fixed 'nextHistoryItem' pointer

After restoring GPS bread crumb trail, the next GPS item logged goes into the first empty slot. If no slots are empty, then find the oldest item and begin overwriting there.

model_gps.h

@@ -230,18 +230,45 @@ class Model {
       }
     }
     logger.info(". Restored %d breadcrumbs from %d lines in CSV file", items_restored, csv_line_number);
-    // logger.info(". Oldest date ", oldest_date);
-    // logger.info(". Newest date ", most_recent_date);
-
-    // This "restore" design will always fill the history[] from 0..N.
-    // So make sure the /next/ GPS point logged goes into the next open slot
-    // and doesn't overwrite any of the existing data
-    if (nextHistoryItem < numHistory) {
-      nextHistoryItem = items_restored;
-    } else {
-      // TODO: find the oldest item and start overwriting there
-      nextHistoryItem = 0;
+
+    // This "restore" design always fills history[] from 0..N.
+    // Make sure the /next/ GPS point logged goes into the next open slot
+    // and doesn't overwrite any historical data.
+    int indexOldest = 0;                             // default to start
+    TimeElements future{59, 59, 23, 0, 1, 1, 255};   // maximum date = year(1970 + 255) = 2,225
+    time_t oldest = makeTime(future);
+
+    int indexNewest = 0;                      // default to start
+    TimeElements past{0, 0, 0, 0, 0, 0, 0};   // minimum date = Jan 1, 1970
+    time_t newest = makeTime(past);
+
+    // find the oldest item (unused slots contain zero and are automatically the oldest)
+    for (int ii = 0; ii < numHistory; ii++) {
+      time_t tm = history[ii].timestamp;
+      if (tm < oldest) {
+        // keep track of oldest GPS bread crumb
+        indexOldest = ii;
+        oldest      = tm;
+      }
+      if (tm > newest) {
+        // keep track of most recent GPS bread crumb, out of curiosity
+        indexNewest = ii;
+        newest      = tm;
+      }
     }
+    // here's the real meat of the potato
+    nextHistoryItem = indexOldest;   
+
+    // report statistics for a visible sanity check to aid debug
+    char sOldest[24], sNewest[24];
+    dateToString(sOldest, sizeof(sOldest), oldest);
+    dateToString(sNewest, sizeof(sNewest), newest);
+
+    char msg1[256], msg2[256];   
+    snprintf(msg1, sizeof(msg1), ". Oldest date = history[%d] = %s", indexOldest, sOldest);
+    snprintf(msg2, sizeof(msg2), ". Newest date = history[%d] = %s", indexNewest, sNewest);
+    logger.info(msg1);
+    logger.info(msg2);
 
     // close file
     config.close();