The deriveUnique function is useful for returning a list of unique items, when these items are not basic enough to be map keys.

Given the following input:

package unique

type Visitor struct {
	UserName   *string
	RemoteAddr string
}

func uniqueVisitors(vs []*Visitor) int {
	return len(deriveUnique(vs))
}

goderive will generate the following code:

// Code generated by goderive DO NOT EDIT.

package unique

// deriveUnique returns a list containing only the unique items from the input list.
// It does this by reusing the input list.
//
// Deprecated: In favour of generics.
func deriveUnique(list []*Visitor) []*Visitor {
	if len(list) == 0 {
		return nil
	}
	table := make(map[uint64][]int)
	u := 0
	for i := 0; i < len(list); i++ {
		contains := false
		hash := deriveHash(list[i])
		indexes := table[hash]
		for _, index := range indexes {
			if deriveEqual(list[index], list[i]) {
				contains = true
				break
			}
		}
		if contains {
			continue
		}
		if i != u {
			list[u] = list[i]
		}
		table[hash] = append(table[hash], u)
		u++
	}
	return list[:u]
}

// deriveEqual returns whether this and that are equal.
func deriveEqual(this, that *Visitor) bool {
	return (this == nil && that == nil) ||
		this != nil && that != nil &&
			((this.UserName == nil && that.UserName == nil) || (this.UserName != nil && that.UserName != nil && *(this.UserName) == *(that.UserName))) &&
			this.RemoteAddr == that.RemoteAddr
}

// deriveHash returns the hash of the object.
func deriveHash(object *Visitor) uint64 {
	if object == nil {
		return 0
	}
	h := uint64(17)
	h = 31*h + deriveHash_(object.UserName)
	h = 31*h + deriveHash_s(object.RemoteAddr)
	return h
}

// deriveHash_ returns the hash of the object.
func deriveHash_(object *string) uint64 {
	if object == nil {
		return 0
	}
	return (31 * 17) + deriveHash_s(*object)
}

// deriveHash_s returns the hash of the object.
func deriveHash_s(object string) uint64 {
	var h uint64
	for _, c := range object {
		h = 31*h + uint64(c)
	}
	return h
}