#include <Arduino.h>
#include <U8g2lib.h>

U8G2_SSD1306_128X64_NONAME_F_SW_I2C u8g2(U8G2_R0, /* clock=*/ A5, /* data=*/ A4, /* reset=*/ U8X8_PIN_NONE);

int16_t offset;				// current offset for the scrolling text
u8g2_uint_t width;			// pixel width of the scrolling text (must be lesser than 128 unless U8G2_16BIT is defined
const char *text = "Shutter Speed Tester";	// scroll this text from right to left

const uint8_t tile_area_x_pos = 1;	// Update area left position (in tiles)
const uint8_t tile_area_y_pos = 1;	// Update area upper position (distance from top in tiles)
const uint8_t tile_area_width = 14;
const uint8_t tile_area_height = 2;	// this will allow cour18 chars to fit into the area

const u8g2_uint_t pixel_area_x_pos = tile_area_x_pos*8;
const u8g2_uint_t pixel_area_y_pos = tile_area_y_pos*8;
const u8g2_uint_t pixel_area_width = tile_area_width*8;
const u8g2_uint_t pixel_area_height = tile_area_height*8;

void setup(void) {
  u8g2.begin();
    
  u8g2.clearBuffer();					// clear the internal memory
  //u8g2.setFont(u8g2_font_helvR10_tr);	// choose a suitable font
  //u8g2.drawStr(0,12,"UpdateDisplayArea");	// write something to the internal memory
  
  // draw a frame, only the content within the frame will be updated
  // the frame is never drawn again, but will stay on the display
  u8g2.drawBox(pixel_area_x_pos-1, pixel_area_y_pos-1, pixel_area_width+2, pixel_area_height+2);
  
  u8g2.sendBuffer();					// transfer internal memory to the display
  
  u8g2.setFont(u8g2_font_tenthinguys_tr);	// set the target font for the text width calculation
  width = u8g2.getUTF8Width(text);		// calculate the pixel width of the text
  offset = width+pixel_area_width;
   
}

void loop(void) {
  u8g2.clearBuffer();						// clear the complete internal memory

  // draw the scrolling text at current offset
  u8g2.setFont(u8g2_font_tenthinguys_tr);		// set the target font
  u8g2.drawUTF8(pixel_area_x_pos-width+offset,pixel_area_y_pos+pixel_area_height+u8g2.getDescent()-1, text);								// draw the scolling text
  // now only update the selected area, the rest of the display content is not changed
  u8g2.updateDisplayArea(tile_area_x_pos, tile_area_y_pos, tile_area_width, tile_area_height);
      
  offset--;								// scroll by one pixel
  if ( offset == 0 )	
    offset = width+pixel_area_width;			// start over again
    
  delay(10);							// do some small delay
}