Builtin pow function support for large arguments

This helps with beeware#47, as I am a first-timer! There are still some issues with very large arguments, but I can file a separate bug for that, assuming it is not already known.
swenson · Aug 15, 2016 · 5bdacc8 · 5bdacc8
1 parent 6cb06ce
commit 5bdacc8
Show file tree

Hide file tree

Showing 2 changed files with 59 additions and 1 deletion.
diff --git a/batavia/core/builtins.js b/batavia/core/builtins.js
@@ -845,7 +845,34 @@ batavia.builtins.pow = function(args) {
         x = args[0];
         y = args[1];
         z = args[2];
-        return Math.pow(x, y) % z;
+
+        if (!batavia.isinstance(x, batavia.types.Int) ||
+            !batavia.isinstance(y, batavia.types.Int) ||
+            !batavia.isinstance(y, batavia.types.Int)) {
+            throw new batavia.builtins.TypeError("pow() requires all arguments be integers when 3 arguments are present");
+        }
+        if (y < 0) {
+          throw new batavia.builtins.TypeError("Builtin Batavia does not support negative exponents")
+        }
+        if (y == 0) {
+          return 1;
+        }
+        if (z == 1) {
+          return 0;
+        }
+
+        // right-to-left exponentiation to reduce memory and time
+        // See https://en.wikipedia.org/wiki/Modular_exponentiation#Right-to-left_binary_method
+        var result = 1;
+        var base = x % z;
+        while (y > 0) {
+          if ((y & 1) == 1) {
+            result = (result * base) % z;
+          }
+          y >>= 1;
+          base = (base * base) % z;
+        }
+        return result;
     } else {
         throw new batavia.builtins.TypeError("pow expected at least 2 arguments, got " + args.length);
     }

diff --git a/tests/builtins/test_pow.py b/tests/builtins/test_pow.py
@@ -66,6 +66,37 @@ def test_float_neg_y_with_z(self):
             print(pow(x, y, z))
         """)
 
+    def test_huge_y(self):
+        self.assertCodeExecution("""
+            x = 2
+            y = 1000000000
+            z = 3
+            print(pow(x, y, z))
+            """)
+
+    def test_lots_of_pows(self):
+        self.assertCodeExecution("""
+            print(pow(967, 441, 8))
+            print(pow(911, 940, 583))
+            print(pow(672, 84, 767))
+            print(pow(237, 30, 789))
+            print(pow(346, 623, 616))
+            print(pow(148, 183, 115))
+            print(pow(14, 487, 965))
+            print(pow(64, 541, 466))
+            print(pow(602, 89, 580))
+            print(pow(269, 556, 645))
+            print(pow(481, 355, 250))
+            print(pow(934, 453, 531))
+            print(pow(19, 508, 6))
+            print(pow(143, 473, 378))
+            print(pow(54, 588, 165))
+            print(pow(557, 144, 938))
+            print(pow(771, 957, 142))
+            print(pow(305, 39, 277))
+            print(pow(807, 177, 155))
+            print(pow(955, 154, 834))
+        """)
 
 class BuiltinPowFunctionTests(BuiltinFunctionTestCase, TranspileTestCase):
     functions = ["pow"]