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function fits_ascii_encode, sum32 |
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;+ |
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; NAME: |
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; FITS_ASCII_ENCODE() |
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; PURPOSE: |
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; Encode an unsigned longword as an ASCII string to insert in a FITS header |
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; EXPLANATION: |
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; Follows the 23 May 2002 version of the FITS checksum proposal at |
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; http://heasarc.gsfc.nasa.gov/docs/heasarc/fits/checksum.html |
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; CALLING SEQUENCE: |
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; result = FITS_ASCII_ENCODE( sum32) |
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; INPUTS: |
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; sum32 - 32bit *unsigned longword* (e.g. as returned by CHECKSUM32) |
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; RESULT: |
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; A 16 character scalar string suitable for the CHECKSUM keyword |
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; EXAMPLE: |
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; A FITS header/data unit has a checksum of 868229149. Encode the |
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; complement of this value (3426738146) into an ASCII string |
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; |
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; IDL> print,FITS_ASCII_ENCODE(3426738146U) |
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; ===> "hcHjjc9ghcEghc9g" |
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; |
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; METHOD: |
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; The 32bit value is interpreted as a sequence of 4 unsigned 8 bit |
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; integers, and divided by 4. Add an offset of 48b (ASCII '0'). |
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; Remove non-alphanumeric ASCII characters (byte values 58-64 and 91-96) |
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; by simultaneously incrementing and decrementing the values in pairs. |
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; Cyclicly shift the string one place to the right. |
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; |
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; REVISION HISTORY: |
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; Written W. Landsman SSAI December 2002 |
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;- |
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if N_Params() LT 1 then begin |
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print,'Syntax - result = FITS_ASCII_ENCODE( sum32)' |
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return,'0' |
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endif |
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|
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; Non-alphanumeric ASCII characters |
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exclude = [58b,59b,60b,61b,62b,63b,64b,91b,92b,93b,94b,95b,96b] |
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ch = bytarr(16) |
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t = byte(sum32,0,4) |
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byteorder,t,/htonl |
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quot = t/4 + 48b |
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for i=0,12,4 do ch[i] = quot |
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|
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remain = t mod 4 |
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ch[0] = ch[0:3] + remain ;Insert the remainder in the first 4 bytes |
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|
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;Step through the 16 bytes, 8 at a time, removing nonalphanumeric characters |
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repeat begin |
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check = 0b |
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for j=0,1 do begin |
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il = j*8 |
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for i=il,il+3 do begin |
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bad = where( (exclude EQ ch[i]) or (exclude Eq ch[i+4]) , Nbad) |
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if Nbad GT 0 then begin |
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ch[i] = ch[i]+1b |
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ch[i+4] = ch[i+4] -1b |
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check=1b |
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endif |
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endfor |
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endfor |
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endrep until (check EQ 0b) |
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|
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return, string( shift(ch,1)) |
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end |
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