D-pad pong on an Arduino Uno

// D-pad pong: the paddle game on an ILI9341, played with a console pad.
//
//   Panel:  CS -> pin 10, DC -> pin 9, SCK -> pin 13, MOSI -> pin 11
//           RST -> 5 V, LED -> 5 V, VCC -> 5 V, GND -> GND
//   Pad:    UP -> 7, DOWN -> 8, LEFT -> 5, RIGHT -> 4, A -> 3, B -> 2, GND -> GND
//   Piezo:  one leg -> pin 6, the other -> GND
//
// Hold LEFT and RIGHT to move the paddle. A serves, and starts a new game when
// you have run out of lives. On screen the pad's buttons are clickable and the
// arrow keys, Z and X work when the pointer is on the canvas.
//
// # A pad is a direction, not a position
//
// This is the whole difference from the thumb-stick version. A pot is an
// absolute position, so the paddle can be put wherever the stick is. Six
// switches say only left, right or neither, so the paddle has a *speed*: held,
// it moves five pixels a frame, and let go it stops. Five pixels at thirty
// frames a second crosses the playfield in about a second, which is fast enough
// to catch a ball and slow enough to aim.
//
// # Why the playfield is a window and not the screen
//
// There is no SPI peripheral on an ATmega328P in Mokxi (docs/uno.md section 3),
// so every bit of every pixel is a pin change made by hand, about fifty
// thousand pixels a second. The whole panel is 76 800 of them, a second and a
// half to clear once. So the game lives in a window in the middle and the rest
// stays the black the controller powers up as. A frame only ever repaints the
// ball and the strip of paddle that moved, which is under two thousand pixels
// and is what makes it playable.

#include "mokxi_ili9341.h"
#include "mokxi_gamepad.h"
#include "mokxi_tone.h"

const uint8_t TFT_CS = 10;
const uint8_t TFT_DC = 9;
const uint8_t TFT_SCK = 13;
const uint8_t TFT_MOSI = 11;
const uint8_t PIEZO_PIN = 6;

SoftSpi spi(TFT_SCK, TFT_MOSI, TFT_CS);
Ili9341 tft(spi, TFT_DC);
//        up  down left right  A  B
Gamepad pad(7, 8, 5, 4, 3, 2);

// The playfield: a window in the middle of the panel, in panel coordinates.
const int16_t FX = 40;
const int16_t FY = 70;
const int16_t FW = 160;
const int16_t FH = 200;

const uint16_t BG = 0x0841;  // a very dark blue, so "cleared" is visible
const uint16_t EDGE = ILI9341_CYAN;
const uint16_t BALL_COLOUR = ILI9341_YELLOW;
const uint16_t PADDLE_COLOUR = ILI9341_GREEN;

const int16_t BALL = 8;
const int16_t PADDLE_W = 36;
const int16_t PADDLE_H = 8;
const int16_t PADDLE_Y = FY + FH - 16;
/** Pixels the paddle moves in a frame while a direction is held. */
const int16_t PADDLE_SPEED = 5;

const int FRAME_MS = 33;  // 30 frames a second
const uint8_t LIVES = 3;

int16_t bx, by, bdx, bdy;
int16_t px, oldPx;
uint8_t lives;
int16_t score, shownScore;
bool playing;
bool started;

static int16_t clampInt(int16_t v, int16_t lo, int16_t hi) {
  return v < lo ? lo : (v > hi ? hi : v);
}

// Kept out of line on purpose. mokxiTone is an inline loop, and three copies of
// it inside a loop() this size push an AVR conditional branch past the 64-word
// reach of R_AVR_7_PCREL and the link fails. One call does the same job.
static void beep(uint16_t hz, uint16_t ms) __attribute__((noinline));
static void beep(uint16_t hz, uint16_t ms) {
  mokxiTone(PIEZO_PIN, hz, ms);
}

// The furniture: a border, the labels, and a cleared playfield. Drawn once.
static void drawFrame() {
  tft.fillRect(FX, FY, FW, FH, BG);
  tft.rect(FX - 2, FY - 2, FW + 4, FH + 4, EDGE, 2);
  tft.text(FX, FY - 26, "D-PAD", ILI9341_WHITE, ILI9341_BLACK, 3);
  tft.text(FX, FY + FH + 10, "SCORE", ILI9341_WHITE, ILI9341_BLACK, 2);
  tft.text(FX + 110, FY + FH + 10, "LIVES", ILI9341_WHITE, ILI9341_BLACK, 2);
}

static void drawScore() {
  tft.number(FX + 64, FY + FH + 10, score, ILI9341_YELLOW, ILI9341_BLACK, 2, 3);
  shownScore = score;
}

static void drawLives() {
  tft.number(FX + 174, FY + FH + 10, lives, ILI9341_ORANGE, ILI9341_BLACK, 2, 1);
}

static void serve() {
  bx = FX + FW / 2 - BALL / 2;
  by = FY + 24;
  // Alternate sides rather than pull in a random number generator: there is
  // 32 KB of flash and this example is already most of an eighth of it.
  bdx = (int16_t)-bdx;
  if (bdx == 0) {
    bdx = 3;
  }
  bdy = 3;
  tft.fillRect(bx, by, BALL, BALL, BALL_COLOUR);
}

static void startGame() {
  score = 0;
  shownScore = -1;
  lives = LIVES;
  playing = true;
  if (started) {
    tft.fillRect(FX, FY, FW, FH, BG);
  }
  started = true;
  drawScore();
  drawLives();
  px = FX + (FW - PADDLE_W) / 2;
  oldPx = px;
  tft.fillRect(px, PADDLE_Y, PADDLE_W, PADDLE_H, PADDLE_COLOUR);
  serve();
}

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Uno: D-pad pong");
  pad.begin();
  mokxiNoTone(PIEZO_PIN);
  tft.begin(0);
  drawFrame();
  startGame();
}

void loop() {
  pad.update();

  if (!playing) {
    if (pad.pressed(PAD_A)) {
      tft.fillRect(FX + 8, FY + FH / 2 - 16, FW - 16, 32, BG);
      startGame();
    }
    delay(FRAME_MS);
    return;
  }

  // A direction, not a position: held left or right moves the paddle at a
  // fixed speed and nothing moves it back.
  int8_t dir = pad.dx();
  if (dir != 0) {
    px = clampInt((int16_t)(px + (int16_t)dir * PADDLE_SPEED), FX,
                  (int16_t)(FX + FW - PADDLE_W));
  }
  if (px != oldPx) {
    // Only the strip that stopped being paddle is erased; the rest is redrawn
    // over itself, which is far less than repainting the whole thing.
    int16_t gap = (int16_t)(px > oldPx ? px - oldPx : oldPx - px);
    if (gap >= PADDLE_W) {
      tft.fillRect(oldPx, PADDLE_Y, PADDLE_W, PADDLE_H, BG);
    } else if (px > oldPx) {
      tft.fillRect(oldPx, PADDLE_Y, gap, PADDLE_H, BG);
    } else {
      tft.fillRect((int16_t)(px + PADDLE_W), PADDLE_Y, gap, PADDLE_H, BG);
    }
    tft.fillRect(px, PADDLE_Y, PADDLE_W, PADDLE_H, PADDLE_COLOUR);
    oldPx = px;
  }

  int16_t nx = (int16_t)(bx + bdx);
  int16_t ny = (int16_t)(by + bdy);
  bool bounced = false;

  if (nx < FX) {
    nx = FX;
    bdx = (int16_t)-bdx;
    bounced = true;
  } else if (nx > FX + FW - BALL) {
    nx = (int16_t)(FX + FW - BALL);
    bdx = (int16_t)-bdx;
    bounced = true;
  }
  if (ny < FY) {
    ny = FY;
    bdy = (int16_t)-bdy;
    bounced = true;
  }

  // The paddle: caught if the ball's foot reaches its top while they overlap.
  if (bdy > 0 && ny + BALL >= PADDLE_Y && by + BALL < PADDLE_Y + PADDLE_H) {
    if (nx + BALL > px && nx < px + PADDLE_W) {
      ny = (int16_t)(PADDLE_Y - BALL);
      bdy = (int16_t)-bdy;
      // Where it hit decides which way it leaves, which is the whole game.
      int16_t offset = (int16_t)((nx + BALL / 2) - (px + PADDLE_W / 2));
      bdx = (int16_t)(offset / 6);
      if (bdx == 0) {
        bdx = 1;
      }
      if (bdx > 4) bdx = 4;
      if (bdx < -4) bdx = -4;
      score++;
      beep(NOTE_C6, 30);
      bounced = false;
    }
  }

  tft.fillRect(bx, by, BALL, BALL, BG);

  if (ny > FY + FH - BALL) {
    // Missed.
    beep(NOTE_C4, 160);
    lives--;
    drawLives();
    if (lives == 0u) {
      playing = false;
      tft.text(FX + 10, FY + FH / 2 - 10, "GAME OVER", ILI9341_RED, BG, 2);
      Serial.println("game over, press A");
      delay(FRAME_MS);
      return;
    }
    serve();
    delay(300);
    return;
  }

  bx = nx;
  by = ny;
  tft.fillRect(bx, by, BALL, BALL, BALL_COLOUR);

  if (score != shownScore) {
    drawScore();
  }
  if (bounced) {
    beep(NOTE_G5, 20);
  }

  delay(FRAME_MS);
}