main/gb: --keys button-script harness + serial_no_eof for OSK testing

--keys "udlrabse." plays scripted joypad taps (each char = one button
held 10 frames then released 10, so the guest sees a clean edge); pairs
with --headless --shot to drive on-screen input and capture the result.
When --keys is active, serial-RX EOF stays pending instead of sending
$04, so a shell blocked on read waits for OSK input instead of exiting.

3 files changed, 31 insertions, 3 deletions

diff --git a/src/gb.c b/src/gb.c
index 3caa986..fc19e25 100644
--- a/src/gb.c
+++ b/src/gb.c
@@ -140,9 +140,11 @@ void gb_write(GB *gb, u16 addr, u8 val) {
                     gb->sc &= ~0x80;
                     gb_request_interrupt(gb, INT_SERIAL);
                 } else if (n == 0) {
-                    gb->sb = 0x04;         // EOF -> EOT (Ctrl-D)
-                    gb->sc &= ~0x80;
-                    gb_request_interrupt(gb, INT_SERIAL);
+                    if (!gb->serial_no_eof) {
+                        gb->sb = 0x04;     // EOF -> EOT (Ctrl-D)
+                        gb->sc &= ~0x80;
+                        gb_request_interrupt(gb, INT_SERIAL);
+                    }                      // else: stay pending (OSK provides input)
                 }
                 // n < 0 (EAGAIN on a live tty): stay pending, poll again
             }
diff --git a/src/gb.h b/src/gb.h
index 7959e60..5192e85 100644
--- a/src/gb.h
+++ b/src/gb.h
@@ -113,6 +113,7 @@ struct GB {
     bool hdma_active;
 
     u64 cycles;        // total T-cycles elapsed
+    bool serial_no_eof; // if true, serial-RX EOF stays pending (don't send $04)
     bool serial_log;   // print serial output to stderr (blargg tests)
     int  serial_out_fd; // if >=0, transmitted serial bytes written here (stdout)
     int  serial_in_fd;  // if >=0, received serial bytes read here (stdin), else 0xFF
diff --git a/src/main.c b/src/main.c
index 6d6379a..40efe29 100644
--- a/src/main.c
+++ b/src/main.c
@@ -28,6 +28,22 @@ static void run_frame(GB *gb) {
         cpu_step(gb);
 }
 
+// Scripted button taps for testing: each char in `keys` = one token occupying
+// 20 frames (pressed for the first 10, released for the next 10, so the guest
+// sees a clean press+release edge). u/d/l/r=dpad, a/b, s=select, e=start, .=wait.
+static u8 keys_mask(const char *keys, int frame) {
+    int tok = frame / 20;
+    if (tok >= (int)strlen(keys)) return 0;
+    if ((frame % 20) >= 10) return 0;
+    switch (keys[tok]) {
+        case 'u': return BTN_UP;    case 'd': return BTN_DOWN;
+        case 'l': return BTN_LEFT;  case 'r': return BTN_RIGHT;
+        case 'a': return BTN_A;     case 'b': return BTN_B;
+        case 's': return BTN_SELECT; case 'e': return BTN_START;
+        default:  return 0;
+    }
+}
+
 int main(int argc, char **argv) {
     if (argc < 2) {
         fprintf(stderr, "usage: %s <rom> [--test seconds] [--sixel [scale]] "
@@ -48,6 +64,7 @@ int main(int argc, char **argv) {
     bool   sixel = false;        // use sixel graphics output
     int    sixel_scale = 2;      // integer pixel zoom for sixel
     bool   headless = false;     // no video; serial <-> stdio (OS debug console)
+    const char *keys = NULL;     // scripted button taps for testing (u d l r a b s e .)
     for (int i = 2; i < argc; i++) {
         if (!strcmp(argv[i], "--test")) {
             test = true;
@@ -55,6 +72,8 @@ int main(int argc, char **argv) {
         } else if (!strcmp(argv[i], "--shot")) {
             if (i + 1 < argc) shot_frames = atoi(argv[++i]);
             if (i + 1 < argc && argv[i+1][0] != '-') shot_path = argv[++i];
+        } else if (!strcmp(argv[i], "--keys")) {
+            if (i + 1 < argc) keys = argv[++i];
         } else if (!strcmp(argv[i], "--fifo")) {
             fifo_path = (i + 1 < argc && argv[i+1][0] != '-')
                         ? argv[++i] : "/tmp/gbc.fifo";
@@ -97,6 +116,8 @@ int main(int argc, char **argv) {
     if (headless) {
         gb->serial_out_fd = STDOUT_FILENO;
         gb->serial_in_fd  = STDIN_FILENO;
+        gb->serial_no_eof = (keys != NULL);   // OSK provides input; don't EOF-exit
+        int keys_frame = 0;
         int fl = fcntl(STDIN_FILENO, F_GETFL, 0);
         if (fl != -1) fcntl(STDIN_FILENO, F_SETFL, fl | O_NONBLOCK);
         signal(SIGINT, on_sig);
@@ -121,6 +142,10 @@ int main(int argc, char **argv) {
         u64 start_cyc = gb->cycles;
         const double HZ = 4194304.0;
         while (running && !gb->poweroff) {
+            if (keys) {
+                gb->buttons = keys_mask(keys, keys_frame);
+                if (keys_frame++ > (int)strlen(keys) * 20 + 40) break;
+            }
             run_frame(gb);              // advance ~a frame's worth of cycles
             if (!uncapped) {
                 double target = start_t + (gb->cycles - start_cyc) / HZ;