run Gittip 89

[chadwhitacre]

88

[clone1018]

Thanks for reminding me it's Thursday Chad. +1

[chadwhitacre]

:-)

[chadwhitacre]

I've marked these two users as suspicious:

• https://www.gittip.com/zachary94971135/
• https://www.gittip.com/nik88888nik/

[chadwhitacre]

And I've notified them on Twitter:

• https://twitter.com/whit537/status/434008856008462336
• https://twitter.com/whit537/status/434008890556956672

[chadwhitacre]

Reviewed 56 accounts.

[chadwhitacre]

Droplet spun up and updated. Taking a backup ...

[chadwhitacre]

Backup verified.

[chadwhitacre]

Script started, parent shell kill 6ed.

[chadwhitacre]

Script ran for {age} (2:35:51.452689).

[chadwhitacre]

Blog post tumbled on Tumblr.

Tweet tweeted on Twitter.

[chadwhitacre]

Log downloaded.

[chadwhitacre]

Droplet destroyed.

[chadwhitacre]

Dang it! MassPay is off by a penny again. :-/

[chadwhitacre]

I've used binary search to find the bad amount amongst the 30 in the MassPay CSV for today. The process is:

1. Commit all three files into my local git repo for masspay.
2. Edit todays *.input.csv file.
3. Rerun ./bin/masspay.py -o
4. Click "Make Changes" in the PayPal UI.
5. Upload the new *.paypal.csv file.
6. Click "Review."
7. GOTO 2

The bad amount is 26.26. We compute a net of 25.74 and a fee of 0.52, but PayPal computes a fee of 0.51 on 25.74.

[chadwhitacre]

Last time we dealt with something like this was #1673 (per comment in code). See also #1958, which we worked around but didn't get to the bottom of.

[chadwhitacre]

$ ./env/bin/python test.py
26.24 0.51 25.73 26.24
26.25 0.51 25.74 26.25
26.26 0.51 25.75 26.26
26.27 0.52 25.75 26.26
26.28 0.52 25.76 26.28
26.29 0.52 25.77 26.29
$

#!/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(2624, 2630):
    amount = D(str(i / 100))
    fee = amount - round_(amount / D('1.02'))
    base = amount - fee
    check = round_(base * D('1.02'))
    print amount, fee, base, check

[chadwhitacre]

The third column is the number we upload to PayPal.
The second column is the fee we expect PayPal to upcharge us.

26.24 0.51 25.73 26.24
26.25 0.51 25.74 26.25
26.26 0.51 25.75 26.26
26.27 0.52 25.75 26.26
26.28 0.52 25.76 26.28
26.29 0.52 25.77 26.29

In this case we're trying to hit 26.27, and we're uploading 25.74.

[chadwhitacre]

We're trying to hit 26.27, which we compute as 25.75, which does hit the special-case we added in #1673.

[chadwhitacre]

Okay! So the number we want to upload is 25.75. Why don't we?

[chadwhitacre]

Why is the special-casing wrong in this case? Why is this case not, in fact, special? And why did we think it was?

[chadwhitacre]

We have two special cases: .25 and .75. The math is opposite in the .75 case, but we treat it just like the .25 case.

[chadwhitacre]

Every 50th one, there is no rounding error.

[chadwhitacre]

I'm doing something wrong.

[chadwhitacre]

Okay, bottom line is if it's 0.75 we don't need to adjust. Giving up on the math. :-(

[chadwhitacre]

Here's where my test.py ended up:

#!/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(2624, 2630):
for i in range(0, 3000):
    amount = D(str(i / 100))
    fee = amount - round_(amount / D('1.02'))
    precise_fee = amount - amount / D('1.02')
    base = amount - fee
    check = round_(base * D('1.02'))

    cents = str(base)[-2:]

    try:
        if cents == '25':
            assert check - amount == D('0.01')
        if cents == '75':
            assert check - amount == D('-0.01')
    except AssertionError:
        print "NOPE"

    print "trying to hit  ", str(amount).rjust(5)
    print "guessing fee is", str(precise_fee)[:7].rjust(8)
    print "so uploading   ", str(base).rjust(5)
    print "right?         ", str(check).rjust(5)

    print

[chadwhitacre]

Masspay done for 30 participants. Escrow rebalanced.