diff --git a/lib/excache/bitmap.ex b/lib/excache/bitmap.ex
new file mode 100644
index 0000000..a3e787e
--- /dev/null
+++ b/lib/excache/bitmap.ex
@@ -0,0 +1,124 @@
+defmodule Excache.Bitmap do
+  @moduledoc false
+
+  import Bitwise
+
+  def getBit(binary, offset) do
+    byteIndex = offset >>> 3
+
+    if byteIndex >= byte_size(binary) do
+      0
+    else
+      bitIndex = 7 - (offset &&& 7)
+      :binary.at(binary, byteIndex) >>> bitIndex &&& 1
+    end
+  end
+
+  def setBit(binary, offset, value) do
+    byteIndex = offset >>> 3
+    bitIndex = 7 - (offset &&& 7)
+    padded = extend(binary, byteIndex + 1)
+    byte = :binary.at(padded, byteIndex)
+    old = byte >>> bitIndex &&& 1
+
+    updated =
+      case value do
+        1 -> byte ||| 1 <<< bitIndex
+        0 -> byte &&& bnot(1 <<< bitIndex)
+      end
+
+    {replaceByte(padded, byteIndex, updated), old}
+  end
+
+  def count(binary, range) do
+    case windowOf(binary, range) do
+      :empty -> 0
+      {window, _absStart} -> popcount(window)
+    end
+  end
+
+  def position(binary, bit, range, endGiven) do
+    case windowOf(binary, range) do
+      :empty ->
+        if bit == 0 and not endGiven, do: bit_size(binary), else: -1
+
+      {window, absStart} ->
+        case findBit(window, bit, 0) do
+          nil ->
+            if bit == 0 and not endGiven, do: bit_size(binary), else: -1
+
+          localIndex ->
+            absStart + localIndex
+        end
+    end
+  end
+
+  def combine(:not, [binary]) do
+    for <<byte <- binary>>, into: <<>>, do: <<bnot(byte) &&& 0xFF>>
+  end
+
+  def combine(operation, binaries) do
+    width = binaries |> Enum.map(&byte_size/1) |> Enum.max(fn -> 0 end)
+    aligned = Enum.map(binaries, &extend(&1, width))
+
+    for index <- 0..(width - 1), width > 0, into: <<>> do
+      bytes = Enum.map(aligned, &:binary.at(&1, index))
+      <<foldBytes(operation, bytes)>>
+    end
+  end
+
+  defp foldBytes(:and, [first | rest]), do: Enum.reduce(rest, first, &(&1 &&& &2))
+  defp foldBytes(:or, [first | rest]), do: Enum.reduce(rest, first, &(&1 ||| &2))
+  defp foldBytes(:xor, [first | rest]), do: Enum.reduce(rest, first, &bxor(&1, &2))
+
+  defp windowOf(binary, :whole) do
+    if binary == <<>>, do: :empty, else: {binary, 0}
+  end
+
+  defp windowOf(binary, {:byte, start, stop}) do
+    size = byte_size(binary)
+    lower = clampIndex(start, size)
+    upper = clampIndex(stop, size)
+
+    if size == 0 or lower >= size or lower > upper do
+      :empty
+    else
+      upper = min(upper, size - 1)
+      {:binary.part(binary, lower, upper - lower + 1), lower * 8}
+    end
+  end
+
+  defp windowOf(binary, {:bit, start, stop}) do
+    size = bit_size(binary)
+    lower = clampIndex(start, size)
+    upper = clampIndex(stop, size)
+
+    if size == 0 or lower >= size or lower > upper do
+      :empty
+    else
+      upper = min(upper, size - 1)
+      len = upper - lower + 1
+      <<_::size(lower), window::bitstring-size(len), _::bitstring>> = binary
+      {window, lower}
+    end
+  end
+
+  defp popcount(window), do: for(<<bit::1 <- window>>, do: bit) |> Enum.sum()
+
+  defp findBit(<<value::1, _rest::bitstring>>, bit, position) when value == bit, do: position
+  defp findBit(<<_::1, rest::bitstring>>, bit, position), do: findBit(rest, bit, position + 1)
+  defp findBit(<<>>, _bit, _position), do: nil
+
+  defp clampIndex(index, size) when index < 0, do: max(size + index, 0)
+  defp clampIndex(index, _size), do: index
+
+  defp extend(binary, width) do
+    missing = width - byte_size(binary)
+    if missing > 0, do: binary <> :binary.copy(<<0>>, missing), else: binary
+  end
+
+  defp replaceByte(binary, index, byte) do
+    <<head::binary-size(index), _::8, tail::binary>> = binary
+    <<head::binary, byte::8, tail::binary>>
+  end
+end