feat: in-RAM pressure history ring buffer with 3h trend

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

WeatherPredictor/WeatherPredictor.ino

@@ -5,6 +5,7 @@
 #include "sensors.h"
 #include "rtc_time.h"
 #include "forecast.h"
+#include "history.h"
 
 static void i2cScan() {
   Serial.println(F("I2C scan:"));
@@ -46,6 +47,18 @@ void setup() {
     Serial.printf("p=%.0f trend=%d -> %c [%s] (%s)\n",
                   c.p, c.tr, f.letter, f.text, categoryShort(f.category));
   }
+
+  // --- history self-test (remove after Task 8) ---
+  historyBegin();
+  uint32_t base = 1000000000UL;
+  for (int i = 0; i < 40; i++)                 // 40 samples @ 5 min = 3h20m, pressure falling
+    historyAdd(base + (uint32_t)i * 300, 1015.0f - i * 0.2f, 20.0f);
+  float d;
+  if (historyTrendDelta(d))
+    Serial.printf("history count=%d  3h delta=%.2f hPa  trend=%d\n",
+                  historyCount(), d, classifyTrend(d));
+  else
+    Serial.println(F("history: not enough data for trend"));
 }
 
 void loop() {

WeatherPredictor/history.cpp

@@ -0,0 +1,38 @@
+#include "config.h"
+#include "history.h"
+
+static Sample s_buf[HISTORY_SIZE];
+static int    s_head  = 0;   // next write position
+static int    s_count = 0;
+
+void historyBegin() { s_head = 0; s_count = 0; }
+
+void historyAdd(uint32_t epoch, float mslHpa, float tempC) {
+  s_buf[s_head] = Sample{ epoch, mslHpa, tempC };
+  s_head = (s_head + 1) % HISTORY_SIZE;
+  if (s_count < HISTORY_SIZE) s_count++;
+}
+
+int historyCount() { return s_count; }
+
+Sample historyGet(int i) {
+  int start = (s_head - s_count + HISTORY_SIZE) % HISTORY_SIZE;
+  return s_buf[(start + i) % HISTORY_SIZE];
+}
+
+Sample historyLatest() { return historyGet(s_count - 1); }
+
+bool historyTrendDelta(float& outDelta) {
+  if (s_count < 2) return false;
+  Sample latest = historyLatest();
+  uint32_t target = latest.epoch - 10800UL;   // 3 h earlier
+  int idx = -1;
+  for (int i = 0; i < s_count; i++) {
+    if (historyGet(i).epoch <= target) idx = i; else break;
+  }
+  if (idx < 0) return false;
+  Sample past = historyGet(idx);
+  if (latest.epoch - past.epoch < 9000UL) return false;  // need >= 2.5 h span
+  outDelta = latest.mslHpa - past.mslHpa;
+  return true;
+}

WeatherPredictor/history.h

@@ -0,0 +1,15 @@
+#pragma once
+#include <Arduino.h>
+
+struct Sample {
+  uint32_t epoch;
+  float    mslHpa;
+  float    tempC;
+};
+
+void   historyBegin();
+void   historyAdd(uint32_t epoch, float mslHpa, float tempC);
+int    historyCount();
+Sample historyGet(int i);      // 0 = oldest
+Sample historyLatest();
+bool   historyTrendDelta(float& outDeltaHpa);