Bit packing of array of integers
I have an array of integers, lets assume they are of type int64_t. Now, I know that only every first n bits of every integer are meaningful (that is, I know that they are limited by some bounds).
What is the most efficient way to convert the array in the way that all unnecessary space is removed (i.e. I have the first integer at a[0], the second one at a[0] + n bits and so on) ?
I would like it to be general as much as possible, because n would vary from time to time, though I guess there might be smart optimizations for specific n like powers of 2 or sth.
Of course I know that I can just iterate value over value, I just want to ask you StackOverflowers if you can think of some more clever way.
Edit:
This question is not about compressing the array to take as least space as possible. I just need to "cut" n bits from every integer and given the array I know the exact n of bits I can safely cut.
Answer
Today I released: PackedArray: Packing Unsigned Integers Tightly (github project).
It implements a random access container where items are packed at the bit-level. In other words, it acts as if you were able to manipulate a e.g. uint9_t or uint17_t array:
PackedArray principle:
. compact storage of <= 32 bits items
. items are tightly packed into a buffer of uint32_t integers
PackedArray requirements:
. you must know in advance how many bits are needed to hold a single item
. you must know in advance how many items you want to store
. when packing, behavior is undefined if items have more than bitsPerItem bits
PackedArray general in memory representation:
|-------------------------------------------------- - - -
| b0 | b1 | b2 |
|-------------------------------------------------- - - -
| i0 | i1 | i2 | i3 | i4 | i5 | i6 | i7 | i8 | i9 |
|-------------------------------------------------- - - -
. items are tightly packed together
. several items end up inside the same buffer cell, e.g. i0, i1, i2
. some items span two buffer cells, e.g. i3, i6