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Fix a rounding bug in display of data sizes

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Eg, showImprecise 1 1.99 returned "1.1" rather than "2". The 9 rounded
upward to 10, and that was wrongly used as the decimal, rather than
carrying the 1.

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Joey Hess 2021-07-30 09:56:04 -04:00
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[[!comment format=mdwn
username="joey"
subject="""comment 1"""
date="2021-07-30T12:59:46Z"
content="""
I am confused by the output you show, because 2147483647 is not any number
of terabytes. It is a couple of gigabytes. This makes me suspect you
arrived at that number in some mistaken way.
Also, when git-annex displays "TB", it's talking about TeraBytes, not
TebiBytes (TiB). But your divisions by 1024 is calulating the latter, not the
former. (Well, actually it's calculating gibibytes..)
If there is a bug, it's presumably in Utility.HumanNumber.showImprecise,
which does some tricky rounding of decimals.
Here's a few simple tests:
# ghci Utility/DataUnits.hs
ghci> roughSize storageUnits True 1000000000000
"1 TB"
ghci> roughSize storageUnits True 1099511627776
"1.1 TB"
Those are the correct results for an even terabyte and a tebibyte,
displayed as terabytes. Now trying the number you provided to see what
git-annex info will display:
ghci> roughSize storageUnits True 2147483647
"2.15 GB"
units confirms that is the correct value, after rounding the decimal:
# units
You have: 2147483647 bytes
You want: gigabytes
* 2.1474836
/ 0.46566129
aIt's also almost exactly 2 gibibytes:
You have: 2147483647 bytes
You want: gibibytes
* 2
/ 0.5
The actual value is 1 byte less than 2, if I do my manual math correctly:
2^30*2 = 2147483648 = 2147483647+1
Asking git-annex for the same using memoryUnits which are the SI units:
ghci> roughSize memoryUnits False 2147483647
"1.1 gibibytes"
Well, this final result looks wrong, and indeed what's going on is
similar to what you hypothesized:
ghci> roughSize' memoryUnits False 10 2147483647
"1.9999999991 gibibytes"
ghci> showImprecise 10 1.9999999990686774
"1.9999999991"
ghci> showImprecise 2 1.9999999990686774
"1.1"
ghci> round 0.9999999990686774 * (10^2)
100
So indeed showImprecise is forgetting to carry the 1.
Excellent diagnosis, but next time, show your work. ;-)
"""]]