vector.txt

/**************************************************************************\

MODULE: vector

SUMMARY:

Template class for dynamic-sized vectors.

The declaration

   Vec<T> v;

creates a zero-length vector.  To grow this vector to length n,
execute

   v.SetLength(n)

This causes space to be allocated for (at least) n elements, and also
causes the delault constructor for T to be called to initialize these
elements.

The current length of a vector is available as v.length().

The i-th vector element (counting from 0) is accessed as v[i].  If the
macro NTL_RANGE_CHECK is defined, code is emitted to test if 0 <= i <
v.length().  This check is not performed by default.

For old-time FORTRAN programmers, the i-th vector element (counting
from 1) is accessed as v(i).

Let n = v.length().  Calling v.SetLength(m) with m <= n sets the
current length of v to m (but does not call any destructors or free
any space).  Calling v.SetLength(m) with m > n will allocate space and
initialize as necessary, but will leave the values of the already
allocated elements unchanged (although their addresses may change).
If T has a user-defined default constructor, that is invoked.
Otherwise, the new memory locations are "default initialized".
In particular, this means that POD types may be uninitialized.

v.MaxLength() is the largest value of n for which v.SetLength(n) was invoked,
and is equal to the number of entries that have been initialized.
v.SetMaxLength(n) will allocate space for and initialize up to n elements,
without changing v.length().

When v's destructor is called, all constructed elements will be
destructed, and all space will be relinquished.

Space is managed using malloc, realloc, and free.  When a vector is
grown, a bit more space may be allocated than was requested for
efficiency reasons.

Note that when a vector is grown, the space is reallocated using
realloc, and thus the addresses of vector elements may change,
possibly creating dangling references to vector elements.  One has to
be especially careful of this when using vectors passed as reference
parameters that may alias one another.

Because realloc is used to grow a vector, the objects stored
in a vector should be "relocatable"---that is, they shouldn't care
what their actual address is, which may change over time.
Most reasonable objects satisfy this constraint.

v.allocated() is the number of elements which have been allocated,
which may be more than the number elements initialized.
Note that if n <= v.allocated(), then v.SetLength(n) is guaranteed
not to cause any memory allocation, or movement of objects.

IMPLEMENTATION DETAILS:

A Vec<T> object is just a pointer to the first element of the array.
There is a control block immediately before the first element that
keeps track of several parameters: 
   len    -- the logical length of the array (returned by length())
   init   -- the number of elements constructed (returned ny MaxLength())
   alloc  -- the number of elements for which space has been allocated
             (returned by allocated())
   fixed  -- flag that indicates that the length is fixed 
             (returned by fixed())

Note that 0 <= len <= init <- alloc

COMPARISON TO STL VECTORS:

When the length of an NTL vector is reduced, no objects are destroyed.
In contrast, when the length of an STL vector is reduced, objects are
destroyed (effectively, maintaining the invariant len == init).

When the length of an NTL vector is increased, and the new value of len
exceeds the current value of alloc, the underying array of objects is
resized using malloc.  This implies that existing objects are moved using
a bit-wise copy.  As mentioned above, this means that objects should
be "relocatable", in the sense that they do not care what their actual
address is.  Most reasonable objects satisfy this constraint.  An example
of an object that does not is one that stores in one data member a pointer
to another data member within the same object.

In contrast, when the length of an STL vector is increased, an new array
is allocated, and objects from the old array are copied to the new array,
and then destroyed in the old array.  This obviously is much more expensive
that NTL's strategy.  However, the new "move semantics", introduced in C++11,
mitigate this issue somewhat.

Because of NTL's relocatability requirement, it is not recommended to use NTL
vectors over classes coming from the standard library, which may not satisfy
the requirement.  In those cases, you could either use an STL vector, or use an
NTL vector and wrap the suspect classes in an NTL smart pointer of some kind
(SmartPtr or OptionalVal).

Note also that Facebook's open source "folly" library also provides
a vector class that uses realloc in a manner very similar to NTL's vector class.
See https://github.com/facebook/folly/blob/master/folly/docs/FBVector.md



\**************************************************************************/


// EXCEPTIONS: all functions below do not throw any exceptions,
//   except as noted

template<class T>
class Vec {  
public:  

   Vec();  // initially length 0

   Vec(const Vec<T>& a); 
   // copy constructor;  uses the assignment operator of T
   // for copying into locations that have already been initialized,
   // and uses the copy constructor for T for initializing new locations.
   
   // EXCEPTIONS: may throw

   Vec& operator=(const Vec<T>& a);  
   // assignment;  uses the assignment operator of T
   // for copying into locations that have already been initialized,
   // and uses the copy constructor for T for initializing new locations.

   // EXCEPTIONS: weak ES (but if it throws, neither length nor MaxLength
   //    will change, although some previously initialized elements
   //    may have been assigned new values).

   ~Vec();  
   // destructor: calls T's destructor for all initialized
   // elements in the vector, and then frees the vector itself
  
   void SetLength(long n);  
   // set current length to n, growing vector if necessary
   // new objects are initialized using the default contructor for T

   // EXCEPTIONS: strong ES (but the vector may have been
   //    reallocated)
  
   void SetLength(long n, const T& a);  
   // set current length to n, growing vector if necessary
   // new objects are initialized using the copy contructor for T

   // EXCEPTIONS: strong ES (but the vector may have been
   //    reallocated)

   template<class F>
   void SetLengthAndApply(long n, F f);
   // set current length to n, growing vector if necessary
   // any new objects are initialized using defauly constructor
   // for T, and after that, f is applied to each new object x
   // as f(x).

   // EXCEPTIONS: strong ES (but the vector may have been
   //    reallocated)

   long length() const;
   // current length
  
   T& operator[](long i);
   const T& operator[](long i) const;
   // indexing operation, starting from 0.
   // The first version is applied to non-const Vec<T>,
   // and returns a non-const reference to a T, while the second version
   // is applied to a const Vec<T> and returns a const reference to a T.

   // EXCEPTIONS: may throw if range checking turned on, strong ES
  
   T& operator()(long i);
   const T& operator()(long i) const;
   // indexing operation, starting from 1
   // The first version is applied to non-const Vec<T>,
   // and returns a non-const reference to a T, while the second version
   // is applied to a const Vec<T> and returns a const reference to a T.

   // EXCEPTIONS: may throw if range checking turned on, strong ES
  
   T* elts();
   const T* elts() const;
   // returns address of first vector element (or 0 if no space has
   // been allocated for this vector).  If a vector potentially has
   // length 0, it is safer to write v.elts() instead of &v[0].
   // The first version is applied to non-const Vec<T>,
   // and returns a non-const pointer to a T, while the second version
   // is applied to a const Vec<T> and returns a const reference to a T.

   
   void swap(Vec<T>& y);
   // swap with y (fast: just swaps pointers)

   // EXCEPTIONS: throws if vectors are fixed and lengths do not match, strong ES

   void append(const T& a);
   // append a to end of vector; uses the assignment operator of T
   // for copying into locations that have already been initialized,
   // and uses the copy constructor for T for initializing new locations.

   // EXCEPTIONS: strong ES if initializing a new element (and in any 
   //    case, if an exception throws, length and MaxLength remain 
   //    unchanged).

   void append(const Vec<T>& w);
   // append w to end of vector; uses the assignment operator of T
   // for copying into locations that have already been initialized,
   // and uses the copy constructor for T for initializing new locations.

   // EXCEPTIONS: strong ES if initializing new elements (and in any 
   //    case, if an exception throws, length and MaxLength remain 
   //    unchanged).


// Alternative access interface 

   const T& get(long i) const; 
   // v.get(i) returns v[i]
 
   void put(long i, const T& a); 
   // v.put(i, a) equivalent to v[i] = q



// Some STL compatibility

   typedef T value_type;
   typedef value_type& reference;
   typedef const value_type& const_reference;
   typedef value_type *iterator;
   typedef const value_type *const_iterator; 

   T* data();
   const T* data() const;
   // v.data() same as v.elts()

   T* begin();
   const T* begin() const;
   // v.begin() same as v.elts()

   T* end();
   const T* end() const;
   // pointer to last element (or NULL)

   T& at(long i);
   const T& at(long i) const;
   // indexing with range checking


// the remaining member functions are a bit esoteric (skip on first
// reading)

   Vec(INIT_SIZE_TYPE, long n);
   // Vec(INIT_SIZE, n) initializes vector with an intial length of n.
   // new objects are initialized using the default contructor for T

   // EXCEPTIONS: may throw

   Vec(INIT_SIZE_TYPE, long n, const T& a);
   // Vec(INIT_SIZE, n, a) initializes vector with an intial length of n.
   // new objects are initialized using the copy contructor for T

   // EXCEPTIONS: may throw

   void kill(); 
   // release space and set to length 0

   void SetMaxLength(long n); 
   // allocates space and initializes up to n elements. Does not change
   // current length

   // EXCEPTIONS: may throw, strong ES

   void FixLength(long n);
   // sets length to n and prohibits all future length changes.
   // FixLength may only be invoked immediately after the default
   // construction or kill.

   // The kill operation is also subsequently prohibited, and swap is
   // allowed on fixed length vectors of the same length.

   // FixLength is provided mainly to implement Mat<T>, to enforce
   // the restriction that all rows have the same length.

   // EXCEPTIONS: may throw, strong ES

   void FixAtCurrentLength();
   // fixes the length at the cuurent length and prohibits
   // all future length changes.  

   // It is required that length() == MaxLength() when called.

   // EXCEPTIONS: if length() != MaxLength() and error is raised;
   // if length() == 0, a memory allocation error may be raised.
   // Strong ES.

   long fixed() const;
   // test if length has been fixed by FixLength() or FixAtCurrentLength()

   long MaxLength() const;
   // maximum length, i.e., number of allocated and initialized elements

   long allocated() const;
   // the number of objects for which space has been allocated, but not
   // necessarily initialized;  this may be larger than MaxLength().

   T& RawGet(long i);
   const T& RawGet(long i) const;
   // indexing with no range checking

   long position(const T& a) const;
   // returns position of a in the vector, or -1 if it is not there.
   // The search is conducted from position 0 to allocated()-1 the vector, 
   // and an error is raised if the object is found at position MaxLength()
   // or higher (in which case a references an uninitialized object).
   // Note that if NTL_CLEAN_PTR flag is set, this routine takes
   // linear time, and otherwise, it takes constant time.

   // EXCEPTIONS: may throw (as indicated above)

   long position1(const T& a) const;
   // returns position of a in the vector, or -1 if it is not there.
   // The search is conducted from position 0 to length()-1 of the vector.
   // Note that if NTL_CLEAN_PTR flag is set, this routine takes
   // linear time, and otherwise, it takes constant time.
         
};   


/**************************************************************************\

                       Some utility routines

\**************************************************************************/

   
template<class T>
void swap(Vec<T>& x, Vec<T>& y);
// swaps x & y; same as x.swap(y)

// EXCEPTIONS: same as for swap member function

template<class T>
void append(Vec<T>& v, const T& a); 
// appends a to the end of v; same as v.append(a)

// EXCEPTIONS: same as for append member function

template<class T>
void append(Vec<T>& v, const Vec<T>& w);
// appends w to the end of v; same as v.append(w)

// EXCEPTIONS: same as for append member function



/**************************************************************************\

                             Input/Output


The I/O format for a vector v with n elements is:

   [v[0] v[1] ... v[n-1]]

Uses corresponding I/O operators for T

\**************************************************************************/

template<class T>
istream& operator>>(istream&, Vec<T>&);  

// EXCEPTIONS: may throw, weak ES

template<class T>
ostream& operator<<(ostream&, const Vec<T>&);  

// EXCEPTIONS: may throw, weak ES



/**************************************************************************\

                              Equality Testing

\**************************************************************************/


template<class T>
long operator==(const Vec<T>& a, const Vec<T>& b);  

template<class T>
long operator!=(const Vec<T>& a, const Vec<T>& b);


/**************************************************************************\

                  Customized Constructors and Destructors
 
Esoteric: skip on first reading...also these interfaces are subject to change

When new elements in a vector need to be constructed, one of the
following routines is called:

   void BlockConstruct(T* p, long n); 
   // invokes T() to initialize p[i] for i = 0..n-1

   void BlockConstructFromVec(T* p, long n, const T* q);
   // invokes T(q[i]) to initialize p[i] for i = 0..n-1;
   // q points to elements from a Vec<T>

   void BlockConstructFromObj(T* p, long n, const T& q);
   // invokes T(q) to initialize p[i] for i = 0..n-1


When a vector is destroyed, the following routine is called:

   void BlockDestroy(T* p, long n);
   // invokes ~T() on p[i] for i = 0..n-1

The default behavior of these routines may be modified by 
overloading these functions with a custom implementation.

EXCEPTIONS:
In order to provide exception safe code, the Construct routines
should provide strong ES; in particular, if any constructor
throws, all newly constructed objects should be destroyed.
Moreover, the BlockDestroy routine should not throw at all.


In NTL, these routines are overridden for the ZZ_p and GF2E classes,
so that many vector entries will be packed into contiguous storage
locations.  This reduces the number of invocations of malloc, and
increases locality of reference.



\**************************************************************************/