undefined | Better HN

story

0 pointslmilcin6y ago0 comments

I develop accounting software for banks, brokerage houses and likes.

Currency, taxes, rebates, etc. handling is NEVER done with floating point.

Whatever you do with money you need predictable, reproducible results. It is norm that calculations are checked by software at two companies on both sides of transaction. Any discrepancies are alarms, bug reports, unhappy customers.

Every significant operation is exactly specified with rounding rules, etc.

For card payments and especially on terminals usually BCD is used.

For everything else usually some kind of arbitrary length decimal library (BigInteger, BigDecimal).

0 comments

chrisseaton6y ago

> Currency, taxes, rebates, etc. handling is NEVER done with floating point.

Nonsense. I’ve seen real banking code at reputable banks that uses floats.

> Whatever you do with money you need predictable, reproducible results.

Floats aren’t random. They’re perfectly deterministic, predictable and reproducible. If you do the same operation in two places you get the same result.

mynegation6y ago

I write real banking code. There is definitely a banking code that uses floats, e.g. valuation of financial instruments. The parent comment talks about software that does transactions and “simpler” calculations, like taxes and fees etc.

When people talk about non-determinism of floating point, what they usually mean is non-associativity, that is (x+y)+z may not be exactly equal to x+(y+z).

throwawaymath6y ago

> When people talk about non-determinism of floating point, what they usually mean is non-associativity, that is (x+y)+z may not be exactly equal to x+(y+z).

Good example of this, in Python 3:

    >>> (0.1 + 0.2) + 0.3
    0.6000000000000001
    >>> 0.1 + (0.2 + 0.3)
    0.6

chrisseaton6y ago

Every single time you run those two statements, you’ll get the same result. Yes they're non-associative. But that's specified and documented. That's not the same thing as non-deterministic in any way.

3 more replies

tsimionescu6y ago

> Floats aren’t random. They’re perfectly deterministic, predictable and reproducible. If you do the same operation in two places you get the same result.

That's not exactly true in real hardware, or at least it wasn't until ~10 years ago. With the x87 FPU, internal precision was 80 bits, while the x86 registers were at most 64 bits. So, depending on the way the program would transfer data between the CPU and FPU your could get different results. It is very likely that different compilers and different optimization decisions could change the way these operations were implemented, so you would get slight differences between different versions of the software.

There are/were also several global FP flags that could get changed by other programs running on the same CPU/FPU that could impact the result of calculations. So, if you want 100% reproducible FP, you would have to either audit all software running on the same machine to ensure it doesn't touch those flags, or set the flags yourself for every FP calculation in your your program.

1 more reply

lmilcinOP6y ago

I did not say floats are random. But when you do accounting you need to be able to sum large sets of numbers and compare results with another sum of different numbers and the sum must match. This just does not work with FP.

Poor souls that use FP for accounting are scourge of the industry and source of jokes.

chrisseaton6y ago

That's what I used to think, then I met these banking types, and they told me 'no we understand their semantics and we use them correctly and we know it is safe for our programs.' These teams have compiler experts on them - they aren't ignorant.

lmilcinOP6y ago

I started working on accounting software in 2002 and right now work for Citi. Compiler experts in accounting? If you are doing HFT you are not doing accounting. Accounting is what happens later when all those transactions need to actually be accounted for and balance calculated

_y5hn6y ago

If you rely on compiler implementations for accounting, you're already lost.

For anything imprecise and scientific, doubles will normally work well.

Accounting rules regarding truncation and rounding as specified, seems unaccounted for by most until they meet such stringent reqs.

j / k navigate · click thread line to collapse

0 comments

chrisseaton6y ago

> Currency, taxes, rebates, etc. handling is NEVER done with floating point.

Nonsense. I’ve seen real banking code at reputable banks that uses floats.

> Whatever you do with money you need predictable, reproducible results.

Floats aren’t random. They’re perfectly deterministic, predictable and reproducible. If you do the same operation in two places you get the same result.

mynegation6y ago

When people talk about non-determinism of floating point, what they usually mean is non-associativity, that is (x+y)+z may not be exactly equal to x+(y+z).

throwawaymath6y ago

> When people talk about non-determinism of floating point, what they usually mean is non-associativity, that is (x+y)+z may not be exactly equal to x+(y+z).

Good example of this, in Python 3:

    >>> (0.1 + 0.2) + 0.3
    0.6000000000000001
    >>> 0.1 + (0.2 + 0.3)
    0.6

chrisseaton6y ago

3 more replies

tsimionescu6y ago

> Floats aren’t random. They’re perfectly deterministic, predictable and reproducible. If you do the same operation in two places you get the same result.

1 more reply

lmilcinOP6y ago

Poor souls that use FP for accounting are scourge of the industry and source of jokes.

chrisseaton6y ago

lmilcinOP6y ago

_y5hn6y ago

If you rely on compiler implementations for accounting, you're already lost.

For anything imprecise and scientific, doubles will normally work well.

Accounting rules regarding truncation and rounding as specified, seems unaccounted for by most until they meet such stringent reqs.

j / k navigate · click thread line to collapse