Ensure new custom sleep image every time (#300)

When picking a random sleep image from a set of custom images, compare
the randomly chosen index against a cached value in settings. If the
value matches, use the next image (rolling over if it's the last image).
Cache the chosen image index to settings in either case.

## Summary

Implements a tweak on the custom sleep image feature that ensures that
the user gets a new image every time the device goes to sleep.
This change adds a new setting (perhaps there's a better place to cache
this?) that stores the most recently used file index. During picking the
random image index, we compare this against the random index and choose
the next one (modulo the number of image files) if it matches, ensuring
we get a new image.

## Additional Context

As mentioned, I used settings to cache this value since it is a
persisted store, perhaps that's overkill. Open to suggestions on if
there's a better way.

src/CrossPointState.cpp

@@ -5,7 +5,7 @@
 #include <Serialization.h>
 
 namespace {
-constexpr uint8_t STATE_FILE_VERSION = 1;
+constexpr uint8_t STATE_FILE_VERSION = 2;
 constexpr char STATE_FILE[] = "/.crosspoint/state.bin";
 }  // namespace
 
@@ -19,6 +19,7 @@ bool CrossPointState::saveToFile() const {
 
   serialization::writePod(outputFile, STATE_FILE_VERSION);
   serialization::writeString(outputFile, openEpubPath);
+  serialization::writePod(outputFile, lastSleepImage);
   outputFile.close();
   return true;
 }
@@ -31,13 +32,18 @@ bool CrossPointState::loadFromFile() {
 
   uint8_t version;
   serialization::readPod(inputFile, version);
-  if (version != STATE_FILE_VERSION) {
+  if (version > STATE_FILE_VERSION) {
     Serial.printf("[%lu] [CPS] Deserialization failed: Unknown version %u\n", millis(), version);
     inputFile.close();
     return false;
   }
 
   serialization::readString(inputFile, openEpubPath);
+  if (version >= 2) {
+    serialization::readPod(inputFile, lastSleepImage);
+  } else {
+    lastSleepImage = 0;
+  }
 
   inputFile.close();
   return true;

src/CrossPointState.h

@@ -8,6 +8,7 @@ class CrossPointState {
 
  public:
   std::string openEpubPath;
+  uint8_t lastSleepImage;
   ~CrossPointState() = default;
 
   // Get singleton instance

src/activities/boot_sleep/SleepActivity.cpp

@@ -80,7 +80,13 @@ void SleepActivity::renderCustomSleepScreen() const {
     const auto numFiles = files.size();
     if (numFiles > 0) {
       // Generate a random number between 1 and numFiles
-      const auto randomFileIndex = random(numFiles);
+      auto randomFileIndex = random(numFiles);
+      // If we picked the same image as last time, reroll
+      while (numFiles > 1 && randomFileIndex == APP_STATE.lastSleepImage) {
+        randomFileIndex = random(numFiles);
+      }
+      APP_STATE.lastSleepImage = randomFileIndex;
+      APP_STATE.saveToFile();
       const auto filename = "/sleep/" + files[randomFileIndex];
       FsFile file;
       if (SdMan.openFileForRead("SLP", filename, file)) {

src/main.cpp

@@ -303,6 +303,7 @@ void setup() {
     // Clear app state to avoid getting into a boot loop if the epub doesn't load
     const auto path = APP_STATE.openEpubPath;
     APP_STATE.openEpubPath = "";
+    APP_STATE.lastSleepImage = 0;
     APP_STATE.saveToFile();
     onGoToReader(path);
   }