Send only the changed dots of a frame to the Nano

Frames were sent as F0 plus one s command per yellow dot (thousands of
characters). During scrolling that overran the Nano's serial buffer, and
half-received frames redrew the whole screen. showBitmap() now sends two B
commands over the changed rectangle (dots off, dots on), or one B command for
a full frame, and the pacing counts the characters sent.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/test_nano/test_main.cpp

@@ -8,8 +8,11 @@
 #include "Arduino.h"
 #include "BROSE9323.h"
 #include "EEPROM.h"
+#include "frame_commands.h" // the real one, from lib/BROSE9323
 
 #include <cstdlib>
+#include <cstring>
+#include <string>
 #include <vector>
 
 // From src/nano/main.cpp
@@ -190,6 +193,101 @@ void test_full_screen_bitmap_is_streamed()
 	TEST_ASSERT_EQUAL(84 * 19, display.count("pixel"));
 }
 
+// The commands the ESP32 driver sends for a frame (lib/BROSE9323/frame_commands.h), run
+// through this parser: the drawn dots must give the wanted image.
+static const int IMG_BYTES = 11 * 19;
+
+static std::string frame(const uint8_t *wanted, const uint8_t *shown, bool instant)
+{
+	std::string text;
+	frameCommands(wanted, shown, 84, 19, instant, [&text](const char *t) { text += t; });
+	return text;
+}
+
+static void setDot(uint8_t *img, int x, int y, bool on)
+{
+	if (on)
+		img[y * 11 + x / 8] |= 0x80 >> (x & 7);
+	else
+		img[y * 11 + x / 8] &= ~(0x80 >> (x & 7));
+}
+
+// The recorded fill and pixel calls applied to an image
+static void applyCalls(uint8_t *img)
+{
+	for (const DisplayCall &call : display.calls)
+		if (call.what == "fill")
+			memset(img, call.a ? 0xFF : 0, IMG_BYTES);
+		else if (call.what == "pixel")
+			setDot(img, call.a, call.b, call.c);
+}
+
+static void pattern(uint8_t *img, int seed)
+{
+	for (int i = 0; i < IMG_BYTES; i++)
+		img[i] = (uint8_t)(i * 37 + seed * 11);
+	for (int y = 0; y < 19; y++) // nothing past the 84th dot
+		img[y * 11 + 10] &= 0xF0;
+}
+
+void test_frame_commands_send_only_the_changed_dots()
+{
+	uint8_t shown[IMG_BYTES], wanted[IMG_BYTES];
+	pattern(shown, 1);
+	memcpy(wanted, shown, IMG_BYTES);
+	int changed = 0;
+	for (int x = 30; x < 50; x++) // a scroll window, 6 rows high
+		for (int y = 12; y < 18; y++)
+			if ((x + y) % 3 == 0)
+			{
+				bool now = (shown[y * 11 + x / 8] >> (7 - (x & 7))) & 1;
+				setDot(wanted, x, y, !now);
+				changed++;
+			}
+
+	std::string text = frame(wanted, shown, true);
+	TEST_ASSERT_TRUE(text.size() < 150); // instead of the whole screen
+	send(text);
+	TEST_ASSERT_FALSE(replied("ERR"));
+	TEST_ASSERT_EQUAL(0, display.count("fill"));
+	TEST_ASSERT_EQUAL(changed, display.count("pixel")); // only the changed dots
+	uint8_t result[IMG_BYTES];
+	memcpy(result, shown, IMG_BYTES);
+	applyCalls(result);
+	TEST_ASSERT_EQUAL_MEMORY(wanted, result, IMG_BYTES);
+	TEST_ASSERT_EQUAL(1, display.count("display"));
+	TEST_ASSERT_EQUAL(1, display.last("display")->b); // D1: instant
+}
+
+void test_frame_commands_full_frame()
+{
+	uint8_t wanted[IMG_BYTES];
+	pattern(wanted, 2);
+	std::string text = frame(wanted, nullptr, false);
+	TEST_ASSERT_TRUE(text.size() < 420);
+	send(text);
+	TEST_ASSERT_FALSE(replied("ERR"));
+	TEST_ASSERT_EQUAL(1, display.count("fill"));
+	uint8_t result[IMG_BYTES];
+	memset(result, 0xAA, IMG_BYTES);
+	applyCalls(result);
+	TEST_ASSERT_EQUAL_MEMORY(wanted, result, IMG_BYTES);
+	TEST_ASSERT_EQUAL(0, display.last("display")->b); // D: with the effect
+}
+
+void test_frame_commands_edge_and_nothing_changed()
+{
+	uint8_t shown[IMG_BYTES] = {}, wanted[IMG_BYTES] = {};
+	TEST_ASSERT_EQUAL_STRING("", frame(wanted, shown, true).c_str());
+	setDot(wanted, 83, 18, true); // the last dot: the B width (4) goes past the edge
+	std::string text = frame(wanted, shown, false);
+	send(text);
+	TEST_ASSERT_FALSE(replied("ERR"));
+	TEST_ASSERT_EQUAL(1, display.count("pixel"));
+	TEST_ASSERT_EQUAL(83, display.last("pixel")->a);
+	TEST_ASSERT_EQUAL(18, display.last("pixel")->b);
+}
+
 void test_invalid_bitmap_data_is_not_run_as_commands()
 {
 	// width 5 is invalid; the data contains the command letters B, D and F
@@ -528,6 +626,9 @@ int main()
 
 	RUN_TEST(test_bitmap_pixels);
 	RUN_TEST(test_full_screen_bitmap_is_streamed);
+	RUN_TEST(test_frame_commands_send_only_the_changed_dots);
+	RUN_TEST(test_frame_commands_full_frame);
+	RUN_TEST(test_frame_commands_edge_and_nothing_changed);
 	RUN_TEST(test_invalid_bitmap_data_is_not_run_as_commands);
 	RUN_TEST(test_incomplete_bitmap_header);