散列反射类型

Question

I'm trying to find a quick way of performing comparisons between two []reflect.Type. Right now I have the following:

func Equal(left, right []reflect.Type) bool {
    if len(left) != len(right) {
        return false
    }
    for i := 0; i < len(left); i++ {
        if left[i] != right[i] {
            return false
        }
    }
    return true
}

Most of the slices don't change. So if I can find a way to hash them, I'd get a huge perf boost.

Background

I'm trying to (for fun) implement a form of function overloading in Go using the reflect package. The first thing I did is to convert each specialised/overloaded function into a signature type.

type Signature struct {
    Variadic bool
    In, Out  []reflect.Type
}

The idea is that, when the overloaded function gets called, I'll convert the arguments into a slice of reflect.Type and then find a Signature where the In types match.

This works, but for each comparison, it's a linear scan which is pretty slow. If I could hash the slice of []reflect.Type I could stick that in a map and get constant time lookups.

Answer 1

I ended up abusing the built-in map to assign unique ids to each reflect.Type. Then I hash those using djb2.

type TypeCode struct {
    seq   int64
    codes map[reflect.Type]int64
}

func (td *TypeCode) TypeID(t reflect.Type) int64 {
    if code, ok := td.codes[t]; ok {
        return code
    }
    td.seq++
    td.codes[t] = td.seq
    return td.seq
}

func (td *TypeCode) SliceTypeID(tt []reflect.Type) int64 {
    id := int64(5381)
    for _, t := range tt {
        id = ((id << 5) + id) + td.TypeID(t)
    }
    return id
}

edit: I switched to a string based approach which is less efficient, but removes any potential for collisions.

type TypeCode struct {
    seq   int64
    codes map[reflect.Type]string
}

func (td *TypeCode) TypeID(t reflect.Type) string {
    if code, ok := td.codes[t]; ok {
        return code
    }
    td.seq++
    id := strconv.FormatInt(td.seq, 10)
    td.codes[t] = id
    return id
}

func (td *TypeCode) SliceTypeID(tt []reflect.Type) string {
    ids := make([]string, len(tt))
    for i := 0; i < len(tt); i++ {
        ids[i] = td.TypeID(tt[i])
    }
    return strings.Join(ids, ".")
}