buildBits

    fn buildBits(
        out_bits: []u4,
        bit_counts: *[16]u16,
        parent_nodes: *[max_nodes]Node.Index,
        sorted: []Node.Index,
        max_bits: u4,
    ) void {
        var internal_node_bits: [max_nodes - max_leafs]u4 = undefined;
        var overflowed: u16 = 0;

        internal_node_bits[sorted[0] - max_leafs] = 0; // root
        for (sorted[1..]) |i| {
            const parent_bits = internal_node_bits[parent_nodes[i] - max_leafs];
            overflowed += @intFromBool(parent_bits == max_bits);
            const bits = parent_bits + @intFromBool(parent_bits != max_bits);
            bit_counts[bits] += @intFromBool(i < max_leafs);
            (if (i >= max_leafs) &internal_node_bits[i - max_leafs] else &out_bits[i]).* = bits;
        }

        if (overflowed == 0) {
            @branchHint(.likely);
            return;
        }

        outer: while (true) {
            var deepest: u4 = max_bits - 1;
            while (bit_counts[deepest] == 0) deepest -= 1;
            while (overflowed != 0) {
                // Insert an internal node under the leaf and move an overflow as its sibling
                bit_counts[deepest] -= 1;
                bit_counts[deepest + 1] += 2;
                // Only overflow moved. Its sibling's depth is one less, however is still >= depth.
                bit_counts[max_bits] -= 1;
                overflowed -= 2;

                if (overflowed == 0) break :outer;
                deepest += 1;
                if (deepest == max_bits) continue :outer;
            }
        }

        // Reassign bit lengths
        assert(bit_counts[0] == 0);
        var i: usize = 0;
        for (1.., bit_counts[1..]) |bits, all| {
            var remaining = all;
            while (remaining != 0) {
                defer i += 1;
                if (sorted[i] >= max_leafs) continue;
                out_bits[sorted[i]] = @intCast(bits);
                remaining -= 1;
            }
        }
        assert(for (sorted[i..]) |n| { // all leafs consumed
            if (n < max_leafs) break false;
        } else true);
    }