Now gives running estimate of XTC length - kill using ctrl-C when happy

with estimate;

xtc-length.c

@@ -3,12 +3,20 @@
 #include <locale.h>
 #include <string.h>
 
+#include <sys/stat.h>
+
 /*
  * Program for finding the number of frames and atoms in a GROMACS XTC file.
  * Inspired by a post on the gmx-developers mailing list in 2012:
  * https://mailman-1.sys.kth.se/pipermail/gromacs.org_gmx-developers/2012-June/005937.html
  */
 
+long file_size(const char *filename){
+    struct stat buffer;
+    int rc = stat(filename, &buffer);
+    return rc == 0 ? buffer.st_size : -1;
+}
+
 uint32_t u4_from_buffer(const uint8_t *b){
     return b[0] << 24 | b[1] << 16 | b[2] << 8 | b[3] << 0;
 }
@@ -21,18 +29,32 @@ void print_header(const uint8_t header[92]){
 }
 
 int get_xtc_num_frames(const char *filename, int *nframes, int *natoms, float *psec){
+    long size = file_size(filename);
+
     FILE *xtc = fopen(filename, "rb");
     if(!xtc) return -1;
 
     uint8_t header[92];
 
     *nframes = 0;
+    double avg_frame_size = 0.;
+    int est_nframes = 0;
     while(fread(header, 92, 1, xtc)){                       // Loop over frames
         //print_header(header);
         (*nframes)++;
         *natoms = u4_from_buffer(header+4);
         uint32_t frame_size = u4_from_buffer(header+88);    // Read frame size from header
         uint32_t skip = (frame_size+3) & ~((uint32_t)3);    // Round up to 4 bytes
+        avg_frame_size += (skip - avg_frame_size + 92) / *nframes;
+        if(*nframes % 10 == 0){
+            est_nframes = size / avg_frame_size;
+            uint32_t ps_tmp = u4_from_buffer(header+12);
+            memcpy(psec, &ps_tmp, 4);
+            *psec *= (double)est_nframes / *nframes;
+            printf("\rEstimated %'d frames (%'d ns) of %'d atoms. - %d%%",
+                    est_nframes, (int)(*psec/1000), *natoms, (int)(100. * *nframes) / est_nframes);
+            fflush(stdout);
+        }
         fseeko(xtc, skip, SEEK_CUR);                         // Skip to next header
     }
     uint32_t ps_tmp = u4_from_buffer(header+12);
@@ -57,6 +79,6 @@ int main(const int argc, const char *argv[]){
         return -1;
     }
 
-    printf("Trajectory contains %'d frames (%'.2f ns) of %'d atoms.\n", nframes, psec/1000, natoms);
+    printf("\rTrajectory contains %'d frames (%'.2f ns) of %'d atoms.\n", nframes, psec/1000, natoms);
     return 0;
 }