Added simple Bijection type

src/core/rudiments-core.scala

@@ -195,6 +195,7 @@ extension [ElemType](array: Array[ElemType])
 
 extension [KeyType, ValueType](map: sc.Map[KeyType, ValueType])
   inline def has(key: KeyType): Boolean = map.contains(key)
+  inline def bijection: Bijection[KeyType, ValueType] = Bijection(map.to(Map))
 
 extension [KeyType, ValueType](map: Map[KeyType, ValueType])
   def upsert(key: KeyType, op: Optional[ValueType] => ValueType): Map[KeyType, ValueType] =

src/core/rudiments.Bijection.scala

@@ -0,0 +1,24 @@
+package rudiments
+
+import scala.collection as sc
+
+object Bijection:
+  def apply[KeyType, ValueType](map: Map[KeyType, ValueType]): Bijection[KeyType, ValueType] =
+    Bijection(map, map.map(_.swap).to(Map))
+
+case class Bijection[KeyType, ValueType]
+    (map: Map[KeyType, ValueType], transposition: Map[ValueType, KeyType])
+extends Iterable[(KeyType, ValueType)], sc.Map[KeyType, ValueType]:
+  private inline def bijection: this.type = this
+  def iterator: Iterator[(KeyType, ValueType)] = map.iterator
+
+  infix def - (key: KeyType): Bijection[KeyType, ValueType] =
+    Bijection(map - key, transposition - map(key))
+
+  infix def - (key1: KeyType, key2: KeyType, keys: Seq[KeyType]): Bijection[KeyType, ValueType] =
+    Bijection(map - key1 - key2 -- keys, transposition - map(key1) - map(key2) -- keys.map(map(_)))
+
+  def get(key: KeyType): Option[ValueType] = map.get(key)
+  def flip: Bijection[ValueType, KeyType] = new Bijection(transposition, map):
+    override def flip: Bijection[KeyType, ValueType] = bijection
+

src/core/rudiments.Indexable.scala

@@ -68,6 +68,16 @@ object Indexable:
       def contains(value: Self, index: KeyType): Boolean = value.contains(index)
       def access(value: Self, index: KeyType): ValueType = value(index)
 
+  given [KeyType, ValueType]
+      => Bijection[KeyType, ValueType] is Indexable by KeyType into ValueType as bijection =
+    new Indexable:
+      type Self = Bijection[KeyType, ValueType]
+      type Operand = KeyType
+      type Result = ValueType
+
+      def contains(value: Self, index: KeyType): Boolean = value.map.contains(index)
+      def access(value: Self, index: KeyType): ValueType = value.map(index)
+
   given [KeyType, ValueType]
       => scm.HashMap[KeyType, ValueType] is Indexable by KeyType into ValueType as hashMap =