Same as regular maps, but also remembers the order in which keys were inserted, akin to Python's collections.OrderedDicts.

It offers the following features:

• optimal runtime performance (all operations are constant time)
• optimal memory usage (only one copy of values, no unnecessary memory allocation)
• allows iterating from newest or oldest keys indifferently, without memory copy, allowing to break the iteration, and in time linear to the number of keys iterated over rather than the total length of the ordered map
• takes and returns generic interface{}s
• idiomatic API, akin to that of container/list

go get -u github.com/wk8/go-ordered-map

Or use your favorite golang vendoring tool!

The full documentation is available on godoc.org.

package main

import (
	"fmt"

	"github.com/wk8/go-ordered-map"
)

func main() {
	om := orderedmap.New()

	om.Set("foo", "bar")
	om.Set("bar", "baz")
	om.Set("coucou", "toi")

	fmt.Println(om.Get("foo"))          // => bar, true
	fmt.Println(om.Get("i dont exist")) // => <nil>, false

	// iterating pairs from oldest to newest:
	for pair := om.Oldest(); pair != nil; pair = pair.Next() {
		fmt.Printf("%s => %s\n", pair.Key, pair.Value)
	} // prints:
	// foo => bar
	// bar => baz
	// coucou => toi

	// iterating over the 2 newest pairs:
	i := 0
	for pair := om.Newest(); pair != nil; pair = pair.Prev() {
		fmt.Printf("%s => %s\n", pair.Key, pair.Value)
		i++
		if i >= 2 {
			break
		}
	} // prints:
	// coucou => toi
	// bar => baz
}

All of OrderedMap's methods accept and return interface{}s, so you can use any type of keys that regular maps accept, as well pack/unpack arbitrary values, e.g.:

type myStruct struct {
	payload string
}

func main() {
	om := orderedmap.New()

	om.Set(12, &myStruct{"foo"})
	om.Set(1, &myStruct{"bar"})

	value, present := om.Get(12)
	if !present {
		panic("should be there!")
	}
	fmt.Println(value.(*myStruct).payload) // => foo

	for pair := om.Oldest(); pair != nil; pair = pair.Next() {
		fmt.Printf("%d => %s\n", pair.Key, pair.Value.(*myStruct).payload)
	} // prints:
	// 12 => foo
	// 1 => bar
}

There are several other ordered map golang implementations out there, but I believe that at the time of writing none of them offer the same functionality as this library; more specifically:

• iancoleman/orderedmap only accepts string keys, its Delete operations are linear
• cevaris/ordered_map uses a channel for iterations, and leaks goroutines if the iteration is interrupted before fully traversing the map
• mantyr/iterator also uses a channel for iterations, and its Delete operations are linear
• samdolan/go-ordered-map adds unnecessary locking (users should add their own locking instead if they need it), its Delete and Get operations are linear, iterations trigger a linear memory allocation