nibbles/nibbles.S

// by Simon Holmberg
//
// This is a document generated by [docco](http://jashkenas.github.io/docco/).
// My report is interleaved with the source code in the form of comments in
// [markdown](http://daringfireball.net/projects/markdown/) format.
//
// 
#define WIDTH 60
#define HEIGHT 40

// .bss
// ====
.bss
// Array 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

// .data
// =====
.data
// Current length of the snake
snakeLength: 	.long 0
// Snake position (starts in the middle of the field)
snakePosX:		.long WIDTH / 2
snakePosY:		.long HEIGHT / 2
// Snake direction (start moving to the right)
snakeDirX:		.long 1
snakeDirY:		.long 0
// Number of apples
numApples:		.long 0
// Game 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"

// .text
// =====
.text
// start_game
// ============
// `start_game(int len, int n_apples)`
.global start_game
start_game:
	// Initialization
	// --------------

	// Save our function arguments for later

	// `snakeLength = len`
	movl 4(%esp), %eax
	movl %eax, snakeLength
	// `numApples = n_apples`
	movl 8(%esp), %eax
	movl %eax, numApples

	// Store all registers so we can restore their state before we return later
	pusha

	// Call nib_init
	call nib_init
	// Seed randomness by calling `srand` with a time value from `time`
	pushl $0
	call time
	addl $4, %esp
	pushl %eax
	call srand
	addl $4, %esp

	// Initialize 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 1b

	// Spawn initial apples
	movl numApples, %ecx
1:
	pushl %ecx
	call spawnApple
	popl %ecx
	loop 1b

	// Game loop
	// ---------
loop:
	// Sleep for a while using `usleep` so the snake doesn't wiggle at the
	// speed of light!
	pushl $250*1000
	call usleep
	addl $4, %esp

	// Jump to `tick`, our updating function, which later chain-jumps to `draw`
	jmp tick

	// Restore all our registers as they were before `start_game` was called
	// and return
	popa
	ret

// tick
// ====
// This is where we update the world.
tick:
	movl $world, %edi
	
	// Increment cell times
	// --------------------
	
	// Loop
	movl snakeLength, %ebx
	xorl %ecx, %ecx
1:
	movl (%edi, %ecx, 4), %eax
	// If the value is negative it's an apple we don't want to disappear
	test %eax, %eax
	js 2f
	// No need to increment if it's already greater than snakeLength
	cmpl %ebx, %eax
	jg 2f
	// Increment the cell value
	incl %eax
	movl %eax, (%edi, %ecx, 4)
2:	incl %ecx // Loop increment
	cmpl $WIDTH*HEIGHT, %ecx
	jne 1b
	
	// Input
	// -----
	
	// Call nib_poll_kbd and check down, up, left and right in succession
	call nib_poll_kbd
	// Down
	movl $258, %ebx
	cmpl %eax, %ebx
	jne 1f
	movl $0, snakeDirX
	movl $1, snakeDirY
	// Up
1:	movl $259, %ebx
	cmpl %eax, %ebx
	jne 1f
	movl $0, snakeDirX
	movl $-1, snakeDirY
	// Left
1:	movl $260, %ebx
	cmpl %eax, %ebx
	jne 1f
	movl $-1, snakeDirX
	movl $0, snakeDirY
	// Right
1:	movl $261, %ebx
	cmpl %eax, %ebx
	jne 1f
	movl $1, snakeDirX
	movl $0, snakeDirY
1:

	// Move snake
	// ----------

	// `snakePosX = (snakePosX + snakeDirY + WIDTH) % WIDTH`
	movl snakePosX, %eax
	movl snakeDirX, %ebx
	addl %ebx, %eax
	// Add width for modulo wrap-around
	addl $WIDTH, %eax
	xorl %edx, %edx
	movl $WIDTH, %ecx 
	divl %ecx
	mov %edx, snakePosX
	// `snakePosY = (snakePosY + snakeDirY + HEIGHT) % HEIGHT`
	movl snakePosY, %eax
	movl snakeDirY, %ebx
	addl %ebx, %eax
	// Add height for modulo wrap-around
	addl $HEIGHT, %eax
	xorl %edx, %edx
	movl $HEIGHT, %ecx 
	divl %ecx
	mov %edx, snakePosY

	// Collision
	// ---------

	// Get value at head position
	movl snakePosY, %eax
	movl $WIDTH, %edx
	mull %edx
	addl snakePosX, %eax
	movl (%edi, %eax, 4), %edx

	// Collision with apple if negative
	test %edx, %edx
	js 1f
	// Compare value with `snakeLength`
	movl snakeLength, %ebx
	cmpl %ebx, %edx
	// End the game if the snake collides with itself, i.e. `%edx < snakeLength`
	jge 2f
	jmp gameOver
	// Spawn a new apple and extend the snake!
1:	pushl %edi
	pushl %eax
	call spawnApple
	popl %eax
	popl %edi
	incl snakeLength
2:

	// Update world
	// ------------

	// 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 draw
	//jmp debug_draw

// 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, %ebx

	// Call `nib_put_scr` depending of the value in a cell

	// `i = x + y * WIDTH`
	//
	// `%eax = %ebx + %ecx * $WIDTH`
	movl %ecx, %eax
	movl $WIDTH, %edx
	mull %edx
	addl %ebx, %eax
	// Fetch the value of the cell
	movl $world, %edi
	movl (%edi, %eax, 4), %eax
	
	// Compare with the snake length
	cmpl snakeLength, %eax
	// If the value is negative, it's an apple
	test %eax, %eax
	js 4f
	// If it's greater or equal to snakeLength, it's empty
	cmpl snakeLength, %eax
	jge 3f
	// Else, it's part of the snake
	pushl $'@'
	jmp 5f
3:	pushl $' '
	jmp 5f
4:	pushl $'o'
5:

	// Push Y-coordinate
	movl %ecx, %eax
	pushl %eax 
	
	// Push X-coordinate
	movl %ebx, %eax
	pushl %eax
	
	// `nib_put_scr(int x, int y, int c)`
	call nib_put_scr
	addl $4*3, %esp

	// Restore loop registers
	popl %ebx
	popl %ecx

	// Decrement X
	decl %ebx
	// End of X-loop
	jnz 2b
	// End of Y-loop
	loop 1b

	// Jump back to the start of our game loop!
	jmp loop

// gameOver
// ========
// Called when the game ends. It displays a game over message
// and score, and quits after **3 seconds**.
gameOver:
	// Calculate message position
	// --------------------------
	
	// Calculate string length
	movl $sgameover_end, %eax
	subl $sgameover, %eax
	// Discout null terminator
	subl $1, %eax

	// Calculate X-pos in the middle of the screen
	movl $2, %ebx
	xorl %edx, %edx
	divl %ebx
	negl %eax
	addl $WIDTH/2, %eax

	// Print game over message
	// -----------------------
	
	// Push `snakeLength` as score
	pushl snakeLength
	// Push printf-string
	pushl $sgameover
	// Push X-coordinate
	pushl %eax
	// Push Y-coordinate
	pushl $HEIGHT/2
	// `mvprintw(int y, int x, const char *fmt, ...)`
	call mvprintw
	add $4*3, %esp
	// Call `refresh` to refresh the screen 
	call refresh

	// Sleep for 3 seconds before calling `nib_end`
	pushl $3000*1000
	call usleep
	addl $4, %esp
	call nib_end

// spawnApple
// ==========
// Spawns 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

	// `i = x + y * WIDTH`
	//
	// `%eax = %ebx + %ecx * $WIDTH`
	movl %ecx, %eax
	movl $WIDTH, %edx
	mull %edx
	addl %ebx, %eax

	// Set the cell negative to represent an apple
	movl $world, %ecx
	movl $-1, %edx
	movl %edx, (%ecx, %eax, 4)

	popl %ebx
	ret

// debug_draw
// =============
// This 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, %ebx

	// Call nib_put_scr

	// `i = x + y * WIDTH`
	//
	// `%edi = %ebx + %ecx * $WIDTH`
	movl %ecx, %eax
	movl $WIDTH, %edx
	mull %edx
	addl %ebx, %eax
	// Fetch the value of the cell
	movl $world, %edi
	movl (%edi, %eax, 4), %eax	
	pushl %eax

	// Y coordinate
	movl %ecx, %eax
	pushl %eax 
	
	// X coordinate
	movl %ebx, %eax
	pushl %eax
	
	call nib_put_scr
	addl $4*3, %esp

	// Restore loop registers
	popl %ebx
	popl %ecx

	// Decrement X
	decl %ebx
	// End of X-loop
	jnz 2b
	// End of Y-loop
	loop 1b

	// Jump back to the start of our game loop!

	jmp loop