Consider adding support for packed BCD

[rybakit]

Hi! I work on a msgpack extension that can pack/unpack fixed-point decimals that are stored in the following format:

+-------+--------------------------------+
| scale | packed BCD (2 digits per byte) |
+-------+--------------------------------+

I implemented packing/unpacking logic in pure PHP, but wonder if it would make sense to add support of packed decimals to this extension. Wdyt?

For example, here is how I pack them now:

$data = $value->toFixed(self::PRECISION);

if ('-' === $data[0]) {
    $nibble = 'd';
    $data = \substr($data, 1);
} else {
    $nibble = 'c';
}

$pieces = \explode('.', $data, 2);
$pieces[1] = \rtrim($pieces[1], '0');

$data = "{$pieces[0]}{$pieces[1]}{$nibble}";
if (0 !== \strlen($data) % 2) {
    $data = '0'.$data;
}

$scale = empty($pieces[1]) ? 0 : \strlen($pieces[1]);

return [$scale, \hex2bin($data)];

[rtheunissen]

How does this compare or relate to how mpdecimal stores decimal values?

[rybakit]

I'm not familiar with mpdecimal and don't know how it stores decimals, but if I'm not mistaken, this is implemented in the decNumber library. Maybe this link will explain better what I need:

• https://github.com/tarantool/doc/issues/992#issuecomment-554912061

And some more links to the implementation:

• https://github.com/tarantool/tarantool/blob/8317189a9b1f8351dabaecc8216a1c65b175d227/src/lib/core/decimal.c#L401
• https://github.com/tarantool/decNumber