fifo: fixed length off-by-one error in implementation

src/device/cdrom/cdrom.cpp

@@ -18,7 +18,7 @@ CDROM::CDROM(System* sys) : sys(sys) {
 
 void CDROM::step() {
     status.transmissionBusy = 0;
-    if (!interruptQueue.empty()) {
+    if (!interruptQueue.is_empty()) {
         if ((interruptEnable & 7) & (interruptQueue.peek() & 7)) {
             sys->interrupt->trigger(interrupt::CDROM);
         }
@@ -152,10 +152,10 @@ uint8_t CDROM::read(uint32_t address) {
     }
     if (address == 1) {  // CD Response
         uint8_t response = 0;
-        if (!CDROM_response.empty()) {
+        if (!CDROM_response.is_empty()) {
             response = CDROM_response.get();
 
-            if (CDROM_response.empty()) {
+            if (CDROM_response.is_empty()) {
                 status.responseFifoEmpty = 0;
             }
         }
@@ -174,7 +174,7 @@ uint8_t CDROM::read(uint32_t address) {
         }
         if (status.index == 1 || status.index == 3) {  // Interrupt flags
             uint8_t _status = 0b11100000;
-            if (!interruptQueue.empty()) {
+            if (!interruptQueue.is_empty()) {
                 _status |= interruptQueue.peek() & 7;
             }
             if (verbose == 2) fmt::print("CDROM: R INTF: 0x{:02x}\n", _status);
@@ -336,7 +336,7 @@ void CDROM::write(uint32_t address, uint8_t data) {
             status.parameterFifoFull = 1;
         }
 
-        if (!interruptQueue.empty()) {
+        if (!interruptQueue.is_empty()) {
             interruptQueue.get();
         }
         if (verbose == 2) fmt::print("CDROM: W INTF: 0x{:02x}\n", data);

src/device/cdrom/cdrom.h

@@ -155,7 +155,7 @@ class CDROM {
     void handleCommand(uint8_t cmd);
 
     void writeResponse(uint8_t byte) {
-        if (CDROM_response.full()) {
+        if (CDROM_response.is_full()) {
             return;
         }
         CDROM_response.add(byte);
@@ -165,7 +165,7 @@ class CDROM {
     uint8_t readParam() {
         uint8_t param = CDROM_params.get();
 
-        status.parameterFifoEmpty = CDROM_params.empty();
+        status.parameterFifoEmpty = CDROM_params.is_empty();
         status.parameterFifoFull = 1;
 
         return param;

src/device/cdrom/commands.cpp

@@ -27,7 +27,7 @@ void CDROM::cmdSetloc() {
 void CDROM::cmdPlay() {
     // Play NOT IMPLEMENTED
     disc::Position pos;
-    if (!CDROM_params.empty()) {
+    if (!CDROM_params.is_empty()) {
         int track = readParam();  // param or setloc used
         if (track >= (int)disc->getTrackCount()) {
             fmt::print("CDROM: Invalid PLAY track parameter ({})\n", track);

src/device/cdrom/fifo.h

@@ -7,6 +7,7 @@ class fifo {
     T data[length] = {};
     size_t write_ptr = 0;
     size_t read_ptr = 0;
+    bool full = false;
 
    public:
     size_t size() const {
@@ -17,34 +18,38 @@ class fifo {
         }
     }
 
-    bool empty() const { return size() == 0; }
+    bool is_empty() const { return write_ptr == read_ptr && !full; }
 
-    bool full() const { return size() == length - 1; }
+    bool is_full() const { return full; }
 
     void clear() {
         write_ptr = 0;
         read_ptr = 0;
+        full = false;
     }
 
     bool add(const T t) {
-        if (full()) {
+        if (is_full()) {
             return false;
         }
 
         data[write_ptr] = t;
-        write_ptr = ++write_ptr % length;
+        write_ptr = (write_ptr + 1) % length;
+
+        full = write_ptr == read_ptr;
 
         return true;
     }
 
     T get() {
-        if (empty()) {
+        if (is_empty()) {
             return 0;
-            // TODO ?
         }
 
         T t = data[read_ptr];
-        read_ptr = ++read_ptr % length;
+        read_ptr = (read_ptr + 1) % length;
+
+        full = false;
 
         return t;
     }
@@ -52,7 +57,6 @@ class fifo {
     T peek(const size_t ptr = 0) const {
         if (ptr >= size()) {
             return 0;
-            // TODO ?
         }
 
         return data[(read_ptr + ptr) % length];
@@ -62,6 +66,6 @@ class fifo {
 
     template <class Archive>
     void serialize(Archive& ar) {
-        ar(data);
+        ar(data, write_ptr, read_ptr, full);
     }
 };