gtreap is an immutable treap implementation in the Go Language

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gtreap implements an immutable treap data structure in golang.

By treap, this data structure is both a heap and a binary search tree.

By immutable, any updates/deletes to a treap will return a new treap which can share internal nodes with the previous treap. All nodes in this implementation are read-only after their creation. This allows concurrent readers to operate safely with concurrent writers as modifications only create new data structures and never modify existing data structures. This is a simple approach to achieving MVCC or multi-version concurrency control.

By heap, items in the treap follow the heap-priority property, where a parent node will have higher priority than its left and right children nodes.

By binary search tree, items are store lexigraphically, ordered by a user-supplied Compare function.

To get a probabilistic O(lg N) tree height, you should use a random priority number during the Upsert() operation.




import (

func stringCompare(a, b interface{}) int {
    return bytes.Compare([]byte(a.(string)), []byte(b.(string)))

t := gtreap.NewTreap(stringCompare)
t = t.Upsert("hi", rand.Int())
t = t.Upsert("hola", rand.Int())
t = t.Upsert("bye", rand.Int())
t = t.Upsert("adios", rand.Int())

hi = t.Get("hi")
bye = t.Get("bye")

// Some example Delete()'s...
t = t.Delete("bye")
nilValueHere = t.Get("bye")
t2 = t.Delete("hi")
nilValueHere2 = t2.Get("hi")

// Since we still hold onto treap t, we can still access "hi".
hiStillExistsInTreapT = t.Get("hi")

t.VisitAscend("cya", func(i Item) bool {
    // This visitor callback will be invoked with every item
    // from "cya" onwards.  So: "hi", "hola".
    // If we want to stop visiting, return false;
    // otherwise a true return result means keep visiting items.
    return true


The Upsert() method takes both an Item (an interface{}) and a heap priority. Usually, that priority should be a random int (math/rand.Int()) or perhaps even a hash of the item. However, if you want to shuffle more commonly accessed items nearer to the top of the treap for faster access, at the potential cost of not approaching a probabilistic O(lg N) tree height, then you might tweak the priority.

See also

For a simple, ordered, key-value storage or persistence library built on immutable treaps, see:

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type Compare

type Compare func(a, b interface{}) int

    Compare returns an integer comparing the two items lexicographically. The result will be 0 if a==b, -1 if a < b, and +1 if a > b.

    type Item

    type Item interface{}

      Item can be anything.

      type ItemVisitor

      type ItemVisitor func(i Item) bool

      type Treap

      type Treap struct {
      	// contains filtered or unexported fields

      func NewTreap

      func NewTreap(c Compare) *Treap

      func (*Treap) Delete

      func (t *Treap) Delete(target Item) *Treap

      func (*Treap) Get

      func (t *Treap) Get(target Item) Item

      func (*Treap) Max

      func (t *Treap) Max() Item

      func (*Treap) Min

      func (t *Treap) Min() Item

      func (*Treap) Upsert

      func (t *Treap) Upsert(item Item, itemPriority int) *Treap

        Note: only the priority of the first insert of an item is used. Priorities from future updates on already existing items are ignored. To change the priority for an item, you need to do a Delete then an Upsert.

        func (*Treap) VisitAscend

        func (t *Treap) VisitAscend(pivot Item, visitor ItemVisitor)

          Visit items greater-than-or-equal to the pivot.

          Source Files