diff --git a/test/excache/skip_list_test.exs b/test/excache/skip_list_test.exs
new file mode 100644
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+++ b/test/excache/skip_list_test.exs
@@ -0,0 +1,149 @@
+defmodule Excache.SkipListTest do
+  use ExUnit.Case, async: true
+
+  alias Excache.SkipList
+
+  test "empty skip list has zero size and no ranks" do
+    list = SkipList.new()
+    assert SkipList.size(list) == 0
+    assert SkipList.to_list(list) == []
+    assert SkipList.rank(list, "ghost") == :error
+    assert SkipList.score(list, "ghost") == :error
+  end
+
+  test "insert keeps ascending score order" do
+    list =
+      SkipList.new()
+      |> SkipList.insert("c", 3)
+      |> SkipList.insert("a", 1)
+      |> SkipList.insert("b", 2)
+
+    assert SkipList.to_list(list) == [{1, "a"}, {2, "b"}, {3, "c"}]
+    assert SkipList.size(list) == 3
+  end
+
+  test "equal scores break ties by member" do
+    list =
+      SkipList.new()
+      |> SkipList.insert("zebra", 5)
+      |> SkipList.insert("apple", 5)
+      |> SkipList.insert("mango", 5)
+
+    assert SkipList.to_list(list) == [{5, "apple"}, {5, "mango"}, {5, "zebra"}]
+  end
+
+  test "insert on an existing member reorders it instead of duplicating" do
+    list =
+      SkipList.new()
+      |> SkipList.insert("a", 1)
+      |> SkipList.insert("b", 2)
+      |> SkipList.insert("a", 10)
+
+    assert SkipList.size(list) == 2
+    assert SkipList.to_list(list) == [{2, "b"}, {10, "a"}]
+    assert SkipList.score(list, "a") == {:ok, 10}
+  end
+
+  test "delete removes a member and closes the gap" do
+    list =
+      SkipList.new()
+      |> SkipList.insert("a", 1)
+      |> SkipList.insert("b", 2)
+      |> SkipList.insert("c", 3)
+      |> SkipList.delete("b")
+
+    assert SkipList.size(list) == 2
+    assert SkipList.to_list(list) == [{1, "a"}, {3, "c"}]
+    assert SkipList.member?(list, "b") == false
+  end
+
+  test "delete on a missing member is a no-op" do
+    list = SkipList.new() |> SkipList.insert("a", 1)
+    unchanged = SkipList.delete(list, "nope")
+    assert SkipList.to_list(unchanged) == [{1, "a"}]
+    assert SkipList.size(unchanged) == 1
+  end
+
+  test "rank matches zero-indexed position in sorted order" do
+    list =
+      SkipList.new()
+      |> SkipList.insert("a", 10)
+      |> SkipList.insert("b", 20)
+      |> SkipList.insert("c", 30)
+      |> SkipList.insert("d", 40)
+
+    assert SkipList.rank(list, "a") == {:ok, 0}
+    assert SkipList.rank(list, "b") == {:ok, 1}
+    assert SkipList.rank(list, "c") == {:ok, 2}
+    assert SkipList.rank(list, "d") == {:ok, 3}
+    assert SkipList.rank(list, "missing") == :error
+  end
+
+  test "range_by_rank matches a plain list slice" do
+    list =
+      Enum.reduce(0..9, SkipList.new(), fn i, acc ->
+        SkipList.insert(acc, "m#{i}", i)
+      end)
+
+    full = SkipList.to_list(list)
+    assert SkipList.range_by_rank(list, 0, 3) == Enum.slice(full, 0, 3)
+    assert SkipList.range_by_rank(list, 3, 4) == Enum.slice(full, 3, 4)
+    assert SkipList.range_by_rank(list, 8, 10) == Enum.slice(full, 8, 10)
+    assert SkipList.range_by_rank(list, 20, 5) == []
+  end
+
+  test "range_by_score respects inclusive and exclusive bounds" do
+    list =
+      Enum.reduce([{"a", 1}, {"b", 2}, {"c", 3}, {"d", 4}, {"e", 5}], SkipList.new(), fn {m, s},
+                                                                                         acc ->
+        SkipList.insert(acc, m, s)
+      end)
+
+    assert SkipList.range_by_score(list, {2, true}, {4, true}) == [{2, "b"}, {3, "c"}, {4, "d"}]
+    assert SkipList.range_by_score(list, {2, false}, {4, false}) == [{3, "c"}]
+    assert SkipList.range_by_score(list, :neg_infinity, :pos_infinity) == SkipList.to_list(list)
+    assert SkipList.range_by_score(list, {10, true}, :pos_infinity) == []
+  end
+
+  test "random insert/delete sequence matches Enum.sort ground truth" do
+    seed = :erlang.phash2(make_ref())
+    :rand.seed(:exsplus, {seed, seed, seed})
+
+    members = for i <- 1..200, do: "member-#{i}"
+
+    {list, ground_truth} =
+      Enum.reduce(members, {SkipList.new(), %{}}, fn member, {acc_list, acc_truth} ->
+        score = :rand.uniform(50)
+        {SkipList.insert(acc_list, member, score), Map.put(acc_truth, member, score)}
+      end)
+
+    expected =
+      ground_truth
+      |> Enum.map(fn {member, score} -> {score, member} end)
+      |> Enum.sort()
+
+    assert SkipList.to_list(list) == expected
+    assert SkipList.size(list) == map_size(ground_truth)
+
+    Enum.each(ground_truth, fn {member, score} ->
+      assert SkipList.score(list, member) == {:ok, score}
+    end)
+
+    {deleted, remaining_truth} = Enum.split(members, 40)
+
+    reduced =
+      Enum.reduce(deleted, list, fn member, acc -> SkipList.delete(acc, member) end)
+
+    remaining_expected =
+      remaining_truth
+      |> Enum.map(fn member -> {Map.fetch!(ground_truth, member), member} end)
+      |> Enum.sort()
+
+    assert SkipList.to_list(reduced) == remaining_expected
+    assert SkipList.size(reduced) == length(remaining_truth)
+
+    Enum.each(deleted, fn member ->
+      assert SkipList.member?(reduced, member) == false
+    end)
+  end
+end