Compact vector

This library provides a bit-packed vector like data structure for storing integer types.

• The number of bits used by each entry is specified either at compile time as a template argument, or at runtime as an argument to the constructor. More optimizations are performed when the number of bits is known at compile time.
• The vector supports storing both signed and unsigned integer types.
• The library provides two variant of vectors (vector and ts_vector) with different multi-threading guarantees.

Installation

Inside a project

Copy the .hpp files from the include directory into your own development tree. This is a header only library, no compilation is necessary.

Globally

To install the library globally on a system, use:

./configure --prefix=...
make install

After installation, use pkg-config --cflags compact_vector get the compiler flags.

Testing

To run the unit tests of the library do

./configure
make check

Usage

A simple partial example:

#include <compact_vector/compact_vector.hpp>

// Vector of signed int, using 6 bits
compact::vector<int, 6> ary;

// Vector of unsigned int, using a number of bits given from the command line
unsigned bits = atoi(argv[1]);
compact::vector<unsigned int> ary(bits);

The rest of the interface should be identical to the vector class from the standard library.

The type compact::vector<unsigned,1> behaves equivalently to the type std::vector<bool>. That is, it is a bit-packed vector where each entry takes 1 bit of space.

Multi-thread guarantees

Most operations on the vector class of the standard library (such as push_back), are not thread safe. A few operations, like accessing different entries of the vectors, are guaranteed to be thread safe.

The compact_vector library offers bit-packed data structure with and without data access guarantees similar to the non-bit packed vector class from the standard library.

Bit-packed vectors

The vector class from the standard library has the following guarantee for multi-threaded code: if i != j, then accessing a[i] and a[j] from 2 different threads is safe.

On the other hand, this does not hold for the specialized vector<bool> class, which is bit-packed. Accessing a[i] and a[j] from two different threads is not safe, even if i and j are not equal, because both values maybe stored in the same word.

The same apply to compact::vector, it is not safe to access elements from two different threads.

Thread safe vector class

The class compact::ts_vector is thread safe in the sense that if i != j, then accessing a[i] and a[j] from two different threads is safe (just like std::vector). There is a performance penalty as updates to the data structure are now performed with CAS (compare and swap) atomic operations.