From 43a4b256bd216345cdf23ce47d520eba90353091 Mon Sep 17 00:00:00 2001
From: Damian Gryski <damian@gryski.com>
Date: Sat, 25 Feb 2023 17:42:51 -0800
Subject: [PATCH] reflect: add Slice()

---
 src/reflect/value.go      | 47 ++++++++++++++++++++++++++++++++++++++-
 src/reflect/value_test.go | 14 ++++++++++++
 2 files changed, 60 insertions(+), 1 deletion(-)

diff --git a/src/reflect/value.go b/src/reflect/value.go
index dce08795..3216ada5 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -382,7 +382,52 @@ func (v Value) Bytes() []byte {
 }
 
 func (v Value) Slice(i, j int) Value {
-	panic("unimplemented: (reflect.Value).Slice()")
+	switch v.Kind() {
+	case Slice:
+		hdr := *(*sliceHeader)(v.value)
+		i, j := uintptr(i), uintptr(j)
+
+		if j < i || hdr.cap < j {
+			slicePanic()
+		}
+
+		elemSize := v.typecode.underlying().elem().Size()
+
+		hdr.len = j - i
+		hdr.cap = hdr.cap - 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 slice of v.elem(), not array of v.elem().
+		// need to be able to look up this "new" type so pointer equality of types still works
+
+	case String:
+		i, j := uintptr(i), uintptr(j)
+		str := *(*stringHeader)(v.value)
+
+		if j < i || str.len < j {
+			slicePanic()
+		}
+
+		hdr := stringHeader{
+			data: unsafe.Add(str.data, i),
+			len:  j - i,
+		}
+
+		return Value{
+			typecode: v.typecode,
+			value:    unsafe.Pointer(&hdr),
+			flags:    v.flags,
+		}
+	}
+
+	panic(&ValueError{"Slice", v.Kind()})
 }
 
 func (v Value) Slice3(i, j, k int) Value {
diff --git a/src/reflect/value_test.go b/src/reflect/value_test.go
index 0c6e43d0..d70984d9 100644
--- a/src/reflect/value_test.go
+++ b/src/reflect/value_test.go
@@ -133,6 +133,20 @@ func TestSlice(t *testing.T) {
 		}
 	}
 
+	s28 := s[2:8]
+	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())
+	}
+
+	for i, got := range s28 {
+		want := int(s28ref.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)