSize := v.typecode.underlying().elem().Size()
+
+		hdr.len = j - i
+		hdr.cap = k - i
+		hdr.data = unsafe.Add(hdr.data, i*elemSize)
+
+		return Value{
+			typecode: v.typecode,
+			value:    unsafe.Pointer(&hdr),
+			flags:    v.flags,
+		}
+
+	case Array:
+		// TODO(dgryski): can't do this yet because the resulting value needs type v.elem(), not array of v.elem().
+		// need to be able to look up this "new" type so pointer equality of types still works
+
+	}
+
 	panic("unimplemented: (reflect.Value).Slice3()")
 }
 
diff --git a/src/reflect/value_test.go b/src/reflect/value_test.go
index d70984d9..d7fdcc30 100644
--- a/src/reflect/value_test.go
+++ b/src/reflect/value_test.go
@@ -147,6 +147,20 @@ func TestSlice(t *testing.T) {
 		}
 	}
 
+	s268 := s[2:6:8]
+	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())
+	}
+
+	for i, got := range s268 {
+		want := int(s268ref.Index(i).Int())
+		if got != want {
+			t.Errorf("s268[%d]=%d, want %d", i, got, want)
+		}
+	}
+
 	refs = MakeSlice(TypeOf(s), 5, 10)
 	s = refs.Interface().([]int)