Display: introduce display_writechar().

You see where the journey is going? This is the equivalent to the
function to write a character to the serial line, so we can
basically swap these two in other functions.

display.c

@@ -13,4 +13,18 @@
   #include "display_ssd1306.c"
 #undef TEACUP_C_INCLUDE
 
-/* No common code so far. */
+
+#ifdef DISPLAY
+
+void display_writestr_P(PGM_P data_P) {
+  uint8_t r, i = 0;
+
+  // Yes, this is *supposed* to be assignment rather than comparison, so we
+  // break when r is assigned zero.
+  while ((r = pgm_read_byte(&data_P[i]))) {
+    display_writechar(r);
+    i++;
+  }
+}
+
+#endif /* DISPLAY */

display.h

@@ -36,6 +36,8 @@ void display_init(void);
 void display_clear(void);
 
 void display_set_cursor(uint8_t line, uint8_t column);
-void display_text_P(PGM_P message_P);
+void display_writechar(uint8_t data);
+
+void display_writestr_P(PGM_P data_P);
 
 #endif /* _DISPLAY_H */

display_ssd1306.c

@@ -99,18 +99,15 @@ void display_set_cursor(uint8_t line, uint8_t column) {
 }
 
 /**
-  Prints the text at the current cursor position.
+  Prints a character at the current cursor position.
 
-  \param message Zero terminated string of the text to be displayed, stored
-                 in program memory.
+  \param data The character to be displayed.
 */
-void display_text_P(PGM_P message) {
-  uint8_t i, index;
+void display_writechar(uint8_t data) {
+  uint8_t i, index = data - 0x20;
 
-  // Render text to bitmap to display.
+  // Write pixels command.
   displaybus_write(0x40, 0);
-  while ((index = pgm_read_byte(message))) {
-    index -= 0x20;
 
   // Send the character bitmap.
   #ifdef FONT_IS_PROPORTIONAL
@@ -122,13 +119,12 @@ void display_text_P(PGM_P message) {
   }
   // Send space between characters.
   for (i = 0; i < FONT_SYMBOL_SPACE; i++) {
-      displaybus_write(0x00, 0);
+    // TODO: we finalise a I2C (or other) bus message after each character
+    //       here because we have no idea on how many more are following. This
+    //       is highly inefficient and makes the displaybus buffer almost
+    //       pointless.
+    displaybus_write(0x00, (i == FONT_SYMBOL_SPACE - 1));
   }
-
-    message++;
-  }
-  // Send another space for transmission end.
-  displaybus_write(0x00, 1);
 }
 
 #endif /* TEACUP_C_INCLUDE && DISPLAY_TYPE_SSD1306 */

i2c_test.c

@@ -25,7 +25,7 @@ static void i2c_test(void) {
     128 columns, so we offset by 32 columns to get it to the center.
   */
   display_set_cursor(1, 32);
-  display_text_P(PSTR("Welcome to Teacup"));
+  display_writestr_P(PSTR("Welcome to Teacup"));
 }
 
 #endif /* I2C_TEST */