Fix sign issues in divmod

1 files changed, 6 insertions, 2 deletions

diff --git a/bigint.h b/bigint.h
index 02f3da3..1e8515a 100644
--- a/bigint.h
+++ b/bigint.h
@@ -26,7 +26,7 @@ private:
 	void normalise();
 	void checkconsistent();
 
-	std::pair<Bigint,Bigint> divmod(const Bigint&,int depth,int maxdepth) const;
+	std::pair<Bigint,Bigint> divmod(const Bigint&,int depth,int maxdepth) const; //ignores all signs
 
 public:
 	Bigint();
@@ -63,7 +63,11 @@ public:
 	Bigint operator*(slongdigit_t) const;
 	Bigint operator<<(int) const;
 	Bigint operator>>(int) const;
-	std::pair<Bigint,Bigint> divmod(const Bigint&) const; //rounds towards zero; returns {quotient,remainder}
+
+	//Uses *mathematical* division-with-remainder.
+	//If we look at {q,r} = x.divmod(y), then x = q*y + r, and 0<=r<|q|. This is *not* compatible with many other
+	//programming languages (and, for that matter, the C++ built-in behaviour).
+	std::pair<Bigint,Bigint> divmod(const Bigint&) const;
 
 	bool operator==(const Bigint&) const;
 	bool operator!=(const Bigint&) const;