#include <stdio.h>
+#include <stdbool.h>
+#include "helpers.c"
+
+#define WIDTH 60
+#define HEIGHT 40
+#define DEBUG false
+
+unsigned int startLength = 0;
+unsigned int startApples = 0;
+
+unsigned int currentTick;
+int snake[WIDTH*HEIGHT];
+unsigned int snakeLength;
+unsigned int snakeX;
+unsigned int snakeY;
+int snakeDirX;
+int snakeDirY;
+unsigned int numApples;
+
+void spawnApple();
+void tick();
+void draw();void start_game(int len, int n_apples)
+{
+ if (startLength == 0)
+ startLength = len;
+ if (startApples == 0)
+ startApples = n_apples;Initialize variables
+ + currentTick = 0;
+ snakeLength = len;
+ numApples = n_apples;
+
+ nib_init();Initialize the snake array +Set every cell to a time value greater than the snake length
+ + for (unsigned int i = 0; i < WIDTH*HEIGHT; ++i) {
+ snake[i] = snakeLength;
+ }Place the snake head in the middle
+ + snakeX = WIDTH / 2;
+ snakeY = HEIGHT / 2;
+ snakeDirX = 0;
+ snakeDirY = -1;Place the initial snake segments, downwards or something
+ + for (unsigned int i = 0; i < snakeLength; ++i) {
+ unsigned int x = snakeX % WIDTH;
+ unsigned int y = (snakeY + i) % HEIGHT;
+ snake[x + y*WIDTH] = i;
+ }Place apples
+ + srand(time(0));
+ for (unsigned int i = 0; i < numApples; ++i) {
+ spawnApple();
+ }
+
+ while (true) {
+ tick();
+ draw();
+ }
+}void spawnApple()
+{
+ unsigned int x = rand() % WIDTH;
+ unsigned int y = rand() % HEIGHT;
+ while (snake[x + y*WIDTH] < snakeLength) {
+ x = rand() % WIDTH;
+ y = rand() % HEIGHT;
+ }
+ snake[x + y*WIDTH] = -1;
+}void tick()
+{
+ currentTick++; for (unsigned int i = 0; i < WIDTH*HEIGHT; ++i) {
+ int val = snake[i];
+
+ if (val < 0) {Apple lifetime
+ + /* snake[i]--; */
+ } else if (val >= 0 && val <= snakeLength) {Other cells
+ + snake[i]++;
+ }
+ } int key = nib_poll_kbd();
+ if (key == 'w' && snakeDirY != 1) {
+ snakeDirX = 0;
+ snakeDirY = -1;
+ }
+ if (key == 's' && snakeDirY != -1) {
+ snakeDirX = 0;
+ snakeDirY = 1;
+ }
+ if (key == 'a' && snakeDirX != 1) {
+ snakeDirX = -1;
+ snakeDirY = 0;
+ }
+ if (key == 'd' && snakeDirX != -1) {
+ snakeDirX = 1;
+ snakeDirY = 0;
+ } snakeX = (snakeX + snakeDirX + WIDTH) % WIDTH;
+ snakeY = (snakeY + snakeDirY + HEIGHT) % HEIGHT;On collision with self
+ + int val = snake[snakeX + snakeY*WIDTH];
+ if (val < snakeLength && val >= 0) {nib_end();
+ + start_game(startLength, startApples);
+ }ON collision with apple
+ + if (val < 0) {Spawn a new apple somewhere
+ + spawnApple();Increase our length!
+ + snakeLength++;
+ } snake[snakeX + snakeY*WIDTH] = 0;
+}void draw()
+{
+ char body[] = "NIBBLES";
+
+ for (int y = -1; y <= HEIGHT; ++y) {
+ for (int x = -1; x <= WIDTH; ++x) {Draw a border outside the playing field
+ + if (x == -1 || y == -1 || x == WIDTH || y == HEIGHT) {
+ unsigned int screenX = x + 1;
+ unsigned int screenY = y + 1;
+ nib_put_scr(screenX, screenY, '#');
+ } else {
+ int snakeVal = snake[x + y*WIDTH];
+ unsigned int screenX = x + 1;
+ unsigned int screenY = y + 1;If the DEBUG flag is set, draw the raw ASCII-value for debugging purposes
if (DEBUG) {
+ nib_put_scr(screenX, screenY, '0' + snakeVal);
+ continue;
+ }If the value is greater than 0 and less than snakeLength, it’s part of the snake
if (snakeVal >= 0 && snakeVal < snakeLength) {
+ if (x == snakeX && y == snakeY) {Draw the head
+ + nib_put_scr(screenX, screenY, '@');
+ } else {Draw the body
+ + nib_put_scr(screenX, screenY, body[(snakeVal - 1) % 7]);
+ }
+ } else if (snakeVal < 0) {If the value is less than 0, it’s an apple
nib_put_scr(screenX, screenY, 'o');
+ } else {Otherwise it’s empty.
+ + nib_put_scr(screenX, screenY, ' ');
+ }
+ }
+ }
+ }
+}by Simon Holmberg
+This is a document generated by docco. +My report is interleaved with the source code in the form of comments in +markdown format.
+ +#define WIDTH 60
+#define HEIGHT 40.bssArray of longs representing the game field
+world is an array of increasing time values. By having the snake head set
+the time value to 0 at its current position, the actual snake will be
+defined by the locations where the time value is lesser than the snake
+length. This makes the tail of the snake automatically “move” visually, and
+allows for a snake with an arbitrary length without complicated arrays of
+coordinates!
Apples are defined by negative time values which do not get incremented +each frame. If we wanted to, we could instead decrease this value and +compare it with a lifetime value, to conveniently make apples disappear +after a set amount of time.
+ +.align 4
+world: .space WIDTH*HEIGHT*4.dataCurrent length of the snake
+ +snakeLength: .long 0Snake position (starts in the middle of the field)
+ +snakePosX: .long WIDTH / 2
+snakePosY: .long HEIGHT / 2Snake direction (start moving to the right)
+ +snakeDirX: .long 1
+snakeDirY: .long 0Number of apples
+ +numApples: .long 0Game over message
+ +sgameover: .string " Game over! Score: %i "
+sgameover_end:Various utility strings mainly used for debugging
+ +string1: .string "Started with snake length %i and %i apples!\n"
+string2: .string "Snake position: (%i, %i)\n"
+stringi: .string "%i\n"
+stringtest: .string "> %i %i %c\n".textstart_game(int len, int n_apples)
.global start_game
+start_game:Save our function arguments for later
+ +snakeLength = len
movl 4(%esp), %eax
+ movl %eax, snakeLengthnumApples = n_apples
movl 8(%esp), %eax
+ movl %eax, numApplesStore all registers so we can restore their state before we return later
+ + pushaCall nib_init
+ + call nib_initSeed randomness by calling srand with a time value from time
pushl $0
+ call time
+ addl $4, %esp
+ pushl %eax
+ call srand
+ addl $4, %espInitialize the snake array by setting every cell to a time value greater
+than snakeLength
movl $world, %edi
+ movl snakeLength, %eax
+ movl $WIDTH*HEIGHT, %ebx
+ xorl %ecx, %ecx
+1:
+ movl %eax, (%edi, %ecx, 4)
+ incl %ecx
+ cmpl %ebx, %ecx
+ jl 1bSpawn initial apples
+ + movl numApples, %ecx
+1:
+ pushl %ecx
+ call spawnApple
+ popl %ecx
+ loop 1bloop:Sleep for a while using usleep so the snake doesn’t wiggle at the
+speed of light!
pushl $250*1000
+ call usleep
+ addl $4, %espJump to tick, our updating function, which later chain-jumps to draw
jmp tickRestore all our registers as they were before start_game was called
+and return
popa
+ retThis is where we update the world.
+ +tick:
+ movl $world, %ediLoop
+ + movl snakeLength, %ebx
+ xorl %ecx, %ecx
+1:
+ movl (%edi, %ecx, 4), %eaxIf the value is negative it’s an apple we don’t want to disappear
+ + test %eax, %eax
+ js 2fNo need to increment if it’s already greater than snakeLength
+ + cmpl %ebx, %eax
+ jg 2fIncrement the cell value
+ + incl %eax
+ movl %eax, (%edi, %ecx, 4)
+2: incl %ecx // Loop increment
+ cmpl $WIDTH*HEIGHT, %ecx
+ jne 1bCall nib_poll_kbd and check down, up, left and right in succession
+ + call nib_poll_kbdDown
+ + movl $258, %ebx
+ cmpl %eax, %ebx
+ jne 1f
+ movl $0, snakeDirX
+ movl $1, snakeDirYUp
+ +1: movl $259, %ebx
+ cmpl %eax, %ebx
+ jne 1f
+ movl $0, snakeDirX
+ movl $-1, snakeDirYLeft
+ +1: movl $260, %ebx
+ cmpl %eax, %ebx
+ jne 1f
+ movl $-1, snakeDirX
+ movl $0, snakeDirYRight
+ +1: movl $261, %ebx
+ cmpl %eax, %ebx
+ jne 1f
+ movl $1, snakeDirX
+ movl $0, snakeDirY
+1:snakePosX = (snakePosX + snakeDirY + WIDTH) % WIDTH
movl snakePosX, %eax
+ movl snakeDirX, %ebx
+ addl %ebx, %eaxAdd width for modulo wrap-around
+ + addl $WIDTH, %eax
+ xorl %edx, %edx
+ movl $WIDTH, %ecx
+ divl %ecx
+ mov %edx, snakePosXsnakePosY = (snakePosY + snakeDirY + HEIGHT) % HEIGHT
movl snakePosY, %eax
+ movl snakeDirY, %ebx
+ addl %ebx, %eaxAdd height for modulo wrap-around
+ + addl $HEIGHT, %eax
+ xorl %edx, %edx
+ movl $HEIGHT, %ecx
+ divl %ecx
+ mov %edx, snakePosYGet value at head position
+ + movl snakePosY, %eax
+ movl $WIDTH, %edx
+ mull %edx
+ addl snakePosX, %eax
+ movl (%edi, %eax, 4), %edxCollision with apple if negative
+ + test %edx, %edx
+ js 1fCompare value with snakeLength
movl snakeLength, %ebx
+ cmpl %ebx, %edxEnd the game if the snake collides with itself, i.e. %edx < snakeLength
jge 2f
+ jmp gameOverSpawn a new apple and extend the snake!
+ +1: pushl %edi
+ pushl %eax
+ call spawnApple
+ popl %eax
+ popl %edi
+ incl snakeLength
+2:Set time value to 0 at snake head position
+i = x + y * WIDTH
%edi = %ebx + %ecx * $WIDTH
movl $0, (%edi, %eax, 4)Proceed to draw
jmp drawjmp debug_draw
+ +draw:Loop on Y-axis
+ + movl $HEIGHT, %ecx
+1:Loop on X-axis
+ + movl $WIDTH, %ebx
+2:Save loop registers
+ + pushl %ecx
+ pushl %ebx
+ subl $1, %ecx
+ subl $1, %ebxCall nib_put_scr depending of the value in a cell
movl %ecx, %eax
+ movl $WIDTH, %edx
+ mull %edx
+ addl %ebx, %eaxFetch the value of the cell
+ + movl $world, %edi
+ movl (%edi, %eax, 4), %eaxCompare with the snake length
+ + cmpl snakeLength, %eaxIf the value is negative, it’s an apple
+ + test %eax, %eax
+ js 4fIf it’s greater or equal to snakeLength, it’s empty
+ + cmpl snakeLength, %eax
+ jge 3fElse, it’s part of the snake
+ + pushl $'@'
+ jmp 5f
+3: pushl $' '
+ jmp 5f
+4: pushl $'o'
+5:Push Y-coordinate
+ + movl %ecx, %eax
+ pushl %eaxPush X-coordinate
+ + movl %ebx, %eax
+ pushl %eaxnib_put_scr(int x, int y, int c)
call nib_put_scr
+ addl $4*3, %espRestore loop registers
+ + popl %ebx
+ popl %ecxDecrement X
+ + decl %ebxEnd of X-loop
+ + jnz 2bEnd of Y-loop
+ + loop 1bJump back to the start of our game loop!
+ + jmp loopCalled when the game ends. It displays a game over message +and score, and quits after 3 seconds.
+ +gameOver:Calculate string length
+ + movl $sgameover_end, %eax
+ subl $sgameover, %eaxDiscout null terminator
+ + subl $1, %eaxCalculate X-pos in the middle of the screen
+ + movl $2, %ebx
+ xorl %edx, %edx
+ divl %ebx
+ negl %eax
+ addl $WIDTH/2, %eaxPush snakeLength as score
pushl snakeLengthPush printf-string
+ + pushl $sgameoverPush X-coordinate
+ + pushl %eaxPush Y-coordinate
+ + pushl $HEIGHT/2mvprintw(int y, int x, const char *fmt, ...)
call mvprintw
+ add $4*3, %espCall refresh to refresh the screen
call refreshSleep for 3 seconds before calling nib_end
pushl $3000*1000
+ call usleep
+ addl $4, %esp
+ call nib_endSpawns an apple in a random location in the world.
spawnApple:
+ pushl %ebx%ecx = rand() % HEIGHT
call rand
+ xorl %edx, %edx
+ movl $HEIGHT, %ebx
+ divl %ebx
+ movl %edx, %ecx%ebx = rand() % WIDTH
pushl %ecx
+ call rand
+ xorl %edx, %edx
+ movl $WIDTH, %ebx
+ divl %ebx
+ movl %edx, %ebx
+ popl %ecx movl %ecx, %eax
+ movl $WIDTH, %edx
+ mull %edx
+ addl %ebx, %eaxSet the cell negative to represent an apple
+ + movl $world, %ecx
+ movl $-1, %edx
+ movl %edx, (%ecx, %eax, 4)
+
+ popl %ebx
+ retThis is basically the same as draw, except it draws the actual ASCII-value
+of the world for debugging purposes.
debug_draw:Loop Y
+ + movl $HEIGHT, %ecx
+1:Loop X
+ + movl $WIDTH, %ebx
+2:Save loop registers
+ + pushl %ecx
+ pushl %ebx
+ subl $1, %ecx
+ subl $1, %ebxCall nib_put_scr
+ + movl %ecx, %eax
+ movl $WIDTH, %edx
+ mull %edx
+ addl %ebx, %eaxFetch the value of the cell
+ + movl $world, %edi
+ movl (%edi, %eax, 4), %eax
+ pushl %eaxY coordinate
+ + movl %ecx, %eax
+ pushl %eaxX coordinate
+ + movl %ebx, %eax
+ pushl %eax
+
+ call nib_put_scr
+ addl $4*3, %espRestore loop registers
+ + popl %ebx
+ popl %ecxDecrement X
+ + decl %ebxEnd of X-loop
+ + jnz 2bEnd of Y-loop
+ + loop 1bJump back to the start of our game loop!
+ +
+ jmp loop.textThe _start symbol. Calls start_game with “reasonable parameters”.
.global _start
+_start:n_apples
pushl $10len
pushl $3start_game(int len, int n_apples)
call start_game
+ addl $4*2, %espExit cleanly with an exit code of 0
+ + pushl $0
+ call _exit