Fix RLE streaming decoder

common/rle-common.h

@@ -6,88 +6,103 @@
 
 // Shared RLE code.
 
-static int _rle_control_byte = -1;
-static int _rle_more_literals = 0;
-
-static int _rle_output_literals(const uint8_t* data, int bytes);
-static void _rle_process(uint8_t control_byte, const uint8_t* data, int bytes);
+// If we're processing a repeat, but don't have the actual byte to be repeated,
+// this stores the number of repeats.  If we're not, it's 0.
+static int _rle_pending_repeats = 0;
+// If we're in the middle of a sequence of literal bytes spread across multiple
+// input buffers, this is the counter for how many are left.  If we're not,
+// it's 0.
+static int _rle_pending_literals = 0;
 
 #if !defined(MIN)
 # define MIN(a, b) ((a) < (b) ? (a) : (b))
 #endif
 
+static int _rle_output_literals(const uint8_t* data, int bytes, int needed) {
+  // We are still copying literal bytes to the output stream.
+  // How many should we copy?  The number we need, or the number we have,
+  // whichever is smaller.
+  int available = MIN(needed, bytes);
+  SRAM_WRITE(data, available);
+  return available;
+}
+
+static void _rle_output_repeats(uint8_t data_byte, int repeats) {
+  for (int i = 0; i < repeats; ++i) {
+    SRAM_WRITE(&data_byte, 1);
+  }
+}
+
 /**
  * Requires these macros:
  *
  * #define SRAM_WRITE(buffer, size)
  */
 static void rle_to_sram(const uint8_t* buffer, int bytes) {
-  if (_rle_control_byte != -1) {
-    // Now we have the data to process this cached control byte from another
-    // callback.
-    int control_byte = _rle_control_byte;
-    _rle_control_byte = -1;  // clear cache
-    _rle_process(control_byte, buffer, bytes);
-  } else if (_rle_more_literals) {
-    int consumed = _rle_output_literals(buffer, bytes);
-    buffer += consumed;
-    bytes -= consumed;
+  if (!bytes) {
+    // This shouldn't happen, but if it did, our logic would break.
+    return;
   }
 
-  if (bytes) {
-    _rle_process(buffer[0], buffer + 1, bytes - 1);
+  if (_rle_pending_repeats) {
+    // Now we have the data to process this pending repeat command from another
+    // input buffer.
+    _rle_output_repeats(*buffer, _rle_pending_repeats);
+    // This consumes one byte from buffer.
+    buffer++;
+    bytes--;
+    // The pending repeat is satisfied.
+    _rle_pending_repeats = 0;
+  } else if (_rle_pending_literals) {
+    // We are still processing literals from another input buffer.
+    int consumed = _rle_output_literals(buffer, bytes, _rle_pending_literals);
+    // This consumed some number of bytes.
+    buffer += consumed;
+    bytes -= consumed;
+    // Our pending literals are also reduced by the same amount.
+    _rle_pending_literals -= consumed;
   }
-}
 
-static void rle_reset() {
-  _rle_control_byte = -1;
-  _rle_more_literals = 0;
-}
-
-static int _rle_output_literals(const uint8_t* data, int bytes) {
-  // We are still copying literal bytes to the output stream.
-  int available = MIN(_rle_more_literals, bytes);
-  SRAM_WRITE(data, available);
-  _rle_more_literals -= available;
-  return available;
-}
+  while (bytes) {
+    // Extract the control byte.
+    uint8_t control_byte = buffer[0];
+    buffer++;
+    bytes--;
 
-static void _rle_process(uint8_t control_byte, const uint8_t* data, int bytes) {
-  while (true) {
+    // Parse it.
     bool repeat = control_byte & 0x80;
     int size = control_byte & 0x7f;
 
     if (repeat) {
       if (!bytes) {
         // We don't have the byte to repeat.
-        // Save the control byte for next time and return.
-        _rle_control_byte = control_byte;
+        // Save the size for next time.
+        _rle_pending_repeats = size;
         return;
       }
 
-      // Output the next byte |size| times.
-      for (int i = 0; i < size; ++i) {
-        SRAM_WRITE(data, 1);
-      }
+      // Repeat the next byte.
+      _rle_output_repeats(*buffer, size);
 
       // Consume that byte.
-      data++;
+      buffer++;
       bytes--;
     } else {
-      _rle_more_literals = size;
-      int consumed = _rle_output_literals(data, bytes);
-      data += consumed;
+      // Output literal bytes, as many as we can.  (This could be zero.)
+      int consumed = _rle_output_literals(buffer, bytes, size);
+
+      // Consume that data.
+      buffer += consumed;
       bytes -= consumed;
-    }
 
-    if (!bytes) {
-      // Nothing left in our input.
-      return;
+      // Store the number of unfulfilled literal bytes we need from the next
+      // buffer.  (This could be zero.)
+      _rle_pending_literals = size - consumed;
     }
-
-    // Set up the next control byte.
-    control_byte = data[0];
-    data++;
-    bytes--;
   }
 }
+
+static void rle_reset() {
+  _rle_pending_repeats = 0;
+  _rle_pending_literals = 0;
+}

emulator-patches/kinetoscope.c

@@ -244,6 +244,10 @@ static bool fetch_range(const char* url, size_t first_byte, size_t size,
 
 static bool fetch_range_to_sram(const char* url, size_t first_byte,
                                 size_t size) {
+  // This shouldn't be necessary, but in case of an incomplete compressed
+  // buffer being processed before this, reset the RLE decoder now.
+  rle_reset();
+
   return fetch_range(url, first_byte, size, http_data_to_sram, NULL);
 }