Add enumerate methods to the multicodec registries.

(This is a logical port of
#169 to follow
#172 .)

I've added more documentation -- particularly, cautionary notes on the
package-scope functions which read the global shared state.

multicodec/defaultRegistry.go

@@ -53,6 +53,22 @@ func LookupEncoder(indicator uint64) (ipld.Encoder, error) {
 	return DefaultRegistry.LookupEncoder(indicator)
 }
 
+// ListEncoders returns a list of multicodec indicators for which an ipld.Encoder is registered.
+// The list is in no particular order.
+// It is a shortcut to the ListEncoders method on the global DefaultRegistry.
+//
+// Be judicious about trying to use this function outside of debugging.
+// Because the global default registry is global and easily modified,
+// and can be changed by any of the transitive dependencies of your program,
+// its contents are not particularly stable.
+// In particular, it is not recommended to make any behaviors of your program conditional
+// based on information returned by this function -- if your program needs conditional
+// behavior based on registred codecs, you may want to consider taking more explicit control
+// and using your own non-default registry.
+func ListEncoders() []uint64 {
+	return DefaultRegistry.ListEncoders()
+}
+
 // RegisterDecoder updates the global DefaultRegistry a map a multicodec indicator number to the given ipld.Decoder function.
 // The decoder functions registered can be subsequently looked up using LookupDecoder.
 // It is a shortcut to the RegisterDecoder method on the global DefaultRegistry.
@@ -83,3 +99,19 @@ func RegisterDecoder(indicator uint64, decodeFunc ipld.Decoder) {
 func LookupDecoder(indicator uint64) (ipld.Decoder, error) {
 	return DefaultRegistry.LookupDecoder(indicator)
 }
+
+// ListDecoders returns a list of multicodec indicators for which an ipld.Decoder is registered.
+// The list is in no particular order.
+// It is a shortcut to the ListDecoders method on the global DefaultRegistry.
+//
+// Be judicious about trying to use this function outside of debugging.
+// Because the global default registry is global and easily modified,
+// and can be changed by any of the transitive dependencies of your program,
+// its contents are not particularly stable.
+// In particular, it is not recommended to make any behaviors of your program conditional
+// based on information returned by this function -- if your program needs conditional
+// behavior based on registred codecs, you may want to consider taking more explicit control
+// and using your own non-default registry.
+func ListDecoders() []uint64 {
+	return DefaultRegistry.ListDecoders()
+}

multicodec/registry.go

@@ -60,6 +60,16 @@ func (r *Registry) LookupEncoder(indicator uint64) (ipld.Encoder, error) {
 	return encodeFunc, nil
 }
 
+// ListEncoders returns a list of multicodec indicators for which an ipld.Encoder is registered.
+// The list is in no particular order.
+func (r *Registry) ListEncoders() []uint64 {
+	encoders := make([]uint64, 0, len(r.encoders))
+	for e := range r.encoders {
+		encoders = append(encoders, e)
+	}
+	return encoders
+}
+
 // RegisterDecoder updates a simple map of multicodec indicator number to ipld.Decoder function.
 // The decoder functions registered can be subsequently looked up using LookupDecoder.
 func (r *Registry) RegisterDecoder(indicator uint64, decodeFunc ipld.Decoder) {
@@ -81,3 +91,13 @@ func (r *Registry) LookupDecoder(indicator uint64) (ipld.Decoder, error) {
 	}
 	return decodeFunc, nil
 }
+
+// ListDecoders returns a list of multicodec indicators for which an ipld.Decoder is registered.
+// The list is in no particular order.
+func (r *Registry) ListDecoders() []uint64 {
+	decoders := make([]uint64, 0, len(r.decoders))
+	for d := range r.decoders {
+		decoders = append(decoders, d)
+	}
+	return decoders
+}