Fix bigint modulo operation. Fixes bug 6056

runtime/vm/bigint_operations.cc

 // Copyright 2012 Google Inc. All Rights Reserved.
 
 #include "vm/bigint_operations.h"
 
 #include "platform/utils.h"
 
 #include "vm/double_internals.h"
 #include "vm/exceptions.h"
 #include "vm/object_store.h"
 #include "vm/zone.h"
@@ skipping 758 matching lines @@
 RawBigint* BigintOperations::Divide(const Bigint& a, const Bigint& b) {
   Bigint& quotient = Bigint::Handle();
   Bigint& remainder = Bigint::Handle();
   DivideRemainder(a, b, &quotient, &remainder);
   return quotient.raw();
 }
 
 
 RawBigint* BigintOperations::Modulo(const Bigint& a, const Bigint& b) {
   Bigint& quotient = Bigint::Handle();
-  Bigint& modulo = Bigint::Handle();
-  DivideRemainder(a, b, &quotient, &modulo);
-  return modulo.raw();
+  Bigint& remainder = Bigint::Handle();
+  DivideRemainder(a, b, &quotient, &remainder);
+  // Emulating code in Integer::ArithmeticOp (Euclidian modulo).
+  if (remainder.IsNegative()) {
+    if (b.IsNegative()) {
+      return BigintOperations::Subtract(remainder, b);
+    } else {
+      return BigintOperations::Add(remainder, b);
+    }
+  }
+  return remainder.raw();
 }
 
 
 RawBigint* BigintOperations::Remainder(const Bigint& a, const Bigint& b) {
   Bigint& quotient = Bigint::Handle();
   Bigint& remainder = Bigint::Handle();
   DivideRemainder(a, b, &quotient, &remainder);
   return remainder.raw();
 }
 
@@ skipping 856 matching lines @@
 int BigintOperations::CountBits(Chunk digit) {
   int result = 0;
   while (digit != 0) {
     digit >>= 1;
     result++;
   }
   return result;
 }
 
 }  // namespace dart