I've written a red-black tree in C as an exercise. The tree works and it is not bad, but it is about 10% slower than std::multimap from libstdc++ which I'm comparing it against.

Full code of my implementation is here. libstdc++'s red-black tree written in C++ is here and here.

I'm using the following code to compare implementations:

test_torture() {
    std::vector<uint64_t> v;
    std::multimap<uint64_t, uint64_t> map;
    uint64_t range = 10 * 1000 * 1000 * 1000LL;
    uint64_t count = 10 * 1000 * 1000;
    for (uint64_t i = 0; i < count; i++) {
        uint64_t key = rand() % range;
        map.insert(std::make_pair(key, key));
    for (uint64_t i = 0; i < count; i++) {
        uint64_t key = v[i]; 
    assert(map.size() == 0);

So it adds 10m random nodes to the tree, then deletes them all again. My code is almost exactly as fast as std::multimap when doing the insertion part, but it lags behind when it comes to erasing nodes. When profiling, the hottest part of my code is my loop for inserting nodes:

rbtree *
rbt_add(rbtree *me, ptr key, ptr value) {
    // Find insertion point.
    rbtree **addr = &me;
    rbtree *parent = NULL;
    while (*addr) {
        parent = *addr;
        addr = &parent->childs[key >= parent->key];
    *addr = rbt_init(parent, key, value);
    return rbt_add_fixup(me, *addr);

But I can't think of any way to improve that. My functions for rebalancing the tree after addition and deletion (which I think are the most interesting part) are below.

static rbtree *
rbt_add_fixup(rbtree *root, rbtree *x) {
    while (x != root && x->parent->is_red) {
        bstdir dir = BST_DIR_OF(x->parent);
        rbtree *y = x->parent->parent->childs[!dir];
        if (y && y->is_red) {
            x->parent->is_red = false;
            y->is_red = false;
            x->parent->parent->is_red = true;
            x = x->parent->parent;
        } else {
            if (BST_DIR_OF(x) != dir) {
                x = x->parent;
                root = rbt_rotate(root, x, dir);
            x->parent->is_red = false;
            x->parent->parent->is_red = true;
            root = rbt_rotate(root, x->parent->parent, !dir);
    root->is_red = false;
    return root;

static rbtree *
rbt_remove_fixup(rbtree *root, rbtree *x, rbtree *x_parent) {
    while (x != root && IS_BLACK(x)) {
        bstdir dir = BST_DIR_OF2(x, x_parent);
        rbtree *w = x_parent->childs[!dir];
        if (w && w->is_red) {
            w->is_red = false;
            x_parent->is_red = true;
            root = rbt_rotate(root, x_parent, dir);
            w = x_parent->childs[!dir];
        if (BOTH_CHILDREN_BLACK(w)) {
            if (w) {
                w->is_red = true;
            x = x_parent;
            x_parent = x->parent;
        } else {
            if (IS_BLACK(w->childs[!dir])) {
                w->childs[dir]->is_red = false;
                w->is_red = true;
                root = rbt_rotate(root, w, !dir);
                w = x_parent->childs[!dir];
            w->is_red = x_parent->is_red;
            x_parent->is_red = false;
            w->childs[!dir]->is_red = false;
            root = rbt_rotate(root, x_parent, dir);
            x = root;
    if (x) {
        x->is_red = false;
    return root;

I'd be very grateful for suggestions on how to increase the performance of the code.

The definition of the node type is:

typedef struct _rbtree {
    struct _rbtree *parent;
    struct _rbtree *childs[2];
    bstkey key;
    bool is_red;
    ptr value;
} rbtree;
  • 9
    \$\begingroup\$ Code in questions should ideally be sufficient to understand on their own. The definition for rbtree would seem to be a key piece of information currently missing from your question. As far as 'hottest part of my code is my loop for inserting', the rest of your description suggests that it's actually the deletion code that is slower so you probably want to be focusing on the deletion profiling, not the insertion. \$\endgroup\$
    – forsvarir
    Nov 6, 2016 at 23:49
  • 1
    \$\begingroup\$ 10% slower I don't see remove() & rbt_rotate(). Include find()in the performance model. One thing to try: Get rid of field parent. rbt_add() seems to leak memory with present keys. For a problem with lower_bound try the root's key in a non-trivial tree. \$\endgroup\$
    – greybeard
    Mar 22 at 10:02


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