pdsinfo.c

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/********************************************************************
 *                                                                  *
 *  Copyright (C) 2007, 2008, 2009                                  *
 *  Charles Darwin University, Darwin, Australia                    *
 *                                                                  *
 *  This program is free software; you can redistribute it and/or   *
 *  modify it under the terms of the GNU General Public License as  *
 *  published by the Free Software Foundation; either version 2 of  *
 *  the License, or (at your option) any later version.             *
 *                                                                  *
 *  This program is distributed in the hope that it will be         *
 *  useful, but WITHOUT ANY WARRANTY; without even the implied      *
 *  warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR         *
 *  PURPOSE.  See the GNU General Public License for more details.  *
 *                                                                  *
 *  You should have received a copy of the GNU General Public       *
 *  License along with this program; if not, write to the Free      *
 *  Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,  *
 *  MA  02111-1307  USA                                             *
 *                                                                  *
 ********************************************************************
 *                                                                  *
 *  provide info about contents of a PDS file                       *
 *                                                                  *
 *  05/12/2007  S W Maier    start of work                          *
 *  12/12/2007  S W Maier    initial version                        *
 *  14/12/2007  S W Maier    added count of missing packets         *
 *  03/06/2008  S W Maier    added count of missing seconds         *
 *  14/08/2008  S W Maier    fix bug in missing seconds calcula-    *
 *                           tion when covering two days            *
 *  10/10/2008  S W Maier    increased data buffer and test for     *
 *                           packet size before reading             *
 *  09/12/2008  S W Maier    test if we have found any valid pkts   *
 *  14/02/2009  S W Maier    output number of day, night and eng.   *
 *                           pkts                                   *
 *  24/03/2009  S W Maier    proper handling of corrupted files     *
 *                                                                  *
 ********************************************************************
 *                                                                  *
 *  usage: pdsinfo <input>                                          *
 *                                                                  *
 ********************************************************************
 *                                                                  *
 * to do:                                                           *
 *  - support other sensors                                         *
 *                                                                  *
 ********************************************************************
 *                                                                  *
 *  build: cc pdsinfo.c -lm -o pdsinfo                              *
 *                                                                  *
 ********************************************************************/
 
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
 
 
/********************************************************************
 *                                                                  *
 *  defines                                                         *
 *                                                                  *
 ********************************************************************/
/* name */
#define NAME "pdsinfo"
/* version */
#define VERSION 1
/* revision */
#define REVISION 6
/* primary header size */
#define PRI_HDR_SIZE 6
/* MODIS secondary header size */
#define MODIS_HDR_SIZE 12
/* Julian Day of MODIS reference date (01/01/1958)*/
#define MODIS_REF_DATE 2436205.0
/* data buffer size */
#define DATA_SIZE 100000
 
 
/********************************************************************
 *                                                                  *
 *  structure definitions                                           *
 *                                                                  *
 ********************************************************************/
 
/* primary header */
struct pri_hdr {
    int version;
    int type;
    int sec_hdr_flag;
    int apid;
    int seq_flags;
    int pkt_count;
    int pkt_length;
};
 
/* APID info */
struct apid_info {
    int apid;
    long int count;
    long int invalid;
    long int missing;
    long int last_pkt_count;
    struct apid_info *next;
};
 
/* MODIS header */
struct modis_hdr {
    int days;
    unsigned long int millisec;
    int microsec;
    int ql;
    int pkt_type;
    int scan_count;
    int mirror_side;
    int src1;
    int src2;
    int conf;
    int sci_state;
    int sci_abnorm;
    int checksum;
};
 
 
/********************************************************************
 *                                                                  *
 *  function declarations                                           *
 *                                                                  *
 ********************************************************************/
int ReadPriHdr(FILE *f, unsigned char *buf);
int DecodePriHdr(unsigned char *buf, struct pri_hdr *hdr);
int DecodeMODISHdr(unsigned char *buf, int len,
        struct modis_hdr *hdr);
struct apid_info *AllocAPIDInfo(int apid);
void AddAPIDInfo(struct apid_info **list, struct apid_info *ai);
void FreeAPIDInfoList(struct apid_info *list);
struct apid_info *FindAPIDInfo(struct apid_info *list, int apid);
void caldat(int *minute, int *hour, int *day, int *month, int *year,
        double jul);
int CalcChecksum12(unsigned char *buf, int n);
 
/********************************************************************
 *                                                                  *
 *  main function                                                   *
 *                                                                  *
 ********************************************************************/
int main(int argc, char *argv[]) {
    /* file pointer */
    FILE *fin;
    /* primary header structure */
    struct pri_hdr hdr;
    /* MODIS header structure */
    struct modis_hdr mhdr;
    /* header buffer */
    unsigned char buf_hdr[PRI_HDR_SIZE];
    /* data buffer */
    unsigned char *buf_data;
    /* pointer to APID Info list */
    struct apid_info *apidlist = NULL;
    /* pointer to current APID Info object */
    struct apid_info *apidinfo = NULL;
    /* first packet date/time */
    long int firstday = 1.E6, firstms = 1.E6, firstmics = 1.E6;
    /* last packet date/time */
    long int lastday = 0, lastms = 0, lastmics = 0;
    /* previous packet date/time */
    long int prevday = 0, prevms = 0;
    /* date buffer */
    int second, minute, hour, day, month, year;
    long int ms;
    double jul;
    /* checksum */
    int chksum;
    /* error code */
    int error;
    /* number of missing packets */
    long int missing;
    /* number of missing seconds */
    long int missingsecs = 0;
    /* millisecs difference between packets */
    long int diffms;
    /* */
    int lastdays, lastmicrosec, lastsrc;
    unsigned long int lastmillisec;
    /* number of day packets */
    long int daypkts1 = 0, daypkts2 = 0;
    /* number of night packets */
    long int nightpkts1 = 0, nightpkts2 = 0;
    /* number of engineering packets */
    long int engpkts1 = 0, engpkts2 = 0;
    /* return value */
    int retvalue = 0;
 
 
    /* print version information */
    fprintf(stderr, "%s V%d.%d ("__DATE__")\n",
            NAME, VERSION, REVISION);
 
    /* check number of arguments */
    if (argc != 2) {
        fprintf(stderr, "USAGE: %s <input>\n", argv[0]);
        return (20);
    }
 
    /* allocate memory */
    if (!(buf_data = malloc(sizeof (unsigned char) * DATA_SIZE))) {
        fprintf(stderr, "not enough memory\n");
        return (10);
    }
 
    /* open input file */
    if (!(fin = fopen(argv[1], "rb"))) {
        fprintf(stderr, "can't open input file (%s)\n", argv[1]);
        return (10);
    }
 
    /* main loop */
    for (;;) {
        /* read primary header */
        if (ReadPriHdr(fin, buf_hdr)) {
            /* end of file? */
            if (feof(fin))
                break;
            else {
                fprintf(stderr,
                        "error 1 reading input file (%s): "
                        "file might be corrupted\n",
                        argv[1]);
                retvalue = 5;
                break;
            }
        }
 
        /* decode primary header */
        switch (error = DecodePriHdr(buf_hdr, &hdr)) {
        case 0:
            break;
        case -1:
            fprintf(stderr,
                    "unsupported packet version (%d): "
                    "file might be corrupted, trying to resyncronise\n",
                    hdr.version);
 
            /* read data block */
            if (fread(buf_data, hdr.pkt_length + 1, 1, fin) != 1) {
                fprintf(stderr,
                        "error 2 reading input file (%s): "
                        "file might be corrupted\n",
                        argv[1]);
                retvalue = 5;
                break;
            }
            continue;
        default:
            fprintf(stderr,
                    "unknown error (%d) while decoding primary header\n",
                    error);
            return (5);
        }
 
        /* do we have a current APID Info object? */
        if (apidinfo != NULL) {
            /* APID different as of current APID Info object? */
            if (apidinfo->apid != hdr.apid) {
                /* find APID Info object */
                if (!(apidinfo = FindAPIDInfo(apidlist, hdr.apid))) {
                    /* allocate an APID Info object */
                    if (!(apidinfo = AllocAPIDInfo(hdr.apid))) {
                        fprintf(stderr, "can't allocate memory\n");
                        return (5);
                    }
 
                    /* add APID Info object to list */
                    AddAPIDInfo(&apidlist, apidinfo);
                }
            }
        } else {
            /* allocate an APID Info object */
            if (!(apidinfo = AllocAPIDInfo(hdr.apid))) {
                fprintf(stderr, "can't allocate memory\n");
                return (5);
            }
 
            /* add APID Info object to list */
            AddAPIDInfo(&apidlist, apidinfo);
        }
 
        /* increase packet counter */
        apidinfo->count = apidinfo->count + 1;
 
        /* check if there are missing packets */
        /* first packet with this APID? */
        if (apidinfo->last_pkt_count != -1) {
            /* calculate number of missing packets */
            missing =
                    (hdr.pkt_count > apidinfo->last_pkt_count) ?
                    (hdr.pkt_count - apidinfo->last_pkt_count - 1) :
                    (hdr.pkt_count - apidinfo->last_pkt_count + 16383);
 
            /* duplicated packet? */
            if (missing == 16383)
                fprintf(stderr, "duplicated packet!!!\n");
 
            /* add to counter for missing packets */
            apidinfo->missing += missing;
        }
 
        /* store packet count */
        apidinfo->last_pkt_count = hdr.pkt_count;
 
        /* read data block */
        if (hdr.pkt_length + 1 > DATA_SIZE) {
            fprintf(stderr,
                    "buffer overflow (%d), "
                    "please contact developer\n",
                    hdr.pkt_length);
            return (20);
        }
        if (fread(buf_data, hdr.pkt_length + 1, 1, fin) != 1) {
            fprintf(stderr,
                    "error 3 reading input file (%s): "
                    "file might be corrupted\n",
                    argv[1]);
            retvalue = 5;
            break;
        }
 
        /* is it a MODIS packet? */
        if ((hdr.apid >= 64) && (hdr.apid <= 127)) {
            /* decode MODIS header */
            DecodeMODISHdr(buf_data, hdr.pkt_length + 1, &mhdr);
 
            /* duplicated packet? */
            if (missing == 16383) {
                fprintf(stderr, "duplicated MODIS packet: %d/%d %ld/%ld %d/%d %d/%d\n",
                        mhdr.days, lastdays,
                        mhdr.millisec, lastmillisec,
                        mhdr.microsec, lastmicrosec,
                        mhdr.src2, lastsrc);
            }
            lastdays = mhdr.days;
            lastmillisec = mhdr.millisec;
            lastmicrosec = mhdr.microsec;
            lastsrc = mhdr.src2;
 
            /* calculate checksum */
            chksum = CalcChecksum12(&(buf_data[MODIS_HDR_SIZE]),
                    (hdr.pkt_length + 1 - MODIS_HDR_SIZE) /
                    1.5 - 1);
 
            /* valid packet? */
            if (chksum != mhdr.checksum) {
                /* increase invalid packet counter */
                apidinfo->invalid = apidinfo->invalid + 1;
            }
 
            /* determine first and last packet date/time */
            if (mhdr.days < firstday) {
                firstday = mhdr.days;
                firstms = mhdr.millisec;
                firstmics = mhdr.microsec;
            } else {
                if (mhdr.days == firstday) {
                    if (mhdr.millisec < firstms) {
                        firstday = mhdr.days;
                        firstms = mhdr.millisec;
                        firstmics = mhdr.microsec;
                    } else {
                        if (mhdr.millisec == firstms) {
                            if (mhdr.microsec < firstmics) {
                                firstday = mhdr.days;
                                firstms = mhdr.millisec;
                                firstmics = mhdr.microsec;
                            }
                        }
                    }
                }
            }
            if (mhdr.days > lastday) {
                lastday = mhdr.days;
                lastms = mhdr.millisec;
                lastmics = mhdr.microsec;
            } else {
                if (mhdr.days == lastday) {
                    if (mhdr.millisec > lastms) {
                        lastday = mhdr.days;
                        lastms = mhdr.millisec;
                        lastmics = mhdr.microsec;
                    } else {
                        if (mhdr.millisec == lastms) {
                            if (mhdr.microsec > lastmics) {
                                lastday = mhdr.days;
                                lastms = mhdr.millisec;
                                lastmics = mhdr.microsec;
                            }
                        }
                    }
                }
            }
 
            /* check if there are missing seconds */
            if (prevday != 0) {
                diffms =
                        mhdr.millisec - prevms +
                        (mhdr.days - prevday) * 86400000;
                missingsecs += diffms / 1000;
            }
            prevday = mhdr.days;
            prevms = mhdr.millisec;
 
            /* earth view packet? */
            if (mhdr.src1 == 0) {
                /* increase packet type counters? */
                switch (mhdr.pkt_type) {
                case 0:
                    daypkts1++;
                    break;
                case 1:
                    nightpkts1++;
                    break;
                case 2:
                case 4:
                    engpkts1++;
                    break;
                }
            } else {
                /* increase packet type counters? */
                switch (mhdr.pkt_type) {
                case 0:
                    daypkts2++;
                    break;
                case 1:
                    nightpkts2++;
                    break;
                case 2:
                case 4:
                    engpkts2++;
                    break;
                }
            }
        }
    }
 
    /* have we read any valid packets? */
    if (apidinfo == NULL) {
        fprintf(stderr, "no valid packets found\n");
        return (5);
    }
 
    /* print APID statistics */
    for (apidinfo = apidlist;
            apidinfo != NULL;
            apidinfo = apidinfo->next) {
        printf("APID %d: count %ld invalid %ld missing %ld\n",
                apidinfo->apid,
                apidinfo->count,
                apidinfo->invalid,
                apidinfo->missing);
    }
 
    /* print first and last packet date/time */
    jul = firstday + MODIS_REF_DATE;
    caldat(&minute, &hour, &day, &month, &year, jul);
    hour = firstms / (1000L * 60L * 60L);
    ms = firstms - hour * 1000L * 60L * 60L;
    minute = ms / (1000L * 60L);
    ms = ms - minute * 1000L * 60L;
    second = ms / 1000L;
    ms = ms - second * 1000L;
    printf("first packet: %04d/%02d/%02d %02d:%02d:%d.%03ld%03ld\n",
            year, month, day, hour, minute, second, ms, firstmics);
    jul = lastday + MODIS_REF_DATE;
    caldat(&minute, &hour, &day, &month, &year, jul);
    hour = lastms / (1000L * 60L * 60L);
    ms = lastms - hour * 1000L * 60L * 60L;
    minute = ms / (1000L * 60L);
    ms = ms - minute * 1000L * 60L;
    second = ms / 1000L;
    ms = ms - second * 1000L;
    printf("last packet: %04d/%02d/%02d %02d:%02d:%d.%03ld%03ld\n",
            year, month, day, hour, minute, second, ms, lastmics);
 
    /* print number of missing secs */
    printf("missing seconds: %ld\n", missingsecs);
 
    /* print number of day packets */
    printf("day packets: %ld/%ld\n", daypkts1, daypkts2);
 
    /* print number of night packets */
    printf("night packets: %ld/%ld\n", nightpkts1, nightpkts2);
 
    /* print number of engineering packets */
    printf("engineering packets: %ld/%ld\n", engpkts1, engpkts2);
 
    /* free APID Info list */
    FreeAPIDInfoList(apidlist);
 
    /* close input file */
    fclose(fin);
 
    /* free memory */
    free(buf_data);
 
    /* Ja das war's. Der Pop-Shop ist zu Ende */
    return (retvalue);
}
 
/********************************************************************
 *                                                                  *
 *  read primary header from file                                   *
 *                                                                  *
 *  f:   file pointer                                               *
 *  buf: pointer to buffer                                          *
 *                                                                  *
 *  result:  0 - ok                                                 *
 *          -1 - read error                                         *
 *                                                                  *
 ********************************************************************/
int ReadPriHdr(FILE *f, unsigned char *buf) {
    /* read header */
    if (fread(buf, PRI_HDR_SIZE, 1, f) != 1)
        return (-1);
 
    /* ois rodger */
    return (0);
}
 
/********************************************************************
 *                                                                  *
 *  decode primary header                                           *
 *                                                                  *
 *  buf: pointer to buffer                                          *
 *  hdr: pointer to header structure                                *
 *                                                                  *
 *  result:  0 - ok                                                 *
 *          -1 - decode error (version not supported)               *
 *                                                                  *
 ********************************************************************/
int DecodePriHdr(unsigned char *buf, struct pri_hdr *hdr) {
    /* version */
    hdr->version = (buf[0] & 0xE0) >> 5;
 
    /* version supported? */
    if (hdr->version != 0)
        return (-1);
 
    /* type */
    hdr->type = (buf[0] & 0x10) >> 4;
 
    /* secondary header flag */
    hdr->sec_hdr_flag = (buf[0] & 0x08) >> 3;
 
    /* APID */
    hdr->apid = (((int) (buf[0] & 0x07)) << 8) + buf[1];
 
    /* sequence flags */
    hdr->seq_flags = (buf[2] & 0xC0) >> 6;
 
    /* packet count per APID */
    hdr->pkt_count = (((int) (buf[2] & 0x3F)) << 8) + buf[3];
 
    /* packet length (length - 1) */
    hdr->pkt_length = (((int) buf[4]) << 8) + buf[5];
 
    /* okeydokey */
    return (0);
}
 
/********************************************************************
 *                                                                  *
 *  allocate and initialise APID Info object                        *
 *                                                                  *
 *  apid: APID                                                      *
 *                                                                  *
 *  result:  pointer to APID Info object                            *
 *           0 - error                                              *
 *                                                                  *
 ********************************************************************/
struct apid_info *AllocAPIDInfo(int apid) {
    /* pointer to APID Info object */
    struct apid_info *ai;
 
 
    /* allocate memory for object */
    if (!(ai = malloc(sizeof (struct apid_info))))
        return (0);
 
    /* store APID */
    ai->apid = apid;
 
    /* initialise packet counter */
    ai->count = 0;
 
    /* initialise invalid packet counter */
    ai->invalid = 0;
 
    /* initialise missing packet counter */
    ai->missing = 0;
 
    /* initialise last packet count value */
    ai->last_pkt_count = -1;
 
    /* initialise pointer to next object */
    ai->next = NULL;
 
    /* all done */
    return (ai);
}
 
/********************************************************************
 *                                                                  *
 *  add APID Info object to list                                    *
 *                                                                  *
 *  list: pointer to pointer to first object in list                *
 *  ai:   pointer to APID Info object to be added                   *
 *                                                                  *
 *  result:  none                                                   *
 *                                                                  *
 ********************************************************************/
void AddAPIDInfo(struct apid_info **list, struct apid_info *ai) {
    /* pointer to APID Info object */
    struct apid_info *p;
 
 
    /* empty list? */
    if (*list == NULL) {
        /* add object as first in list */
        *list = ai;
 
        /* ciao */
        return;
    }
 
    /* first object? */
    if ((*list)->apid > ai->apid) {
        /* add object as first in list */
        ai->next = *list;
        *list = ai;
 
        /* Tschuess */
        return;
    }
 
    /* find right place in list */
    for (p = *list; p->next != NULL; p = p->next) {
        /* reached right position */
        if (p->next->apid > ai->apid) {
            /* add object to list */
            ai->next = p->next;
            p->next = ai;
 
            /* all good */
            return;
        }
    }
 
    /* add object as last in list */
    p->next = ai;
 
    /* hasta la vista*/
    return;
}
 
/********************************************************************
 *                                                                  *
 *  free memory in APID list                                        *
 *                                                                  *
 *  list: pointer to first object in list                           *
 *                                                                  *
 *  result:  none                                                   *
 *                                                                  *
 ********************************************************************/
void FreeAPIDInfoList(struct apid_info *list) {
    /* pointers to APID Info objects */
    struct apid_info *p1, *p2;
 
 
    /* go through list */
    for (p1 = list; p1 != NULL; p1 = p2) {
        /* get pointer to next object */
        p2 = p1->next;
 
        /* free memory of current object */
        free(p1);
    }
}
 
/********************************************************************
 *                                                                  *
 *  find APID Info object                                           *
 *                                                                  *
 *  list: pointer to first object in list                           *
 *  apid: APID                                                      *
 *                                                                  *
 *  result:  pointer to APID Info object                            *
 *           0 - APID Info object not found                         *
 *                                                                  *
 ********************************************************************/
struct apid_info *FindAPIDInfo(struct apid_info *list, int apid) {
    /* pointer to APID Info object */
    struct apid_info *p;
 
 
    /* go through list */
    for (p = list; p != NULL; p = p->next) {
        /* right APID? */
        if (p->apid == apid)
            /* return object address */
            return (p);
    }
 
    /* couldn't find object */
    return (NULL);
}
 
/********************************************************************
 *                                                                  *
 *  decode MODIS header                                             *
 *                                                                  *
 *  buf: pointer to buffer                                          *
 *  len: data lengh                                                 *
 *  hdr: pointer to header structure                                *
 *                                                                  *
 *  result:  0 - ok                                                 *
 *          -1 - decode error                                       *
 *          -2 - decode error                                       *
 *                                                                  *
 ********************************************************************/
int DecodeMODISHdr(unsigned char *buf, int len,
        struct modis_hdr *hdr) {
    /* days since 01/01/1958 */
    hdr->days =
            (((int) buf[0]) << 8) +
            (((int) buf[1]));
 
    /* milliseconds of day */
    hdr->millisec =
            (((unsigned long int) buf[2]) << 24) +
            (((unsigned long int) buf[3]) << 16) +
            (((unsigned long int) buf[4]) << 8) +
            (((unsigned long int) buf[5]));
 
    /* microseconds of milliseconds */
    hdr->microsec =
            (((int) buf[6]) << 8) +
            (((int) buf[7]));
 
    /* quicklook flag */
    hdr->ql = (buf[8] & 0x80) >> 7;
 
    /* packet type (000 = day, 001 = night, 010 = eng1, 100 = eng2 */
    hdr->pkt_type = (buf[8] & 0x70) >> 4;
 
    /* scan count */
    hdr->scan_count = (buf[8] & 0x0E) >> 1;
 
    /* mirror side */
    hdr->mirror_side = (buf[8] & 1);
 
    /* source identification (0 = earth, 1 = calibration) */
    hdr->src1 = (buf[9] & 0x80) >> 7;
 
    /* source identification (0 = eng., 1 to 1354 = sample count) */
    hdr->src2 =
            (((int) buf[9] & 0x7F) << 4) +
            (((int) buf[10] & 0xF0) >> 4);
 
    /* FPA/AEM config */
    hdr->conf =
            (((int) buf[10] & 0x0F) << 6) +
            (((int) buf[11] & 0xFC) >> 2);
 
    /* sci state */
    hdr->sci_state = (((int) buf[11] & 0x02) >> 1);
 
    /* sci abnorm */
    hdr->sci_abnorm = (((int) buf[11] & 0x01));
 
    /* check sum */
    hdr->checksum =
            (((int) buf[len - 2] & 0x0F) << 8) +
            (((int) buf[len - 1]));
 
    return 0;
}
 
/********************************************************************
 *                                                                  *
 *  convert julian day to calendar date                             *
 *                                                                  *
 *  minute: pointer to store minute                                 *
 *  hour:   pointer to store hour                                   *
 *  day:    pointer to store day                                    *
 *  month:  pointer to store month                                  *
 *  year:   pointer to store year                                   *
 *  jul:    julian day                                              *
 *                                                                  *
 *  result: none                                                    *
 *                                                                  *
 ********************************************************************/
void caldat(int *minute, int *hour, int *day, int *month, int *year,
        double jul) {
    /* julian day as long */
    long ljul;
    /* helping variables */
    long ja, jalpha, jb, jc, jd, je;
 
 
    ljul = (long) floor(jul);
    jul -= (double) ljul;
    *hour = (int) (floor(jul * 24.0));
    jul -= (double) *hour / 24.0;
    *minute = (int) (floor(jul * 1440.0));
 
    if (ljul >= 2299161) {
        jalpha = (long) (((float) (ljul - 1867216) - 0.25) / 36524.25);
        ja = ljul + 1 + jalpha - (long) (0.25 * jalpha);
    } else
        ja = ljul;
    jb = ja + 1524;
    jc = (long) (6680.0 + ((float) (jb - 2439870) - 122.1) / 365.25);
    jd = (long) (365 * jc + (0.25 * jc));
    je = (long) ((jb - jd) / 30.6001);
    *day = jb - jd - (long) (30.6001 * je);
    *month = je - 1;
    if (*month > 12)
        *month -= 12;
    *year = jc - 4715;
    if (*month > 2)
        --(*year);
    if (*year <= 0)
        --(*year);
}
 
/********************************************************************
 *                                                                  *
 *  calculate 12bit checksum                                        *
 *                                                                  *
 *  buf: pointer to data buffer                                     *
 *  n:   number of bytes in buffer                                  *
 *                                                                  *
 *  result: checksum                                                *
 *                                                                  *
 ********************************************************************/
int CalcChecksum12(unsigned char *buf, int n) {
    /* counter */
    int i;
    /* data value */
    unsigned long x;
    /* checksum */
    unsigned long s = 0;
 
 
    /* main loop */
    for (i = 0; i < n; i++) {
        /* get 1. value */
        x =
                (((unsigned long) buf[(int) (1.5 * i)]) << 4) +
                (((unsigned long) buf[(int) (1.5 * i) + 1] & 0xF0) >> 4);
 
        /* add to checksum */
        s = s + x;
 
        /* increase counter */
        i++;
 
        /* do we have a second value */
        if (i >= n)
            break;
 
        /* get 2. value */
        x =
                (((unsigned long) buf[(int) (1.5 * (i - 1)) + 1] & 0x0F) << 8) +
                (((unsigned long) buf[(int) (1.5 * (i - 1)) + 2]));
 
        /* add to checksum */
        s = s + x;
    }
 
 
    s = s >> 4;
    s = s & 0xFFF;
 
    /* return checksum */
    return (s);
}