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Unpacking ICAO Codes etc from BGL File

Discussion in 'Tools programming' started by scruffyduck, 22/11/06.

  1. scruffyduck

    scruffyduck Administrator Staff Member FSDevConf team Resource contributor

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    wales
    My head hurts :)

    I am trying to understand how the packing of ICAO codes works in the bgl file format.

    Reading Winfried's documentation each letter or number has a value between 0...37 With:
    blank 00
    digits 0 .. 9 02 .. 11
    letters A .. Z 12 .. 37

    I am to take the leftmost letter/number coded as above and multiply it by 38. I then add the next letter/number code to the result and multiply that by 38. Keep going until all the letters/numbers have been consumed. Multiply the result by 0x20 to left shift it by 5 bytes and hey presto that is the packed number!

    Well not here it ain't!! So I must be doing something wrong - consistently.

    Take MXA3 - M=24, X=35, A=12 and 3=5. The result in the bgl file for this is 43773729 but if I use the above calculation (WITHOUT the shift) I get 51981302. Compare the two numbers and mine is 1.187499973 times the number stored in the bgl.

    I tried 6 other ICAOs and in each case got a value 1.18749973 times the value in the BGL file. Clearly I don't understand how this works so any help would be much appreciated.
    Last edited: 22/11/06
  2. Manuel Ambulo

    Manuel Ambulo

    Joined:
    29/9/06
    Messages:
    162
    Country:
    panama
    Hi, scruffyduck,

    In my FS2004's program, for get an airport's ICAO_ID, i use the structure (below) for airport, where i get the "dwICAO_ID" member, and pass it to the "Get_ICAO_ID" subroutine (and also, i pass an empty string which will be filled with the decoded ICAO_ID of the airport). Note, that i just get the lastest 27 bits of the DWORD containing the ICAO_ID....the first 5 bits is unused for me.

    // FS2004's Airport
    struct FS2004_Airport
    {
    unsigned short wType; // (0x0003 = Airport)
    unsigned long dwSize;
    unsigned char Total_runways;
    unsigned char Total_com_freqs;
    unsigned char Total_startlocations;
    unsigned char unknown1;
    unsigned char unknown2;
    unsigned char Total_helipads;
    unsigned long Lon;
    unsigned long Lat;
    unsigned long Alt;
    unsigned long Tower_Lon;
    unsigned long Tower_Lat;
    unsigned long Tower_Alt;
    float mag_var;
    unsigned long dw1 :5; // (5 bits)--> Unknown
    unsigned long dwICAO_ID :27; // (27 bits)--> Airport's ICAO_ID
    unsigned long unknown3;
    unsigned long unknown4;
    };

    void Get_ICAO_ID(char szId[6], unsigned long dwId)
    {
    static char szIdChar[] = " 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    szId[0] = '\0';

    for (int nIndex = 0; nIndex < 5 && dwId > 0; nIndex++)
    {
    memmove(szId + 1, szId, 5);
    szId[0] = szIdChar[dwId % 38];
    dwId /= 38;
    }
    }


    24 * 38 = 912
    912 + 35 * 38 = 35986
    35986 + 12 * 38 = 1367924
    1367924 + 5 * 38 = 51981302 <--- Here is the value you get, BUT!..the problem is...

    The lastest multiplied "38"...see...if i dont multiply the last digit by "38", you will get -->
    1367924 + 5 = **1367929**.........instead of 51981302.

    If i convert the result "1367929" from decimal to binary i will get:

    101001101111101111001

    And if i get the value in the *.bgl file, "43773729", and convert it to binary:

    10100110111110111100100001

    Compare both values and you will see they are the SAME, except for the first 5 bits in the "43773729" value, See:

    101001101111101111001-------------> 1367929
    101001101111101111001---00001----> 43773729
    --------------------------(5 bits)----

    In others words, to the result you get from your calculations will be "1367929", if you do a Left shift (<<) of 5 bits:

    1367929 << 5 = 43773728; // For add the 5 bits to your value...

    You will get ---> 43773728 (pretty much closer to the value you get in the *.bgl file).

    101001101111101111001----00000----> 43773728 (calculations results..)
    101001101111101111001----00001----> 43773729 (value from *.bgl file.)
    ---------------------------(5 bits)-----------

    Hope this help you :)

    Best Regards,

    Manuel Ambulo
    Last edited: 23/11/06
  3. scruffyduck

    scruffyduck Administrator Staff Member FSDevConf team Resource contributor

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    Hi Manuel and many many thanks - that is the conclusion I came to at 2am this morning when I could not sleep! I was multiplying the last letter/number by 38 when I should not. 38/32 is 1.1875 and that is the error I found :)


    Update: Ok that is working fine now - thanks again!
    Last edited: 23/11/06
  4. VADS

    VADS

    Joined:
    12/12/06
    Messages:
    177
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    southafrica
    Unpacking ICAO Codes

    I need to query when they say shift to the left 5 bytes - how does one convert the result to get what you're looking for?

    as i had the idea somehow that if i have 51981302 that i'd convert
    the last three by multiplying by 0.20

    302 x 0.20 = 60.4 is this calculation correct or have i misunderstood something along the way?

    Gareth - Vads
    Virtual Aerospace Design South Africa
  5. berendt

    berendt

    Joined:
    27/2/07
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    germany
    Unpacking ICAO Codes ...

    Hi,

    presumably, your question is completely answered by now; however, if you feel so, you might have a look at my tool <Find_ICAO.zip> which I uploaded to the Avsim Library last week.

    Regards, Gerhard.
    Last edited: 3/3/07
  6. scruffyduck

    scruffyduck Administrator Staff Member FSDevConf team Resource contributor

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    Thanks Gerhard

    Yes SDE can now pack and unpack ICAO codes, both those that are shifted and those that are not. :)

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