Comment #0 by bearophile_hugs — 2013-06-02T05:08:32Z
Split from Issue 4124.
I suggest to add some useful methods to BitArray:
- reset all bits
- set all bits
- are all bit set?
- are all bit reset?
- set n-th bit (this can be a little more efficient than bs[n]=1;)
- reset n-th bit (this can be a little more efficient than bs[n]=1;)
- flip n-th bit
After Issue 4124 "%b" (unlike "%s") produces a string output that's not usable to build a new BitArray, this is another enhancement request (underscores and leading-trailing whitespace should be ignored):
import std.stdio, std.bitmanip, std.conv, std.string;
void main() {
BitArray b1;
b1.init([0, 0, 0, 0, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1]);
writefln("%b", b1); // Prints: 00001111_00001111
BitArray b2;
// Error: function std.bitmanip.BitArray.init (bool[] ba)
// is not callable using argument types (string)
b2.init("00001111_00001111");
}
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The usefulness of isSet(), set(), and reset() methods is visible with two benchmarks. They implement the same algorithm (a simple sieve), the first uses a BitArray, and the second a manually implemented bit array. On my 32 bit system the second program is about two times faster than the first one.
import std.stdio, std.algorithm, std.range, std.math, std.bitmanip, std.datetime;
uint[] primes(uint n) {
if (n < 2) return [];
BitArray F;
F.length = n + 1;
F[0] = true;
F[1] = true;
foreach (i; 2 .. cast(uint)sqrt(n))
if (!F[i])
for (uint j = i * i; j <= n; j += i)
F[j] = true;
return array(filter!((i){ return !F[i]; })(iota(n+1)));
}
void main() {
StopWatch sw;
sw.start();
primes(30_000_000);
sw.stop();
writeln(sw.peek().msecs / 1000.0);
}
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import std.stdio, std.algorithm, std.range, std.math, std.datetime;
size_t[] primes(in size_t n) {
enum size_t bpc = size_t.sizeof * 8;
auto F = new size_t[n / bpc + 1];
F[] = size_t.max;
bool isSet(in size_t i) nothrow {
immutable size_t offset = i / bpc;
immutable size_t mask = 1 << (i % bpc);
return (F[offset] & mask) != 0;
}
void clear(in size_t i) nothrow {
immutable size_t offset = i / bpc;
immutable size_t mask = 1 << (i % bpc);
if ((F[offset] & mask) != 0)
F[offset] = F[offset] ^ mask;
}
clear(0);
clear(1);
foreach (i; 2 .. cast(size_t)sqrt(n))
if (isSet(i))
for (uint j = i * i; j <= n; j += i)
clear(j);
return array(filter!isSet(iota(n + 1)));
}
void main() {
StopWatch sw;
sw.start();
size_t n = primes(30_000_000).length;
sw.stop();
writeln(sw.peek().msecs / 1000.0);
writeln("n primes: ", n);
}
Comment #1 by bearophile_hugs — 2013-06-02T05:11:01Z
See also Issue 10239
Comment #2 by robert.schadek — 2024-12-01T16:17:50Z