Comment bigint (and fix indentation in gf28)

1 files changed, 23 insertions, 18 deletions

diff --git a/bigint.h b/bigint.h
index a3c5be3..cff7bd4 100644
--- a/bigint.h
+++ b/bigint.h
@@ -6,24 +6,30 @@
 #include <utility>
 #include <cstdint>
 
+//A bigint implementation based on unsigned 32-bit digits.
+//The implementation relies on having native 64-bit arithmetic (or specifically,
+//(2*digit_bits)-bit arithmetic).
 class Bigint{
 public:
 	typedef uint32_t digit_t;
-	typedef int32_t sdigit_t;
-	typedef uint64_t longdigit_t;
-	typedef int64_t slongdigit_t;
+	typedef int32_t sdigit_t; //signed version
+	typedef uint64_t longdigit_t; //should be twice as large as digit_t
+	typedef int64_t slongdigit_t; //signed version
 	static const int digit_bits=8*sizeof(digit_t);
 
 private:
-	std::vector<digit_t> digits;
-	int sign;
+	std::vector<digit_t> digits; //little-endian order
+	int sign; //-1 for negative, 1 for positive; -0 is not a valid state
 
 	static void add(Bigint&,const Bigint&); //ignores sign of arguments
-	static void subtract(Bigint&,const Bigint&); //ignores sign of arguments; assumes a>=b
+	static void subtract(Bigint&,const Bigint&); //ignores sign of arguments
 	static Bigint product(const Bigint&,const Bigint&);
 
-	void shrink();
-	void normalise();
+	void shrink(); //prune unnecessary zero digits on top
+	void normalise(); //make sure -0 doesn't occur
+
+	//This function is called from all relevant return points of the member functions. It's
+	//basically a debug check, but it can't hurt to have it since it takes few cpu cycles.
 	void checkconsistent();
 
 	std::pair<Bigint,Bigint> divmod(const Bigint&,int depth,int maxdepth) const; //ignores all signs
@@ -32,8 +38,9 @@ public:
 	Bigint();
 	Bigint(const Bigint&)=default;
 	Bigint(Bigint&&);
-	explicit Bigint(const std::string&);
-	explicit Bigint(sdigit_t);
+
+	explicit Bigint(const std::string&); //Initialise with a large value
+	explicit Bigint(sdigit_t); //Initialise with a small value
 	explicit Bigint(digit_t);
 	explicit Bigint(slongdigit_t);
 	explicit Bigint(longdigit_t);
@@ -54,6 +61,8 @@ public:
 	Bigint& operator<<=(int);
 	Bigint& operator>>=(int);
 	Bigint& negate();
+	//Division and modulo operators are explicitly NOT included, to promote clever usage
+	//of the divmod function, which combines both.
 
 	Bigint operator+(const Bigint&) const;
 	Bigint operator-(const Bigint&) const;
@@ -93,22 +102,18 @@ public:
 	bool odd() const;
 
 	std::string serialiseMantissa() const; //stores everything but the sign
-
-	//restores non-negative number; can throw invalid_argument
-	void deserialiseMantissa(const std::string&);
-	std::vector<bool> bits() const;
+	void deserialiseMantissa(const std::string&); //restores non-negative number; can throw invalid_argument
+	std::vector<bool> bits() const; //Stores bits in little-endian order
 
 	friend std::istream& operator>>(std::istream&,Bigint&);
 	friend std::ostream& operator<<(std::ostream&,Bigint);
 
-	digit_t _digit(int idx) const;
-
 
-	static Bigint mone;
+	static Bigint mone; //Some small values to save a constructor call
 	static Bigint zero;
 	static Bigint one;
 	static Bigint two;
 };
 
 std::istream& operator>>(std::istream&,Bigint&);
-std::ostream& operator<<(std::ostream&,Bigint);
+std::ostream& operator<<(std::ostream&,Bigint); //The output operator supports the `hex` modifier