json: codec functions now use codecCache

structs are not the only type kind which is capable of cycles:
of relevance to json, at least maps, slices,
and array pointers can be as well.

Thus we're restructuring the "seen" parameter concept from being
struct-specific to more broadly expressed in terms of codecs.

Author: extemporalgenome

json/codec.go

@@ -114,8 +114,9 @@ func typeid(t reflect.Type) unsafe.Pointer {
 	return (*iface)(unsafe.Pointer(&t)).ptr
 }
 
-func constructCachedCodec(t reflect.Type, cache map[unsafe.Pointer]codec) codec {
-	c := constructCodec(t, map[reflect.Type]*structType{}, t.Kind() == reflect.Ptr)
+func constructCachedCodec(t reflect.Type, cache codecCache) codec {
+	seen := make(codecCache)
+	c := constructCodec(t, seen, t.Kind() == reflect.Ptr)
 
 	if inlined(t) {
 		c.encode = constructInlineValueEncodeFunc(c.encode)
@@ -125,7 +126,7 @@ func constructCachedCodec(t reflect.Type, cache map[unsafe.Pointer]codec) codec
 	return c
 }
 
-func constructCodec(t reflect.Type, seen map[reflect.Type]*structType, canAddr bool) (c codec) {
+func constructCodec(t reflect.Type, seen codecCache, canAddr bool) (c codec) {
 	switch t {
 	case nullType, nil:
 		c = codec{encode: encoder.encodeNull, decode: decoder.decodeNull}
@@ -261,7 +262,7 @@ func constructCodec(t reflect.Type, seen map[reflect.Type]*structType, canAddr b
 	return
 }
 
-func constructStringCodec(t reflect.Type, seen map[reflect.Type]*structType, canAddr bool) codec {
+func constructStringCodec(t reflect.Type, seen codecCache, canAddr bool) codec {
 	c := constructCodec(t, seen, canAddr)
 	return codec{
 		encode: constructStringEncodeFunc(c.encode),
@@ -287,7 +288,7 @@ func constructStringToIntDecodeFunc(t reflect.Type, decode decodeFunc) decodeFun
 	}
 }
 
-func constructArrayCodec(t reflect.Type, seen map[reflect.Type]*structType, canAddr bool) codec {
+func constructArrayCodec(t reflect.Type, seen codecCache, canAddr bool) codec {
 	e := t.Elem()
 	c := constructCodec(e, seen, canAddr)
 	s := alignedSize(e)
@@ -311,7 +312,7 @@ func constructArrayDecodeFunc(size uintptr, t reflect.Type, decode decodeFunc) d
 	}
 }
 
-func constructSliceCodec(t reflect.Type, seen map[reflect.Type]*structType) codec {
+func constructSliceCodec(t reflect.Type, seen codecCache) codec {
 	e := t.Elem()
 	s := alignedSize(e)
 
@@ -378,7 +379,7 @@ func constructSliceDecodeFunc(size uintptr, t reflect.Type, decode decodeFunc) d
 	}
 }
 
-func constructMapCodec(t reflect.Type, seen map[reflect.Type]*structType) codec {
+func constructMapCodec(t reflect.Type, seen codecCache) codec {
 	var sortKeys sortFunc
 	k := t.Key()
 	v := t.Elem()
@@ -496,15 +497,15 @@ func constructMapDecodeFunc(t reflect.Type, decodeKey, decodeValue decodeFunc) d
 	}
 }
 
-func constructStructCodec(t reflect.Type, seen map[reflect.Type]*structType, canAddr bool) codec {
+func constructStructCodec(t reflect.Type, seen codecCache, canAddr bool) codec {
 	st := constructStructType(t, seen, canAddr)
 	return codec{
 		encode: constructStructEncodeFunc(st),
 		decode: constructStructDecodeFunc(st),
 	}
 }
 
-func constructStructType(t reflect.Type, seen map[reflect.Type]*structType, canAddr bool) *structType {
+func constructStructType(t reflect.Type, seen codecCache, canAddr bool) *structType {
 	// Used for preventing infinite recursion on types that have pointers to
 	// themselves.
 	st := seen[t]
@@ -577,7 +578,7 @@ func constructEmbeddedStructPointerDecodeFunc(t reflect.Type, unexported bool, o
 	}
 }
 
-func appendStructFields(fields []structField, t reflect.Type, offset uintptr, seen map[reflect.Type]*structType, canAddr bool) []structField {
+func appendStructFields(fields []structField, t reflect.Type, offset uintptr, seen codecCache, canAddr bool) []structField {
 	type embeddedField struct {
 		index      int
 		offset     uintptr
@@ -778,7 +779,7 @@ func encodeKeyFragment(s string, flags AppendFlags) string {
 	return *(*string)(unsafe.Pointer(&b))
 }
 
-func constructPointerCodec(t reflect.Type, seen map[reflect.Type]*structType) codec {
+func constructPointerCodec(t reflect.Type, seen codecCache) codec {
 	e := t.Elem()
 	c := constructCodec(e, seen, true)
 	return codec{