std.bitmanip.FixedBitArray

[dlangBugzillaToGithub]

bearophile_hugs reported this on 2013-07-15T17:00:54Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=10650

Description

This is a struct with no defined constructor that uses a bit array of statically known size:


import std.bitmanip: BitArray;
struct Foo {
    enum nBits = 1_024;
    size_t[nBits / size_t.sizeof] buffer;
    BitArray bitSet;
    bool isInitialized = false;

    void bar() /*pure nothrow*/ {
        if (!isInitialized) {
             bitSet.init(buffer, nBits);
             isInitialized = true;
         }

        // .......
    }
}
void main() {}



A statically known size is useful to reduce pressure a bit on the GC, to increase cache locality, etc. So I suggest to introduce in std.bitmanip a simple FixedBitArray based on BitArray that offers a simpler usage for statically known sizes of bit arrays:


struct FixedBitArray(size_t nBits) {
    private size_t[nBits / size_t.sizeof + (nBits % size_t.sizeof) ? 1 : 0] buffer;
...
}


import std.bitmanip: FixedBitArray;
struct Foo {
    FixedBitArray!(1_024) bitSet;

    void bar() pure nothrow {
        // .......
    }
}
void main() {}


An alternative name is "BoundedBitArray" as in the Ada 2012 bounded collections.

An alternative is to modify BitArray to allow both usages nicely.