calculate fee properly in MassPay script

[chadwhitacre]

It's calculated as a percentage on the individual payments, not on the total.

Reticketed from #1671 (comment) :

MassPay is off by a penny. :-/

[chadwhitacre]

I tried levying the fee on the individual payments and am not seeing that it worked. :-/

I'm going to try uploading 12 spreadsheets, one for each payment, to compare the fee for each so I can pinpoint the problem.

[chadwhitacre]

Found it! One of the twelve is off by one.

[chadwhitacre]

And only that one is off.

[chadwhitacre]

The target amount is 15.55, and we're calculating a 0.30 fee, but PayPal calculates a 0.31 fee on 15.25.

[chadwhitacre]

15.25 * 0.02 == .305

[chadwhitacre]

15.25 * 1.02 == 15.555
15.24 * 1.02 == 15.5448

[chadwhitacre]

Sure feels like there's some math going on here ...

[chadwhitacre]

So if we drop back to 15.24 we come out a penny under, and if we go with 15.25 we are a penny over.

[chadwhitacre]

Is this the only number this happens for, or are there others?

[chadwhitacre]

If this is provably the only number this happens for, we can special case it (and good work, @evbogue, for hitting it).

If it's not the only number then (still good work, @evbogue, and) we need a formula for the general case.

[chadwhitacre]

Interesting. Could be a rounding error:

$ ./brute.py 
15.54 0.30470588235294117647058824 15.23529411764705882352941176 15.54000000000000000000000000
15.55 0.30490196078431372549019608 15.24509803921568627450980392 15.55000000000000000000000000
15.56 0.30509803921568627450980392 15.25490196078431372549019608 15.56000000000000000000000000

#!/usr/bin/python
from __future__ import division

from decimal import Decimal as D


for i in range(1554, 1557):
    amount = D(str(i / 100))
    fee = amount - (amount / D('1.02'))
    print amount, fee, amount - fee, (amount - fee) * D('1.02') 

[chadwhitacre]

That is, we do get from the target back to the target there.

[chadwhitacre]

It's related to the quantize call in our rounding.

[chadwhitacre]

But even then, can we give PayPal a dollar amount with more than two digits after the decimal point?

[chadwhitacre]

No.

[chadwhitacre]

That was for 15.24509803921568627450980392. How about 15.245?

[chadwhitacre]

Nope. Same error. Okay!

[chadwhitacre]

Back to square one: is 15.55 the perfect number here?

[chadwhitacre]

Many numbers:

$ ./brute.py 
0.25 0.00 0.25 0.26
0.77 0.02 0.75 0.76
1.27 0.02 1.25 1.28
1.79 0.04 1.75 1.78
2.29 0.04 2.25 2.30
2.81 0.06 2.75 2.80
3.31 0.06 3.25 3.32
3.83 0.08 3.75 3.82
4.33 0.08 4.25 4.34
4.85 0.10 4.75 4.84
5.35 0.10 5.25 5.36
5.87 0.12 5.75 5.86
6.37 0.12 6.25 6.38
6.89 0.14 6.75 6.88
7.39 0.14 7.25 7.40
7.91 0.16 7.75 7.90
8.41 0.16 8.25 8.42
8.93 0.18 8.75 8.92
9.43 0.18 9.25 9.44
9.95 0.20 9.75 9.94
10.45 0.20 10.25 10.46
10.97 0.22 10.75 10.96
11.47 0.22 11.25 11.48
11.99 0.24 11.75 11.98
12.49 0.24 12.25 12.50
13.01 0.26 12.75 13.00
13.51 0.26 13.25 13.52
14.03 0.28 13.75 14.02
14.53 0.28 14.25 14.54
15.05 0.30 14.75 15.04
15.55 0.30 15.25 15.56
16.07 0.32 15.75 16.06
16.57 0.32 16.25 16.58
17.09 0.34 16.75 17.08
17.59 0.34 17.25 17.60
18.11 0.36 17.75 18.10
18.61 0.36 18.25 18.62
19.13 0.38 18.75 19.12
19.63 0.38 19.25 19.64
$

That's with:

#!/usr/bin/python
from __future__ import division

from decimal import Decimal as D


def round_(d):
    return d.quantize(D('0.01'))


for i in range(0, 2000):
    amount = D(str(i / 100))
    fee = amount - round_(amount / D('1.02'))
    base = amount - fee
    check = round_(base * D('1.02'))
    if amount == check:
        continue
    print amount, fee, base, check

[chadwhitacre]

Look at that! If x % 1 in (0.25, 0.75), we need to ... adjust ... some ... how ...

[chadwhitacre]

We can't hit the target. That's the bottom line. I repeat: we can't hit the target.

[chadwhitacre]

So what do we do?

[chadwhitacre]

Ev has $15.55 to withdraw. We compute $15.25 to pass to PayPal. This results in a 31¢ fee where we intended a 30¢ fee.

Options?

• We could leave a penny in his Gittip account. Payout based on 15.54. This would be confusing for him if he ever noticed it.

[chadwhitacre]

I don't see another way to do it. If we accept the extra penny charge from PayPal then we are leaking escrow, and that's Bad™.

[chadwhitacre]

What if we upload the spreadsheet and then take whatever the fee actually is and then as an additional step feed that back into computing the spreadsheet from which we POST exchanges back to Gittip?

The problem is that PayPal calculates fees per-receiver. That still doesn't help us hit the target.

[chadwhitacre]

Thanks! :-)