diff --git a/commons/src/collection/IndexedSet.scala b/commons/src/collection/IndexedSet.scala
new file mode 100644
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+++ b/commons/src/collection/IndexedSet.scala
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+/*
+ * Copyright 2017-2019 Marconi Lanna
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package scail.commons.collection
+
+import scala.collection.AbstractSet
+import scala.collection.SetLike
+import scala.collection.breakOut
+
+/**
+ * This class implements immutable indexed sets using a hash trie.
+ *
+ * An indexed set is like a set, however elements are indexed (hashed) by a given key.
+ * A function `indexedBy: A => B` is used to map elements to their keys.
+ * The key can be used to test for membership or retrieve the element, like in a map.
+ *
+ * Indexed sets guarantee no two elements share the same key.
+ *
+ * @tparam A the type of the elements contained in this indexed set
+ * @tparam B the type of the keys in this indexed set
+ */
+final class IndexedSet[A, B] private (map: Map[B, A], indexedBy: A => B)
+  extends AbstractSet[A]
+  with SetLike[A, IndexedSet[A, B]] {
+  def contains(elem: A): Boolean = map.get(indexedBy(elem)).contains(elem)
+  def iterator: Iterator[A] = map.valuesIterator
+  // scalastyle:off method.name ensure.single.space.after.token
+  def +(elem: A): IndexedSet[A, B] = new IndexedSet(map + (indexedBy(elem) -> elem), indexedBy)
+  def -(elem: A): IndexedSet[A, B] = new IndexedSet(map - indexedBy(elem), indexedBy)
+  // scalastyle:on
+
+  override def empty: IndexedSet[A, B] = IndexedSet.empty(indexedBy)
+  override def foreach[U](f: A => U): Unit = map foreach { case (_, v) => f(v) }
+  override def size: Int = map.size
+
+  /**
+   * Tests if some key is contained in this set.
+   *
+   * @param key the key to test for membership
+   * @return `true` if `key` is contained in this set, `false` otherwise
+   */
+  def containsKey(key: B): Boolean = map.contains(key)
+
+  /**
+   * Optionally returns the element associated with a key.
+   *
+   * @param key the key
+   * @return the option value containing the element associated with `key` in this set,
+   *         or `None` if none exists
+   */
+  def get(key: B): Option[A] = map.get(key)
+}
+
+/**
+ * This object provides a set of operations needed to create `IndexedSet` values.
+ */
+object IndexedSet {
+  /**
+   * Creates a collection with the specified elements.
+   *
+   * @param indexedBy a function mapping an element to its key
+   * @param elems the elements of the created collection
+   * @tparam A the type of the elements contained in this indexed set
+   * @tparam B the type of the keys in this indexed set
+   * @return the new collection with elements elems
+   */
+  def apply[A, B](indexedBy: A => B)(elems: A*): IndexedSet[A, B] = {
+    val map: Map[B, A] = elems.map { e => (indexedBy(e), e) } (breakOut)
+    new IndexedSet(map, indexedBy)
+  }
+
+  /**
+   * An empty collection of type Set[A].
+   *
+   * @param indexedBy a function mapping an element to its key
+   * @tparam A the type of the elements contained in this indexed set
+   * @tparam B the type of the keys in this indexed set
+   * @return the empty `IndexedSet`
+   */
+  def empty[A, B](indexedBy: A => B): IndexedSet[A, B] = new IndexedSet(Map.empty[B, A], indexedBy)
+}
diff --git a/commons/test/collection/IndexedSetSpec.scala b/commons/test/collection/IndexedSetSpec.scala
new file mode 100644
index 0000000..fbe15d7
--- /dev/null
+++ b/commons/test/collection/IndexedSetSpec.scala
@@ -0,0 +1,118 @@
+/*
+ * Copyright 2017-2019 Marconi Lanna
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package scail.commons.collection
+
+import scail.commons.Constants.Warts
+import scail.commons.Spec
+
+@SuppressWarnings(Array(Warts.GenTraversableLikeOps, Warts.TraversableOps))
+class IndexedSetSpec extends Spec {
+  "IndexedSet should:" - {
+    "Create empty collection" in new Context {
+      val result = IndexedSet.empty(f)
+
+      assert(result.isEmpty)
+      assert(result.iterator.isEmpty)
+      assert(result.size == 0)
+    }
+
+    "Create collection from elements" in new Context {
+      val result = IndexedSet(f)(elems: _*)
+
+      assert(result.nonEmpty)
+      assert(result.iterator.nonEmpty)
+      assert(result.size == elems.size)
+
+      elems foreach { e =>
+        f(e) was called
+      }
+    }
+
+    "Tell whether collection contains element" in new Context {
+      val result = IndexedSet(f)(elems: _*)
+
+      elems foreach { e =>
+        assert(result.contains(e))
+      }
+
+      assert(!result.contains(other))
+    }
+
+    "Return collection iterator" in new Context {
+      val result: Iterator[String] = IndexedSet(f)(elems: _*).iterator
+
+      assert(result.nonEmpty)
+      assert(result.toSet == elems.toSet)
+    }
+
+    "Add element to collection" in new Context {
+      val result = IndexedSet(f)(elems: _*) + other
+
+      assert(result.size == elems.size + 1)
+      assert(result.toSet == elems.toSet + other)
+    }
+
+    "Remove element from collection" in new Context {
+      val result = IndexedSet(f)(elems: _*) - elems.head
+
+      assert(result.size == elems.size - 1)
+      assert(result.toSet == elems.tail.toSet)
+    }
+
+    "Iterate through collection" in new Context {
+      val result = IndexedSet(f)(elems: _*)
+
+      val g = mock[String => Unit]
+
+      result foreach g
+
+      elems foreach { e =>
+        g(e) was called
+      }
+    }
+
+    "Tell whether collection contains key" in new Context {
+      val result = IndexedSet(f)(elems: _*)
+
+      elems foreach { e =>
+        assert(result.containsKey(f(e)))
+      }
+
+      assert(!result.containsKey(f(other)))
+    }
+
+    "Return an element by its key" in new Context {
+      val result = IndexedSet(f)(elems: _*)
+
+      assert(result.get(f(elems.head)).contains(elems.head))
+      assert(result.get(f(other)).isEmpty)
+    }
+  }
+
+  class Context {
+    // shared objects
+    val elems = Seq("a", "bc", "def")
+    val other = "ghij"
+
+    // shared mocks
+    val f = mock[String => Int]
+
+    // common expectations
+    f(any[String]) answers { s: String =>
+      s.size
+    }
+  }
+}