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In a reply to Netflix (or on handling large amounts of data efficiently in perl), I suggested that Judy::1/Judy1(3) was a sparse bit vector which might be interesting as a replacement for vec(). Perl's built-in vec can also be used as a bit vector but it's not sparse - all bits between the zeroth to the highest ever set are instantiated inside a perl string. This is only convenient if your bit vector isn't too big in ram.

## Example code

A simple perl bit vector

```use constant MEGABYTE => 2 ** 20;
my \$vector = '';
# 5,000 bits. Set every 100th bit between 15 million and 20 million.
for ( 15_000_000 .. 20_000_000 ) {
if ( ! ( \$_ % 100 ) ) {
vec( \$vector, \$_, 1 ) = 1;
}
}
printf "%0.1fM\n", length( \$vector ) / MEGABYTE;

A simple Judy::1 bit vector

```use constant KILOBYTE => 2 ** 10;
use Judy::1 qw( Set MemUsed );
my \$vec;
# 5,000 bits. Set every 100th bit between 15 million and 20 million.
for ( 15_000_000 .. 20_000_000 ) {
if ( ! ( \$_ % 100 ) ) {
Set( \$judy, \$_ );
}
}
printf "%0.1fK\n", MemUsed( \$judy ) / KILOBYTE;

## Memory "benchmarks"

Densityvec()Judy::1
Tests using the entire range 0..20,000,000
Every bit2.3M618K
Densityvec()Judy::1
Tests using range 15,000,000..20,000,000
Every bit2.3M162K
Every 10th bit2.3M772K
Every 100th bit2.3M158K
Every 1000th bit2.3M65K

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In reply to Compact and sparse bit vector by diotalevi

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