diff --git a/lib/excache/skip_list.ex b/lib/excache/skip_list.ex
new file mode 100644
index 0000000..9fa600d
--- /dev/null
+++ b/lib/excache/skip_list.ex
@@ -0,0 +1,358 @@
+defmodule Excache.SkipList do
+  @moduledoc false
+
+  # Faithful port of the classic Redis zskiplist algorithm (t_zset.c):
+  # leveled forward pointers plus per-edge span counters so rank and
+  # positional range queries stay O(log n) instead of O(n).
+
+  @max_level 32
+  @probability 0.25
+
+  defmodule Node do
+    @moduledoc false
+    defstruct [:member, :score, :forward, :span]
+  end
+
+  defstruct nodes: %{},
+            index: %{},
+            header_forward: List.to_tuple(List.duplicate(nil, 32)),
+            header_span: List.to_tuple(List.duplicate(0, 32)),
+            level: 1,
+            length: 0,
+            next_id: 0
+
+  def new, do: %__MODULE__{}
+
+  def size(%__MODULE__{length: length}), do: length
+
+  def member?(%__MODULE__{index: index}, member), do: Map.has_key?(index, member)
+
+  def score(%__MODULE__{index: index} = list, member) do
+    case Map.fetch(index, member) do
+      {:ok, id} -> {:ok, node!(list, id).score}
+      :error -> :error
+    end
+  end
+
+  def rank(list, member) do
+    case Map.fetch(list.index, member) do
+      :error ->
+        :error
+
+      {:ok, id} ->
+        node = node!(list, id)
+        {_updates, ranks} = locate(list, node.score, node.member)
+        {:ok, Map.fetch!(ranks, 0)}
+    end
+  end
+
+  def insert(list, member, score) do
+    list
+    |> delete(member)
+    |> do_insert(member, score)
+  end
+
+  def delete(list, member) do
+    case Map.fetch(list.index, member) do
+      :error ->
+        list
+
+      {:ok, id} ->
+        node = node!(list, id)
+        {updates, _ranks} = locate(list, node.score, node.member)
+
+        list
+        |> splice(updates, id, node)
+        |> shrink_level()
+        |> forget(id, member)
+    end
+  end
+
+  def to_list(list) do
+    collect_from(list, forward_at(list, :header, 0), [])
+  end
+
+  def range_by_rank(_list, lower, _count) when lower < 0, do: []
+  def range_by_rank(_list, _lower, count) when count <= 0, do: []
+
+  def range_by_rank(list, lower, count) do
+    case at_rank(list, lower) do
+      nil -> []
+      id -> take_forward(list, id, count, [])
+    end
+  end
+
+  def range_by_score(list, min_bound, max_bound) do
+    case first_at_or_after(list, min_bound) do
+      nil -> []
+      id -> collect_while(list, id, max_bound, [])
+    end
+  end
+
+  defp do_insert(list, member, score) do
+    {updates, ranks} = locate(list, score, member)
+    new_level = random_level()
+
+    {updates, ranks, list} =
+      if new_level > list.level do
+        extended_updates =
+          Enum.reduce(list.level..(new_level - 1), updates, &Map.put(&2, &1, :header))
+
+        extended_ranks = Enum.reduce(list.level..(new_level - 1), ranks, &Map.put(&2, &1, 0))
+        {extended_updates, extended_ranks, %{list | level: new_level}}
+      else
+        {updates, ranks, list}
+      end
+
+    id = list.next_id
+    base_rank = Map.fetch!(ranks, 0)
+
+    forward =
+      for i <- 0..(new_level - 1), into: [], do: forward_at(list, Map.fetch!(updates, i), i)
+
+    span =
+      for i <- 0..(new_level - 1), into: [] do
+        span_at(list, Map.fetch!(updates, i), i) - (base_rank - Map.fetch!(ranks, i))
+      end
+
+    node = %Node{
+      member: member,
+      score: score,
+      forward: List.to_tuple(forward),
+      span: List.to_tuple(span)
+    }
+
+    list
+    |> put_node(id, node)
+    |> rewire_updates(updates, ranks, base_rank, new_level, id)
+    |> bump_untouched_spans(updates, new_level)
+    |> remember(id, member)
+  end
+
+  defp locate(list, score, member) do
+    top = list.level - 1
+
+    Enum.reduce(top..0//-1, {%{}, %{}, :header}, fn i, {updates, ranks, x} ->
+      rank_above = Map.get(ranks, i + 1, 0)
+      {x, rank_here} = advance(list, x, i, score, member, rank_above)
+      {Map.put(updates, i, x), Map.put(ranks, i, rank_here), x}
+    end)
+    |> then(fn {updates, ranks, _x} -> {updates, ranks} end)
+  end
+
+  defp advance(list, x, i, score, member, rank_acc) do
+    case forward_at(list, x, i) do
+      nil ->
+        {x, rank_acc}
+
+      next_id ->
+        next = node!(list, next_id)
+
+        if before_target?(next.score, next.member, score, member) do
+          advance(list, next_id, i, score, member, rank_acc + span_at(list, x, i))
+        else
+          {x, rank_acc}
+        end
+    end
+  end
+
+  defp before_target?(score, member, target_score, target_member) do
+    score < target_score or (score == target_score and member < target_member)
+  end
+
+  defp rewire_updates(list, updates, ranks, base_rank, new_level, new_id) do
+    Enum.reduce(0..(new_level - 1), list, fn i, acc ->
+      update_id = Map.fetch!(updates, i)
+      new_span = base_rank - Map.fetch!(ranks, i) + 1
+
+      acc
+      |> set_forward(update_id, i, new_id)
+      |> set_span(update_id, i, new_span)
+    end)
+  end
+
+  defp bump_untouched_spans(list, updates, new_level) do
+    if new_level > list.level - 1 do
+      list
+    else
+      Enum.reduce(new_level..(list.level - 1), list, fn i, acc ->
+        bump_span(acc, Map.fetch!(updates, i), i, 1)
+      end)
+    end
+  end
+
+  defp splice(list, updates, id, node) do
+    Enum.reduce(0..(list.level - 1), list, fn i, acc ->
+      update_id = Map.fetch!(updates, i)
+
+      if forward_at(acc, update_id, i) == id do
+        new_span = span_at(acc, update_id, i) + elem(node.span, i) - 1
+
+        acc
+        |> set_forward(update_id, i, elem(node.forward, i))
+        |> set_span(update_id, i, new_span)
+      else
+        bump_span(acc, update_id, i, -1)
+      end
+    end)
+  end
+
+  defp shrink_level(list) do
+    if list.level > 1 and forward_at(list, :header, list.level - 1) == nil do
+      shrink_level(%{list | level: list.level - 1})
+    else
+      list
+    end
+  end
+
+  defp at_rank(list, rank) when rank < 0 or rank >= list.length, do: nil
+
+  defp at_rank(list, rank) do
+    target = rank + 1
+    top = list.level - 1
+
+    {_traversed, x} =
+      Enum.reduce(top..0//-1, {0, :header}, fn i, {traversed, x} ->
+        advance_to_rank(list, x, i, traversed, target)
+      end)
+
+    case x do
+      :header -> nil
+      id -> id
+    end
+  end
+
+  defp advance_to_rank(list, x, i, traversed, rank) do
+    case forward_at(list, x, i) do
+      nil ->
+        {traversed, x}
+
+      next_id ->
+        span = span_at(list, x, i)
+
+        if traversed + span <= rank do
+          advance_to_rank(list, next_id, i, traversed + span, rank)
+        else
+          {traversed, x}
+        end
+    end
+  end
+
+  defp take_forward(_list, nil, _count, acc), do: Enum.reverse(acc)
+  defp take_forward(_list, _id, 0, acc), do: Enum.reverse(acc)
+
+  defp take_forward(list, id, count, acc) do
+    node = node!(list, id)
+    take_forward(list, elem(node.forward, 0), count - 1, [{node.score, node.member} | acc])
+  end
+
+  defp first_at_or_after(list, bound) do
+    top = list.level - 1
+
+    {_traversed, x} =
+      Enum.reduce(top..0//-1, {0, :header}, fn i, {traversed, x} ->
+        skip_below_bound(list, x, i, traversed, bound)
+      end)
+
+    forward_at(list, x, 0)
+  end
+
+  defp skip_below_bound(list, x, i, traversed, bound) do
+    case forward_at(list, x, i) do
+      nil ->
+        {traversed, x}
+
+      next_id ->
+        next = node!(list, next_id)
+
+        if below_bound?(next.score, bound) do
+          skip_below_bound(list, next_id, i, traversed + span_at(list, x, i), bound)
+        else
+          {traversed, x}
+        end
+    end
+  end
+
+  defp collect_while(list, id, max_bound, acc) do
+    node = node!(list, id)
+
+    if within_upper?(node.score, max_bound) do
+      updated = [{node.score, node.member} | acc]
+
+      case elem(node.forward, 0) do
+        nil -> Enum.reverse(updated)
+        next_id -> collect_while(list, next_id, max_bound, updated)
+      end
+    else
+      Enum.reverse(acc)
+    end
+  end
+
+  defp collect_from(_list, nil, acc), do: Enum.reverse(acc)
+
+  defp collect_from(list, id, acc) do
+    node = node!(list, id)
+    collect_from(list, elem(node.forward, 0), [{node.score, node.member} | acc])
+  end
+
+  defp below_bound?(_score, :neg_infinity), do: false
+  defp below_bound?(_score, :pos_infinity), do: true
+  defp below_bound?(score, {value, true}), do: score < value
+  defp below_bound?(score, {value, false}), do: score <= value
+
+  defp within_upper?(_score, :pos_infinity), do: true
+  defp within_upper?(_score, :neg_infinity), do: false
+  defp within_upper?(score, {value, true}), do: score <= value
+  defp within_upper?(score, {value, false}), do: score < value
+
+  defp forward_at(list, :header, i), do: elem(list.header_forward, i)
+  defp forward_at(list, id, i), do: elem(node!(list, id).forward, i)
+
+  defp span_at(list, :header, i), do: elem(list.header_span, i)
+  defp span_at(list, id, i), do: elem(node!(list, id).span, i)
+
+  defp set_forward(list, :header, i, value),
+    do: %{list | header_forward: put_elem(list.header_forward, i, value)}
+
+  defp set_forward(list, id, i, value) do
+    node = node!(list, id)
+    %{list | nodes: Map.put(list.nodes, id, %{node | forward: put_elem(node.forward, i, value)})}
+  end
+
+  defp set_span(list, :header, i, value),
+    do: %{list | header_span: put_elem(list.header_span, i, value)}
+
+  defp set_span(list, id, i, value) do
+    node = node!(list, id)
+    %{list | nodes: Map.put(list.nodes, id, %{node | span: put_elem(node.span, i, value)})}
+  end
+
+  defp bump_span(list, target, i, delta),
+    do: set_span(list, target, i, span_at(list, target, i) + delta)
+
+  defp put_node(list, id, node), do: %{list | nodes: Map.put(list.nodes, id, node)}
+
+  defp remember(list, id, member),
+    do: %{list | index: Map.put(list.index, member, id), length: list.length + 1, next_id: id + 1}
+
+  defp forget(list, id, member) do
+    %{
+      list
+      | nodes: Map.delete(list.nodes, id),
+        index: Map.delete(list.index, member),
+        length: list.length - 1
+    }
+  end
+
+  defp node!(list, id), do: Map.fetch!(list.nodes, id)
+
+  defp random_level, do: grow(1)
+
+  defp grow(level) do
+    if level < @max_level and :rand.uniform() < @probability do
+      grow(level + 1)
+    else
+      level
+    end
+  end
+end