From a1bb7f0d42eddc32f7d44a138ad52cf8d3b485ad Mon Sep 17 00:00:00 2001
From: Michal Schwarz <michal.schwarz@gmail.com>
Date: Fri, 27 Jan 2023 20:59:40 +0100
Subject: [PATCH] checksum support

---
 README.md           |  2 +-
 client/src/main.cpp | 93 ++++++++++++++++++++++++---------------------
 2 files changed, 51 insertions(+), 44 deletions(-)

diff --git a/README.md b/README.md
index 3de3897..a44baa0 100644
--- a/README.md
+++ b/README.md
@@ -15,7 +15,7 @@ I choose this approach because it's easier (and more fun) for me to implement th
 1. ~~Add support for iCal.~~
 1. ~~Fix the icons at the bottom, make more of them available, make them more random.~~
 1. ~~Indicate possible WiFi outage or server unavailability on the display.~~
-1. Refresh the display only if image has changed (=check image checksum against the previous value). This should allow the portal calendar to ask server periodically more often but still sleep a lot and preserve energy.
+1. ~~Refresh the display only if image has changed (=check image checksum against the previous value). This should allow the portal calendar to ask server periodically more often but still sleep a lot and preserve energy.~~
 1. Maybe add support for a weather forecast (but I'll probably create a different project just for this purpose).
 1. Replace the ESP8266 ePaper module with what [original project](https://github.com/wuspy/portal_calendar) uses, i.e. specific low power ESP32 board + separate e-Paper hat [^1].
 
diff --git a/client/src/main.cpp b/client/src/main.cpp
index f38c46d..53990a8 100644
--- a/client/src/main.cpp
+++ b/client/src/main.cpp
@@ -59,6 +59,7 @@ GxEPD2_BW<GxEPD2_750_T7, GxEPD2_750_T7::HEIGHT / 2> display(
 
 WiFiManager wifiManager;
 WiFiClient wifiClient;  // for HTTP requests
+String lastChecksum = "";
 
 void setup() {
   Serial.begin(115200);
@@ -116,7 +117,6 @@ void showRawBitmapFrom_HTTP(const char* host,
                             int16_t bytes_per_row,
                             int16_t rows_at_once) {
   bool connection_ok = false;
-  bool valid = false;  // valid format to be handled
   uint32_t startTime = millis();
   if ((x >= display.epd2.WIDTH) || (y >= display.epd2.HEIGHT))
     return;
@@ -158,54 +158,61 @@ void showRawBitmapFrom_HTTP(const char* host,
   }
 
   DEBUG_PRINT("Parsing bitmap header");
-  if (read16(wifiClient) == 0x4D4D)  // "MM" signature
+  String line = wifiClient.readStringUntil('\n');
+  if (line != "MM")  // signature
   {
-    uint32_t bytes_read = 2;  // read so far
-    DEBUG_PRINT("w=%d, h=%d", w, h);
-
-    valid = true;
-    // display.clearScreen();
-
-    for (uint16_t row = 0; row < h; row += rows_at_once)  // for each line
-    {
-      if (!connection_ok || !(wifiClient.connected() || wifiClient.available()))
-        break;
-      delay(1);  // yield() to avoid WDT
-      yield();
-
-      uint32_t got = read8n(wifiClient, input_row_mono_buffer,
-                            bytes_per_row * rows_at_once);
-      bytes_read += got;
-
-      if (!connection_ok) {
-        Serial.print("Error: got no more after ");
-        Serial.print(bytes_read);
-        Serial.println(" bytes read!");
-        error("Read from HTTP server failed.");
-        break;
-      }
+    Serial.println("bitmap format not handled.");
+    error("Invalid bitmap received.");
+  }
+
+  line = wifiClient.readStringUntil('\n');  // checksum
+  DEBUG_PRINT("Last checksum was: %s", lastChecksum.c_str());
+  DEBUG_PRINT("New checksum is: %s", line.c_str());
+  if (line == lastChecksum) {
+    DEBUG_PRINT("Not refreshing, image is unchanged");
+    return;
+  };
+  lastChecksum = line;
+
+  // display.clearScreen();
+
+  uint32_t bytes_read = 0;                              // read so far
+  for (uint16_t row = 0; row < h; row += rows_at_once)  // for each line
+  {
+    if (!connection_ok || !(wifiClient.connected() || wifiClient.available()))
+      break;
+    delay(1);  // yield() to avoid WDT
+    yield();
+
+    uint32_t got =
+        read8n(wifiClient, input_row_mono_buffer, bytes_per_row * rows_at_once);
+    bytes_read += got;
+
+    if (!connection_ok) {
+      Serial.print("Error: got no more after ");
+      Serial.print(bytes_read);
+      Serial.println(" bytes read!");
+      error("Read from HTTP server failed.");
+      break;
+    }
 
 #ifdef USE_GRAYSCALE_DISPLAY
-      // https://github.com/ZinggJM/GxEPD2_4G/blob/master/src/epd/GxEPD2_750_T7.cpp
-      display.writeImage_4G(input_row_mono_buffer, 8, x, y + row, w, 1, false,
-                            false, false);
+    // https://github.com/ZinggJM/GxEPD2_4G/blob/master/src/epd/GxEPD2_750_T7.cpp
+    display.writeImage_4G(input_row_mono_buffer, 8, x, y + row, w, 1, false,
+                          false, false);
 #else
-      display.writeImage(input_row_mono_buffer, x, y + row, w, rows_at_once);
+    display.writeImage(input_row_mono_buffer, x, y + row, w, rows_at_once);
 #endif
-    }  // end line
-    Serial.print("downloaded and displayed in ");
-    Serial.print(millis() - startTime);
-    Serial.println(" ms");
-    display.refresh();
-
-    Serial.print("bytes read ");
-    Serial.println(bytes_read);
-  }
+  }  // end line
+  Serial.print("downloaded and displayed in ");
+  Serial.print(millis() - startTime);
+  Serial.println(" ms");
+  display.refresh();
+
+  Serial.print("bytes read ");
+  Serial.println(bytes_read);
+
   wifiClient.stop();
-  if (!valid) {
-    Serial.println("bitmap format not handled.");
-    error("Invalid bitmap received.");
-  }
 }
 
 void stopWiFi() {