Implement integer modular exponentiation using BigInteger#mod_pow

CHANGELOG.md

@@ -14,6 +14,7 @@ Compatibility:
 Performance:
 
 * Enable lazy translation from the parser AST to the Truffle AST for user code by default. This should improve application startup time (#1992).
+* Implement integer modular exponentiation using `BigInteger#mod_pow` (#1999)
 
 # 20.1.0
 

src/main/java/org/truffleruby/core/numeric/IntegerNodes.java

@@ -1755,6 +1755,19 @@ protected int getLimit() {
 
     }
 
+    @Primitive(name = "mod_pow", lowerFixnum = { 0, 1, 2 })
+    public abstract static class ModPowNode extends PrimitiveArrayArgumentsNode {
+        @Child private FixnumOrBignumNode fixnumOrBignum = new FixnumOrBignumNode();
+
+        @Specialization
+        protected Object mod_pow(Integer base, Integer exponent, Integer modulo) {
+            BigInteger result = BigInteger
+                    .valueOf(base)
+                    .modPow(BigInteger.valueOf(exponent), BigInteger.valueOf(modulo));
+            return fixnumOrBignum.fixnumOrBignum(result);
+        }
+    }
+
     @CoreMethod(
             names = "downto",
             needsBlock = true,

src/main/ruby/truffleruby/core/integer.rb

@@ -122,8 +122,9 @@ def nobits?(mask)
 
   def pow(e, m=undefined)
     return self ** e if Primitive.undefined?(m)
+    raise TypeError, '2nd argument not allowed unless a 1st argument is integer' unless Primitive.object_kind_of?(e, Integer)
     raise TypeError, '2nd argument not allowed unless all arguments are integers' unless Primitive.object_kind_of?(m, Integer)
-    (self ** e) % m
+    Primitive.mod_pow(self, e, m)
   end
 
   def times