Use Set(T) instead of Hash(T, Bool) (#13611)

src/array.cr

@@ -256,7 +256,7 @@ class Array(T)
     return Array(T).new if self.empty? || other.empty?
 
     # Heuristic: for small arrays we do a linear scan, which is usually
-    # faster than creating an intermediate Hash.
+    # faster than creating an intermediate Set.
     if self.size + other.size <= SMALL_ARRAY_SIZE * 2
       ary = Array(T).new
       each do |elem|
@@ -265,20 +265,13 @@ class Array(T)
       return ary
     end
 
-    hash = other.to_lookup_hash
-    hash_size = hash.size
+    set = other.to_set
     Array(T).build(Math.min(size, other.size)) do |buffer|
-      i = 0
+      appender = buffer.appender
       each do |obj|
-        hash.delete(obj)
-        new_hash_size = hash.size
-        if hash_size != new_hash_size
-          hash_size = new_hash_size
-          buffer[i] = obj
-          i += 1
-        end
+        appender << obj if set.delete(obj)
       end
-      i
+      appender.size.to_i
     end
   end
 
@@ -292,7 +285,7 @@ class Array(T)
   # See also: `#uniq`.
   def |(other : Array(U)) : Array(T | U) forall U
     # Heuristic: if the combined size is small we just do a linear scan
-    # instead of using a Hash for lookup.
+    # instead of using a Set for lookup.
     if size + other.size <= SMALL_ARRAY_SIZE
       ary = Array(T | U).new
       each do |elem|
@@ -305,23 +298,15 @@ class Array(T)
     end
 
     Array(T | U).build(size + other.size) do |buffer|
-      hash = Hash(T, Bool).new
-      i = 0
+      set = Set(T).new
+      appender = buffer.appender
       each do |obj|
-        unless hash.has_key?(obj)
-          buffer[i] = obj
-          hash[obj] = true
-          i += 1
-        end
+        appender << obj if set.add?(obj)
       end
       other.each do |obj|
-        unless hash.has_key?(obj)
-          buffer[i] = obj
-          hash[obj] = true
-          i += 1
-        end
+        appender << obj if set.add?(obj)
       end
-      i
+      appender.size.to_i
     end
   end
 
@@ -356,7 +341,7 @@ class Array(T)
   # ```
   def -(other : Array(U)) : Array(T) forall U
     # Heuristic: if any of the arrays is small we just do a linear scan
-    # instead of using a Hash for lookup.
+    # instead of using a Set for lookup.
     if size <= SMALL_ARRAY_SIZE || other.size <= SMALL_ARRAY_SIZE
       ary = Array(T).new
       each do |elem|
@@ -366,9 +351,9 @@ class Array(T)
     end
 
     ary = Array(T).new(Math.max(size - other.size, 0))
-    hash = other.to_lookup_hash
+    set = other.to_set
     each do |obj|
-      ary << obj unless hash.has_key?(obj)
+      ary << obj unless set.includes?(obj)
     end
     ary
   end
@@ -1858,7 +1843,7 @@ class Array(T)
     end
 
     # Heuristic: for a small array it's faster to do a linear scan
-    # than creating a Hash to find out duplicates.
+    # than creating a Set to find out duplicates.
     if size <= SMALL_ARRAY_SIZE
       ary = Array(T).new
       each do |elem|
@@ -1867,8 +1852,8 @@ class Array(T)
       return ary
     end
 
-    # Convert the Array into a Hash and then ask for its values
-    to_lookup_hash.values
+    # Convert the Array into a Set and then ask for its values
+    to_set.to_a
   end
 
   # Returns a new `Array` by removing duplicate values in `self`, using the block's
@@ -2176,10 +2161,6 @@ class Array(T)
     Slice.new(@buffer + start, count)
   end
 
-  protected def to_lookup_hash
-    to_lookup_hash { |elem| elem }
-  end
-
   protected def to_lookup_hash(& : T -> U) forall U
     each_with_object(Hash(U, T).new) do |o, h|
       key = yield o

src/iterator.cr

@@ -1283,18 +1283,14 @@ module Iterator(T)
     include IteratorWrapper
 
     def initialize(@iterator : I, @func : T -> U)
-      @hash = {} of U => Bool
+      @set = Set(U).new
     end
 
     def next
       while true
         value = wrapped_next
         transformed = @func.call value
-
-        unless @hash[transformed]?
-          @hash[transformed] = true
-          return value
-        end
+        return value if @set.add?(transformed)
       end
     end
   end

src/reference.cr

@@ -139,7 +139,8 @@ class Reference
 
   # :nodoc:
   module ExecRecursive
-    alias Registry = Hash({UInt64, Symbol}, Bool)
+    # NOTE: can't use `Set` here because of prelude require order
+    alias Registry = Hash({UInt64, Symbol}, Nil)
 
     {% if flag?(:preview_mt) %}
       @@exec_recursive = Crystal::ThreadLocalValue(Registry).new
@@ -159,14 +160,12 @@ class Reference
   private def exec_recursive(method, &)
     hash = ExecRecursive.hash
     key = {object_id, method}
-    if hash[key]?
-      false
-    else
-      hash[key] = true
+    hash.put(key, nil) do
       yield
       hash.delete(key)
-      true
+      return true
     end
+    false
   end
 
   # :nodoc: