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2835 :: integer literal cosmetics and sign extension

• jordan (orginal) [2017-03-08T12:47:09+01:00] view original

  Is there another way of writing

  var iVar = 0x0ffff0000.int
  Run

  just (with a suffix or so) like

  var i32var = 0x0ffff0000i32
  Run

  Fiddling with bit set variables from data protocols it would be nice to write something like

  iVar == 0x0ffff0000
  Run

  which bails out because the rhs becomes an int64 rather than an int. I could use

  iVar == 0x0ffff0000i32
  Run

  but this would be extended to 0xffffffff'ffff0000 when

  int.sizeof == int64.sizeof
  Run

  Any thoughts?

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• jordan (orginal) [2017-03-09T00:50:26+01:00] view original

  I found a solution which is semi elegant only but does the job. It was trickier (for me at least) than originally anticipated:

  proc `!`(n: int64): int {.compileTime.} =
    when int.sizeof == int64.sizeof:
      (n and 0xffffffff).int
    else:
      if (n and 0x80000000) != 0:
        (n or (not 0xffffffff)).int
      else:
        n.int
  Run

  This allows me to write something like

  proc tryLayers(v: int): MpgWhat {.inline.} =
    block dontKnow:
      case v and !0xfffe0000:
      of !0xfffa0000,                       # MP3, M1A  (layer III), v1
         !0xfffc0000,                       # MP2, M1A  (layer II),  v1
         
         # MPA, M1A starts with fffe => utf16le BOM
         !0xfff20000,                       # MP3, M2A  (layer III), v2
         !0xfff40000,                       # MP2, M2A  (layer II),  v2
         !0xfff60000,                       # MPA, M2A  (layer I),   v2
         !0xffe20000:                       # MP3, M25A (layer III), v2.5
      [...]
  Run

  which works regardless of architecture for 32bit values loaded into int:

  when int.sizeof == int64.sizeof:
    doAssert !0xffffffffff == 0xffffffff
  else:
    doAssert !0xffffffff == -1
    doAssert !0x7fffffff == int.high
  Run

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• mashingan (orginal) [2017-03-09T01:24:13+01:00] view original

  @jordan, CMIIW, are you perhaps don't want to write it like this?

  var i32var = 0xffff0000'i32
  Run

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• jordan (orginal) [2017-03-09T02:33:21+01:00] view original

  :)

  yes I would like but prefer to work in integer and not provide two code versions for 64 and 32 bit architecture (unless I got something wrong)

  the problem is the sign extension when i32 is propagated to 64 bit int, and u32 opens a can of worms (sign propagation is what I normally want but not here)

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