feat: persist pressure history to LittleFS across reboots

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

WeatherPredictor/WeatherPredictor.ino

@@ -13,6 +13,7 @@ AppState g_state;
 
 static unsigned long tSample  = 0;
 static unsigned long tHistory = 0;
+static unsigned long tFlush   = 0;
 
 static void sampleNow() {
   g_state.now    = rtcNow();
@@ -47,12 +48,14 @@ void setup() {
   }
   settingsBegin();
   historyBegin();
+  historyLoad();   // restore prior samples (empty on first ever boot)
+  Serial.printf("history restored: %d samples\n", historyCount());
 
   // Seed first sample immediately.
   sampleNow();
   historyAdd(rtcEpoch(), g_state.mslHpa, g_state.tempC);
   displayRender(g_state);
-  tSample = tHistory = millis();
+  tSample = tHistory = tFlush = millis();
 }
 
 void loop() {
@@ -68,4 +71,10 @@ void loop() {
     tHistory = now;
     historyAdd(rtcEpoch(), g_state.mslHpa, g_state.tempC);
   }
+
+  if (now - tFlush >= FLUSH_INTERVAL_MS) {
+    tFlush = now;
+    historySave();
+    Serial.printf("history flushed: %d samples\n", historyCount());
+  }
 }

WeatherPredictor/history.cpp

@@ -1,3 +1,4 @@
+#include <LittleFS.h>
 #include "config.h"
 #include "history.h"
 
@@ -36,3 +37,35 @@ bool historyTrendDelta(float& outDelta) {
   outDelta = latest.mslHpa - past.mslHpa;
   return true;
 }
+
+static const char* HPATH = "/history.dat";
+
+// Binary format: [int32 count][count * Sample], stored oldest-first.
+bool historySave() {
+  File f = LittleFS.open(HPATH, "w");
+  if (!f) return false;
+  int32_t n = s_count;
+  f.write((const uint8_t*)&n, sizeof(n));
+  for (int i = 0; i < s_count; i++) {
+    Sample s = historyGet(i);
+    f.write((const uint8_t*)&s, sizeof(s));
+  }
+  f.close();
+  return true;
+}
+
+bool historyLoad() {
+  File f = LittleFS.open(HPATH, "r");
+  if (!f) return false;
+  int32_t n = 0;
+  if (f.read((uint8_t*)&n, sizeof(n)) != sizeof(n)) { f.close(); return false; }
+  if (n < 0 || n > HISTORY_SIZE) { f.close(); return false; }
+  s_head = 0; s_count = 0;
+  for (int i = 0; i < n; i++) {
+    Sample s;
+    if (f.read((uint8_t*)&s, sizeof(s)) != sizeof(s)) break;
+    historyAdd(s.epoch, s.mslHpa, s.tempC);
+  }
+  f.close();
+  return true;
+}

WeatherPredictor/history.h

@@ -13,3 +13,5 @@ int    historyCount();
 Sample historyGet(int i);      // 0 = oldest
 Sample historyLatest();
 bool   historyTrendDelta(float& outDeltaHpa);
+bool   historySave();
+bool   historyLoad();