I need a simple way to hash strings into 64bit integers so I started some research. Initially I was focusing on xxHash but it's slightly harder to implement than other newer hashes. I ended up landing on the amazing Komihash which bills itself as: "a very fast 64-bit hash function, mainly designed for hash-table, hash-map, and bloom-filter uses".

Komihash is available as a single Komihash.h file and is extremely easy to implement into existing code.

#include <stdio.h>
#include <stdlib.h>
#include "komihash.h"

int main(int argc, char *argv[]) {
    int seed        = 0;
    const char *buf = "Hello world";

    uint64_t hash_num = komihash(buf, strlen(buf), seed);

    printf("Komihash: %s = %llu\n", buf, hash_num); // 3745467240760726046
}

As a bonus it also comes with a PRNG which is equally simple to implement:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include "komihash.h"

int main(int argc, char *argv[]) {
    uint64_t seed1 = time(NULL);
    uint64_t seed2 = (seed1 << 32);

    for (int i = 0; i < 5; i++) {
        uint64_t rand64 = komirand(&seed1, &seed2);
        printf("Komirand: %llu\n", rand64);
    }
}

In fact, I liked it so much I ported it to Perl in Crypt::Komihash.

I'm working with a 32bit PRNG and I'd like to get a 64bit integer out of it. You can take two random 32bit integers and combine them into a 64bit one this way:

uint64_t high = rand32();
uint64_t low  = rand32();

uint64_t num64 = (high << 32) | low;

Someone on Reddit suggested using this cool hash to perform a similar task to "upgrade" a 32bit number into a 64bit one by hashing it:

// Borrowed from: https://elixir.bootlin.com/linux/v6.11.5/source/include/linux/hash.h
static inline uint64_t hash64(uint64_t val, unsigned bits) {
        return (val * 0x61c8864680b583ebull) >> (64 - bits);
}

I needed this functionality in Perl so got some help from Reddit and wrote it up:

sub hash64 {
    my ($val, $bits)   = @_;
    my $magic_constant = 7046029254386353131;

    use integer; # This forces integer math and overflow
    my $result = ($val * $magic_constant) >> (64 - $bits);

    # Convert from signed int to unsigned
    if ($result < 0) {
        no integer;
        $result += 18446744073709551615; # 2**64 - 1
        $result += 1;
    }

    return $result;
}

I'm a big fan of .ini files because they're human readable, and also very machine readable. I wrote a quick function to convert a hashref to a simple .ini string.

my $str = hash_to_ini({ data => { 'name' => 'scott', animal => 'dog' }});

sub hash_to_ini {
    my $x   = $_[0];
    my $ret = '';

    foreach my $key (sort(keys(%$x))) {
        my $val = $x->{$key};

        if (ref($val) eq "HASH") {
            $ret .= "[$key]\n";
            foreach my $k (sort(keys(%$val))) { $ret .= "$k = " . $val->{$k} . "\n"; }
        } else { $ret .= "$key = $val\n" }
    }

    $ret =~ s/\n\[/\n\n[/; # Add a space between sections
    return $ret;
}

I also wrote a version in PHP

function hash_to_ini($x) {
    $ret = '';

    foreach (array_keys($x) as $key) {
        $val = $x[$key];

        if (is_array($val)) {
            $ret .= "[$key]\n";
            foreach (array_keys($val) as $k) { $ret .= "$k = " . $val[$k] . "\n"; }
        } else { $ret .= "$key = $val\n"; }
    }

    $ret = preg_replace("/\n\[/", "\n\n[", $ret);
    return $ret;
}

Note: see also parse_ini()

This syntax still doesn't make much sense to me but here is how you sort a perl hash by value. This returns a list of all the keys of the hash sorted in the order you want. To reverse the sort simply change $a and $b locations with each other.

my @sort = sort{ $unique{$a} <=> $unique{$b} } keys %unique;