#include <stdio.h>
#include <sys/param.h>

#define distance(x,y)   (((x)<(y))?((y)-(x)):((x)-(y)))

#define SNEDEBUG 1

// This is the core code, stand-alone, to compute SNE from seqno
// >> replace this function with your own code to test alternates
unsigned long compute_sne(unsigned long seqno) {
    // INPUT: 32-bit unsigned sequence number (low bits)
    // OUTPUT: 32-bit unsigned SNE (high bits)

    // variables used in this code example to compute SNE:

    static unsigned long
      RCV_SNE = 0;        // high-watermark SNE
    static int
      RCV_SNE_FLAG = 1;   // set during first half rollover
                          // (prevents re-rollover)
    static unsigned long
      RCV_PREV_SEQ = 0;   // high-watermark SEQ
    unsigned long
      holdSNE;            // temp copy of output

    holdSNE = RCV_SNE;                // use current SNE to start
    if (distance(seqno,RCV_PREV_SEQ) < 0x80000000) {
        // both in same SNE range?
        if ((seqno >= 0x80000000) && (RCV_PREV_SEQ < 0x80000000)) {
            // jumps fwd over N/2?
            RCV_SNE_FLAG = 0;         // reset wrap increment flag
        }
        RCV_PREV_SEQ = MAX(seqno,RCV_PREV_SEQ);
                                      // move prev forward if needed
    } else {
            // both in diff SNE ranges
            if (seqno < 0x80000000) {
                // jumps forward over zero?
                RCV_PREV_SEQ = seqno; // update prev
                if (RCV_SNE_FLAG == 0) {
                    // first jump over zero? (wrap)
                    RCV_SNE_FLAG = 1;
                              // set flag so we increment once
                    RCV_SNE = RCV_SNE + 1;
                              // increment window
                    holdSNE = RCV_SNE;
                              // use updated SNE value
                }
            } else {
                // jump backward over zero
                holdSNE = RCV_SNE - 1;
                              // use pre-rollover SNE value
            }
    }
    #ifdef SNEDEBUG
    fprintf(stderr,"state RCV_SNE_FLAG =        %1d\n",
      RCV_SNE_FLAG);
    fprintf(stderr,"state      RCV_SNE = %08lx\n", RCV_SNE);
    fprintf(stderr,"state RCV_PREV_SEQ = %08lx\n", RCV_PREV_SEQ);
    #endif
    return holdSNE;
}

int main() {
    // variables used as input and output:
    unsigned long SEG_SEQ;        // input - received SEQ
    unsigned long SNE;            // output - SNE corresponding
                                  // to received SEQ

    // variables used to validate the computed SNE:
    unsigned long SEG_HIGH;       // input - xmitter side SNE
                                  // -> SNE should match this value
    unsigned long long BIG_PREV;  // prev 64-bit total seqno
    unsigned long long BIG_THIS = 0;  // current 64-bit total seqno
                                  // -> THIS, PREV should never jump
                                  //    more than half the SEQ space

   char *prompt = "Input hex numbers only (0x is optional)\n\n")
                  "\tHex input\n"
                  "\t(2 hex numbers separated by whitespace,"
                  "each with 8 or fewer digits)";

    fprintf(stderr,"%s\n",prompt);

    while (scanf("%lx %lx",&SEG_HIGH,&SEG_SEQ) == 2) {
        BIG_PREV = BIG_THIS;
        BIG_THIS = (((unsigned long long)SEG_HIGH) << 32)
                  | ((unsigned long long)SEG_SEQ);

        // given SEG_SEQ, compute SNE
        SNE = compute_sne(SEG_SEQ);

        fprintf(stderr,"           SEG_SEQ = %08lx\n", SEG_SEQ);
        fprintf(stderr,"               SNE = %08lx\n", SNE);
        fprintf(stderr,"          SEG_HIGH = %08lx %s\n",SEG_HIGH,
                (SEG_HIGH == SNE)? " - OK" : " - ERROR !!!!!!!");
        fprintf(stderr,"\t\tthe jump was %16llx %s %s\n",
                distance(BIG_PREV,BIG_THIS),
                ((BIG_PREV < BIG_THIS)?"+":"-"),
                (((distance(BIG_PREV,BIG_THIS)) > 0x7FFFFFFF)
                 ? "ILLEGAL JUMP" : "."));
        fprintf(stderr,"\n");
        fprintf(stderr,"\n");

        fprintf(stderr,"%s\n",prompt);

    }
}

00000000 00000000
00000000 30000000
00000000 90000000
00000000 70000000
00000000 a0000000
00000001 00000001
00000000 e0000000
00000001 00000000
00000001 7fffffff
00000001 00000000
00000001 50000000
00000001 80000000
00000001 00000001
00000001 40000000
00000001 90000000
00000001 b0000000
00000002 0fffffff
00000002 20000000
00000002 90000000
00000002 70000000
00000002 A0000000
00000003 00004000
00000002 D0000000
00000003 20000000
00000003 90000000
00000003 70000000
00000003 A0000000
00000004 00004000
00000003 D0000000