Golang Ordered Maps

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
• supports any generic types for both keys and values. If you're running go < 1.18, you can use version 1 that takes and returns generic interface{}s instead of using generics
• idiomatic API, akin to that of container/list
• support for JSON and YAML marshalling

Documentation

The full documentation is available on pkg.go.dev.

Installation

go get -u github.com/wk8/go-ordered-map/v2

Or use your favorite golang vendoring tool!

Supported go versions

Go >= 1.23 is required to use version >= 2.2.0 of this library, as it uses generics and iterators.

if you're running go < 1.23, you can use version 2.1.8 instead.

If you're running go < 1.18, you can use version 1 instead.

Example / usage

package main

import (
	"fmt"

	"github.com/wk8/go-ordered-map/v2"
)

func main() {
	om := orderedmap.New[string, string]()

	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")) // => "", 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
}

An OrderedMap's keys must implement comparable, and its values can be anything, for example:

type myStruct struct {
	payload string
}

func main() {
	om := orderedmap.New[int, *myStruct]()

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

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

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

Also worth noting that you can provision ordered maps with a capacity hint, as you would do by passing an optional hint to make(map[K]V, capacity):

om := orderedmap.New[int, *myStruct](28)

You can also pass in some initial data to store in the map:

om := orderedmap.New[int, string](orderedmap.WithInitialData[int, string](
	orderedmap.Pair[int, string]{
		Key:   12,
		Value: "foo",
	},
	orderedmap.Pair[int, string]{
		Key:   28,
		Value: "bar",
	},
))

OrderedMaps also support JSON serialization/deserialization, and preserves order:

// serialization
data, err := json.Marshal(om)
...

// deserialization
om := orderedmap.New[string, string]() // or orderedmap.New[int, any](), or any type you expect
err := json.Unmarshal(data, &om)
...

Similarly, it also supports YAML serialization/deserialization using the yaml.v3 package, which also preserves order:

// serialization
data, err := yaml.Marshal(om)
...

// deserialization
om := orderedmap.New[string, string]() // or orderedmap.New[int, any](), or any type you expect
err := yaml.Unmarshal(data, &om)
...

Iterator support (go >= 1.23)

The FromOldest, FromNewest, KeysFromOldest, KeysFromNewest, ValuesFromOldest and ValuesFromNewest methods return iterators over the map's pairs, starting from the oldest or newest pair, respectively.

For example:

om := orderedmap.New[int, string]()
om.Set(1, "foo")
om.Set(2, "bar")
om.Set(3, "baz")

for k, v := range om.FromOldest() {
	fmt.Printf("%d => %s\n", k, v)
}

// prints:
// 1 => foo
// 2 => bar
// 3 => baz

for k := range om.KeysNewest() {
	fmt.Printf("%d\n", k)
}

// prints:
// 3
// 2
// 1

From is a convenience function that creates a new OrderedMap from an iterator over key-value pairs.

om := orderedmap.New[int, string]()
om.Set(1, "foo")
om.Set(2, "bar")
om.Set(3, "baz")

om2 := orderedmap.From(om.FromOldest())

for k, v := range om2.FromOldest() {
	fmt.Printf("%d => %s\n", k, v)
}

// prints:
// 1 => foo
// 2 => bar
// 3 => baz

Alternatives

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