On Wed, Feb 12, 2020 at 12:09 PM Jacek Pliszka <jacek.plis...@gmail.com> wrote: > > Hi! > > ARROW-3329 - we can discuss there. > > > It seems like it makes sense to implement both lossless safe casts > > (when all zeros after the decimal point) and lossy casts (fractional > > part discarded) from decimal to integer, do I have that right? > > Yes, though if I understood your examples are the same case - in both > cases fractional part is discarded - just it is all 0s in the first > case. > > The key question is whether CastFunctor in cast.cc has access to scale > of the decimal? If yes how?
Yes, it's in the type of the input array. Here's a kernel implementation that uses the TimestampType metadata of the input https://github.com/apache/arrow/blob/master/cpp/src/arrow/compute/kernels/cast.cc#L521 > > If not - these are the options I've came up with: > > Let's assume Decimal128Type value is n > > Then I expect that base call > .cast('int64') will return overflow for n beyond int64 values, value > otherwise > > Option 1: > > .cast('int64', decimal_scale=s) would calculate n/10**s and return > overflow if it is beyond int64, value otherwise > > Option 2: > > .cast('int64', bytes_group=0) would return n & 0x00000000FFFFFFFF > .cast('int64', bytes_group=1) would return (n >> 64) & 0x00000000FFFFFFFF > .cast('int64') would have default value bytes_group=0 > > Option 3: > > cast has no CastOptions but we add multiply compute kernel and have > something like this instead: > > .compute('multiply', 10**-s).cast('int64') > > BR, > > Jacek
