## Let's `float`

I'll jump straight into the demonstration, to get you all motivated. Imagine
the following situation: I am binge-reading Twitter, and suddenly, among the
flood of cute kitten videos, here comes a special offer: a fictitious uShop
gives 10% off if you spend at least 0.8 credits. (Credits are, obviously, a
pretty strong currency.)

This of course draws my attention. I click through to uShop and after a
short while stumble upon the ideal product: the most recent uPad max with a
smart case for a special price. And it all perfectly sums up to 0.8 which
should grant me a 10% discount.

I don't hesitate for long. Until I find out that there is no discount. To
add to my confusion, uShop tells me to spend another 0.0 credits. Huh?
What's wrong?

You might have guessed it already: uShop is using floats.

## The floating box of crap

Imagine a different situation: I am a uShop developer and receive a ticket
about the obscure behaviour. I find it challenging and dive right into it, and
soon become equally confused as the customer: What is the problem? I mean, if
you put `0.7 + 0.1`

to PHP console, it gives you the expected
answer:

```
var_dump(0.7 + 0.1);
> float(0.8)
```

…or, does it, really?

```
var_dump(0.7 + 0.1 === 0.8);
> bool(false)
```

That's pretty weird. But okay, PHP is clearly flawed and I start persuading
the management that we need to rewrite uShop in Node.js. I've wanted to try that
for some time anyway. And JavaScript is so hip, they must do elementary-school
maths right, right?

```
console.log(0.7 + 0.1);
> 0.7999999999999999
```

Um. Okay. At least JavaScript doesn't bullshit me like PHP did. But what the
hell is going on?

## What is a `float`

?

The thing is you'd still end up with the same result if you rewrote uShop in
pretty much any programming language since the very beginning of programming
languages. The problem lies deeper, beyond languages, in how computers store
data, particularly numbers. Let's start with a little theory of how numbers are
represented in general.

Wikipedia
says that floating-point arithmetic uses “formulaic representation of real
numbers as an approximation,” and goes on to explain the formula:

A number is, in general, represented approximately to a fixed number of
significant digits (the significand) and scaled using an exponent in some fixed
base. A number that can be represented exactly is of the following form:

1.2345 = 12345 × 10^{−4}

### What is a `float`

to a computer?

With the high school recap done, let's see how those numbers are represented
in computers. That is dictated by IEEE 754, a standard established in 1985 and
followed by almost all computing technology ever since then.

IEEE 754 defines many different formats. The one most relevant to us is the
double precision format, which is used by countless languages and virtual
machines including PHP and JavaScript.

Double precision is a binary format – meaning that the base is 2 – that
occupies 64 bits: with one for the signum and eleven for the exponent, 52 bits
are left for the significand. One of its bits is encoded implicitly, so
that's effectively 53 bits of information. What does it mean?

### Storing a number in double precision format

Perhaps the most important part is that double precision is a binary format.
Since we've already revised our elementary and high school math skills, the only
thing left is to remember those boring computer science classes at the
university, and convert a decimal number `0.7`

to binary.

Decimals are converted in two steps: first the integral part, then the
fractional part. The integral part is zero, so that's easy. The fraction not so
much: we'll have to multiply it by two, subtract the integral part of the
result, and keep doing this until we reach zero:

- 0.7 × 2 = 1.4 → we'll subtract 1 and keep it;
- 0.4 × 2 = 0.8 → 0
- 0.8 × 2 = 1.6 → 1
- 0.6 × 2 = 1.2 → 1
- 0.2 × 2 = 0.4 → 0
- 0.4 × 2 = 0.8 → 0
- 0.8 × 2 = wait a minute, we're back at step three!

It turns out that decimal 0.7 converts to binary 0.10(1100) recurring. In
other words, the number doesn't have a finite binary expansion. Incidentally,
the same applies to 0.1 which is converted to binary 0.0(0011) recurring. With
the 53 bits the double precision format gives us, the representation has to be
cut off at some point:

- 0.7 → 0.699999999999999955591079014994
- 0.1 → 0.100000000000000005551115123126

Those two long numbers are the closest values that a computer is capable of
keeping in memory. They add up to a number that is slightly lesser than 0.8 –
the difference doesn't manifest itself until the sixteenth decimal place, but
it's there nonetheless.

## Let's do arbitrary precision math

Now we know what's wrong – and that it is not, in fact, wrong. But even
though it's technically a feature, not a bug, it's a rather impractical one.
Oh yes, there are use cases in which we want to gain performance at the cost of
losing a tiny bit of precision (in games, for example). But there are many more
where things are at stake, ranging from tax inspection fines when miscalculating
money, to credibility in case of scientific research, to the lives of US
soldiers in the Gulf
War.

Unfortunately, PHP doesn't have a native API like, for instance, Java's
`BigDecimal`

, but there are two extensions that can be used for
arbitrary precision math: GMP and BC Math. The latter is generally more usable
(as in developer-friendly) while the former is presumably faster. But why even
bother with choosing the best low-level solution when there are nice
abstractions.

My favourite one is `brick/math`

. Internally, it
works with GMP or BC Math, whichever is available, or if none is available, it
provides its own PHP implementation which basically uses the same algorithm as
if you did the math on paper. Publicly, it exposes an API not unlike that of
Java, which is a well-thought one:

```
BigInteger::of('7');
BigDecimal::of('0.7');
BigRational::of('7/10');
```

It provides three classes for three different use cases: one for integers of
any length, one for numbers with finite decimal expansion, and one for all
rational numbers. The last one is particularly interesting: it keeps track of
the numerator and denominator separately, so that it can *exactly*
represent numbers such as two thirds.

Now if you want to add up two numbers, it's easy and quite intuitive:

```
$uPad = BigDecimal::of('0.7');
$case = BigDecimal::of('0.1');
$total = $uPad->plus($case);
```

You might note that in this API, all three classes ensure immutability –
if you add the price of the case to that of the uPad, it returns a new instance
with the result, but `$uPad`

keeps its value of `0.7`

.

And if you compare the result, I finally get the 10% off my new uPad!

```
if ($total->isGreaterThanOrEqualTo('0.8')) {
$total = $total->multipliedBy('0.9');
}
```

## I've got 99 problems, and a dollar ain't 0.9999974645547

Martin Fowler, in his bible Patterns of Enterprise Application Architecture,
says this about
Money:

A large proportion of the computers in this world manipulate money, so
it's always puzzled me that money isn't actually a first class data type in any
mainstream programming language. The lack of a type causes problems, the most
obvious surrounding currencies. If all your calculations are done in a single
currency, this isn't a huge problem, but once you involve multiple currencies
you want to avoid adding your dollars to your yen without taking the currency
differences into account. The more subtle problem is with rounding. Monetary
calculations are often rounded to the smallest currency unit. When you do this
it's easy to lose pennies (or your local equivalent) because of rounding
errors.

We now have a way to overcome one problem Fowler mentioned: rounding errors.
So what about the other one? If you work in a multi-currency setup, you
certainly don't want to mix up a strong currency such as credits with a weak one
such as US dollars. Imagine sending somebody $699, an equivalent of 0.7 credits
for the uPad, and them receiving 699 credits instead. It's certainly nice for
the recipient, but not for the bank, and the least for the developer responsible
for this.

That's why there is another package in the Brick family: `brick/money`

. As you might
have guessed, it builds on top of `brick/math`

and pretty much
mirrors its API, adding support for currencies:

```
$credits = new Currency('XCR', 0, 'Credits', 5);
$uPad = Money::of('0.7', $credits);
$case = Money::of('0.1', $credits);
$total = $uPad->plus($case);
```

Now if you try to apply a $100 voucher, you'll get a
`MoneyMismatchException`

:

```
$voucherValue = Money::of('100', 'USD');
$totalWithVoucher = $total->minus($voucherValue);
```

Another great feature is the ability to use money in specific contexts, the
most useful example being cash rounding. For example, in the Czech Republic we
round cash to nearest whole crown, while Ireland and Switzerland round to
nearest 5 cent. This can be represented using `CashContext`

and
again, an exception is thrown if you try to mix such value up with an
incompatible one:

```
$cash = Money::of('99', 'CZK', new CashContext(100));
$discount = Money::of('9.99', 'CZK');
$cash->minus($discount);
```

## Databases are no exception

Another topic we need to discuss is storing numbers in the database. The
obvious choice would be `FLOAT`

, wouldn't it? Well, the float in the
database is represented in IEEE 754, so it suffers from the same issues as
floats in the application codebase. In PDO, it even maps to a PHP
`float`

.

A much better choice is the `NUMERIC`

type (also known as
`DECIMAL`

). It maps to a PHP string, so it's safe for pretty much
any value, and it works with a fixed-point precision. That is both its pro and
its con: in some databases, you need to define the precision and scale of the
`NUMERIC`

column when creating it, and while not a single
world's standard currency uses a scale larger than four, you can easily get to
18 with some cryptos (ethers, namely).

Also, the SQL standard is one thing and its implementations are another. For
example in PostgreSQL, `NUMERIC`

without given precision nor scale
can store a value of any precision – which is perfect. However, the standard
in this case rather impractically implies that the scale is zero and such column
can only store integers. Also, the maximum precision of a `NUMERIC`

column differs across database systems. So please be aware of the features and
limitations of the particular one you're using.

An alternative approach might be to use a `BIGINT`

column to store
the amount in minor units. While it certainly works, one thing to consider is
that you cannot immediately tell the scale of the value which might complicate
calculations, and again, with some cryptocurrencies (ethers again), you can run
into the limits of integer values, even on 64-bit platforms.

That's the overview of how to store numbers. Monetary values, specifically,
also need to keep track of the currency. I suggest a `CHAR(3)`

column with the currency code, because it is unique and human-readable at the
same time. You can also create a composite type if your database
supports it:

```
CREATE TYPE price AS (
amount NUMERIC,
currency CHAR(3)
);
```

Some databases also have a native `MONEY`

type. One example is the
SQL Server, another one is PostgreSQL. I only know about Postgres's
`MONEY`

type and I strongly recommend not to use it, because
everything about it – from the fractional precision to the output format –
is determined by the database locale settings, so you might have a hard time
working with it from the application.

`float`

no more

I believe I've covered everything about working with fractional numbers and
monetary values in PHP and databases. If not, feel free to ask further
questions, add suggestions and share stories in the comments section below.

I'll leave you with a one last tip: Roave has created a PHPStan
plugin that detects and forbids `float`

s in your codebase. That
might be a good starting point of refactoring away from `float`

s. You
can find it and learn more in its Github repo.