Fastmsgpack is a Golang msgpack decoder. It is fast, but lacks features you might need. It can be used in combination with other msgpack libraries for e.g. encoding.

• It is very fast to query a list of fields from a nested msgpack structure.
• It is zero copy for strings and []byte.
• No reflection usage when decoding.

• It can only encode Go builtin types.
• The return value might contain pointers to the original data, so you can't modify the input data until you're done with the return value.
• It uses unsafe (to cast []byte to string without copying).
• It can't deserialize into your structs.
• It only supports strings as map keys.
• It decodes all ints as a Go int, including 64 bit ones, so it doesn't work on 32-bit platforms.

• It supports extension -1 and decodes such values into a time.Time.
• It supports extension -128 (interned strings). I didn't find any documentation for it, but I've based it on https://github.com/vmihailenco/msgpack. You can pass a dict to the decoder and it will replace indexes into that dict with the strings from the dict.
• It introduces extension 17 which wraps map and arrays. Because extension headers contain the byte-length, partial decoding can efficiently skip over those values.
• It introduces extension 18 which is like a Switch statement inside the data. (Whether that's a good idea is up for debate.) We use this to pack data for multiple locales in one value.
• It introduces extension 19 which encodes void. When decoding a void as for example a map value the key and value are treated as non-existent.
• It introduces extension 20 which injects a piece of msgpack in that place. It's a placeholder for data to be specified when decoding.

Decode returns an any, which is either int, float32, float64, string, []byte, []any, map[string]any or time.Time.

(*Resolver).Resolve returns a list of such anys, one for each field requested.

r := fastmsgpack.NewResolver([]string{"person.properties.firstName", "person.properties.age"}, nil)
found, err := r.Resolve(data)
firstName, ok := found[0].(string)
age, ok := found[1].(int)