Zombie

Monster data
HP	1
Points	100

Entity data
Entity pointer	$81:87F8 (normal) $81:88CA (hard)

The zombie is a respawning monster. There are two different types of zombies: normal and hard. They behave mostly the same, but there are slight differences that make the hard version more aggressive.

Behavior

Zombies can be in one of three states:

• Wandering: Walk in a straight line in a random one of the four orthogonal directions. Zombies start in this state.
• Following wall: Go counterclockwise around the perimeter of a wall. A zombie will enter this state if it touches a wall (i.e. a tile that is solid to monsters) while in the wandering state.
• Targeting: Move towards the nearest targetable sprite. A zombie enters this state when it comes within a certain distance of a targetable sprite. The zombie will leave this state and go back to the wandering state if there is no longer a targetable sprite within another (slightly larger) distance.

Zombies will despawn if they are a distance of 208 pixels or more on a single axis from the closest player, and are not within target range of any targetable sprites. The SNES screen has a width of 256 pixels, so this means it is possible for a zombie to despawn while still on screen if all players are on the very left or right of the screen and the zombie is on the opposite side.

Differences in hard version

Normal (left) vs. hard (right) targeting through a wall

• Start targeting at distance of 159 pixels (instead of 64 pixels for normal). They continue targeting while there is a target within 179 pixels (instead of 69 pixels for normal).
• Moves at a speed of 1 px/frame when wandering and following wall (instead of 0.5 px/frame for normal). The speed when targeting is 1 px/frame for both versions.
• Position update logic is different when targeting. When targeting through a wall, normal zombies will get stuck, but the hard version will still follow along the wall.
• The angle to rotate when following a wall is stored in a variable instead of being hardcoded. However, this variable is never initialized, so it will retain whatever value happened to be in RAM previously. This generally results in the zombie sticking to the wall instead of following it.
• Will switch to the wandering state if they are unable to move towards the target when targeting (because there is a wall in the way). The zombie will still likely be within target range, so it will usually immediately switch back to targeting, resulting in the zombie turning in place spastically.

Animation

Zombies have a four frame animation cycle, with 8 frames in between each animation frame. There is a separate set of animation frames for each of the 4 orthogonal directions, chosen based on which direction the zombie is moving. If the zombie is moving diagonal, it uses the left or right animation.

RAM map

Entity arguments

Address	Length	Type	Name	Description
$00	2	int16	x	X position
$02	2	int16	y	Y position

Entity memory

Address	Length	Type	Name	Description
$08	2	pointer16	sprite	Pointer to sprite
$0A	2	uint16	cyclesUntilAnimUpdate	Number of 2-frame cycles until next animation frame
$0C	2	uint16 x2	animationFrame	Current animation frame
$0E	2	direction x2	direction	Current direction
$10	2	direction x2	newDirection	New direction
$12	2	uint16	deathStatus	Death status. 0 = alive, 0xF5F5 = killed, other = despawned
$14	2	pointer16	state	Current state subroutine pointer
$16	2	int16	x	X position
$18	2	int16	y	Y position
$1A	2	int16	newX	New X position
$1C	2	int16	newY	New Y position
$1E	2	int16 0-based	health	Health
$20	2	direction x4	targetDirection	Direction to target
$22	2	sprite type	collidedSpriteType	Type of sprite collided with
$24	2	unused	unused24	Value is set and used, but appears to have no effect (hard only)
$26	2	int16	tempX	Temp X position (hard only)
$28	2	int16	tempY	Temp Y position (hard only)
$2A	2	uint16	moveCounter	Number of times left to move when targeting (hard only)
$2C	2	int16	wallRotateAngle	Amount to rotate when following wall (hard only)
$2E	2	pointer16	targetSprite	Pointer to target sprite (not used) (normal only)
$7E	2	int16 0-based	freezeTimer	Amount of time to stay frozen

Pseudocode

moveAmount = { {  0,  0 },
               {  0, -1 },
               {  1, -1 },
               {  1,  0 },
               {  1,  1 },
               {  0,  1 },
               { -1,  1 },
               { -1,  0 },
               { -1, -1 } }

newPositionIsBlocked() {
	return bgSolidToMonsters(this.newX, this.newY) ||
	       touchingSpriteSolidToMonsters(this.sprite, this.newX, this.newY)
}

wanderInRandomDirection() {
	this.direction = (getRandomByte() % 4) * 2 + 1
	this.setStateToWandering()
}

setStateToWandering() {
	this.state = this.wanderingState
	this.wanderingState()
}

wanderingState() {
	this.newX = this.x + this.moveAmount[this.direction][0]
	this.newY = this.y + this.moveAmount[this.direction][1]
	if (!bgSolidToMonsters(this.newX, this.newY)) {
		if (!touchingSpriteSolidToMonsters(this.sprite, this.newX, this.newY)) {
			this.x = this.newX
			this.sprite.x = this.newX
			this.y = this.newY
			this.sprite.y = this.newY
		}
	} else {
		this.followWall()
	}
}

followWall() {
	this.direction = ((this.direction - 1) + 2) % 8 + 1 // Rotate 90 degrees clockwise
	this.setStateToFollowingWall()
}

setStateToFollowingWall() {
	this.state = this.followingWallState
}

followingWallState() {
	this.newDirection = ((this.direction - 1) - 2) % 8 + 1 // Rotate 90 degrees counterclockwise
	this.newX = this.x + this.moveAmount[this.newDirection][0]
	this.newY = this.y + this.moveAmount[this.newDirection][1]
	if (!this.newPositionIsBlocked()) {
		this.direction = this.newDirection
	}

	this.newX = this.x + this.moveAmount[this.direction][0]
	this.newY = this.y + this.moveAmount[this.direction][1]
	if (!this.newPositionIsBlocked()) {
		this.x = this.newX
		this.sprite.x = this.newX
		this.y = this.newY
		this.sprite.y = this.newY
	} else {
		this.followWall()
	}
}

setStateToTargeting() {
	this.state = this.targetingState
}

targetingState() {
	distance, this.targetSprite = findTarget(this.x, this.y)
	if (distance >= 70) {
		this.wanderInRandomDirection()
		return
	}

	snapToTarget(this.sprite, this.targetSprite)
	this.direction = getDirectionToTarget(this.sprite, this.targetSprite)
	if (this.direction == 0) {
		this.wanderInRandomDirection()
		return
	}
	this.targetDirection = this.direction

	for (i = 0; i < 2; i++) {
		this.newX = this.x + this.moveAmount[this.targetDirection][0]
		this.newY = this.y + this.moveAmount[this.targetDirection][1]
		if (!this.newPositionIsBlocked()) {
			this.x = this.newX
			this.sprite.x = this.newX
			this.y = this.newY
			this.sprite.y = this.newY
		}
	}
}

checkForTarget() {
	distance = findTarget(this.x, this.y)
	if (distance < 65) {
		this.setStateToTargeting()
		return
	}

	direction = findPlayerInRange(208, this.x, this.y)
	if (direction == 0) {
		this.deathStatus -= 1
	}
}

killed() {
	findPlayerInRange(this.collidedSpriteType & 0x8000, 100)
	this.deathStatus = 0xF5F5
}

updateAnimation() {
	this.cyclesUntilAnimUpdate -= 1
	if (this.cyclesUntilAnimUpdate == 0) {
		this.cyclesUntilAnimUpdate = 4
		this.animationFrame = (this.animationFrame + 1) % 4

		this.sprite.tileData.lowBytes = this.animations[this.direction][this.animationFrame]
		this.sprite.tileData.bank = 0x90
		if (this.direction < DOWN_LEFT) {
			this.sprite.flipX = false
		} else {
			this.sprite.flipY = true
		}
	}
}

animations = { { 0xCABC, 0xCADD, 0xCAFE, 0xCB1F },
               { 0xCB40, 0xCB61, 0xCB82, 0xCBA3 },
               { 0xCA28, 0xCA51, 0xCA72, 0xCA9B },
               { 0xCA28, 0xCA51, 0xCA72, 0xCA9B },
               { 0xCA28, 0xCA51, 0xCA72, 0xCA9B },
               { 0xCABC, 0xCADD, 0xCAFE, 0xCB1F },
               { 0xCA28, 0xCA51, 0xCA72, 0xCA9B },
               { 0xCA28, 0xCA51, 0xCA72, 0xCA9B },
               { 0xCA28, 0xCA51, 0xCA72, 0xCA9B } }

init() {
	createMonsterSprite()
	this.animationFrame = 0
	this.x = args.x
	this.sprite.x = args.x
	this.sprite.z = 0
	this.y = args.y
	this.sprite.y = args.y
	this.sprite.tileData = $90:C98A
	this.sprite.visible = true
	this.sprite.alternatePalette = 6
	this.collidedSpriteType = 0
	this.freezeTimer = 0
}

mainNormal() {
	$7E:00DE += 0x14
	this.cyclesUntilAnimUpdate = 7
	this.init()
	showAnimation(this.spawnAnimation)
	this.sprite.type = MONSTER
	this.wanderInRandomDirection()
	this.health = 0
	setCollisionHandler(this.collisionHandler)
	this.deathStatus = 0

	while (this.deathStatus == 0) {
		waitFrames(2)
		this.checkForTarget()
		this.state()
		this.updateAnimation()
	}

	if (this.deathStatus == 0xF5F5) {
		zombiesKilled += 1
		showMonsterDeath(mummy.deathAnimation, this.deathStatus, 0x90)
	}

	$7E:00DE -= 0x14
	while ($7E:00DE < 0) { }
	deleteSprite(this.sprite)
}

spawnAnimation = { { 0xC98A, 10 }, { 0xC993, 10 }, { 0xC9A4, 10 }, { 0xC9B5, 10 },
                   { 0xC9D6, 10 }, { 0xC9FF, 10 }, { 0, 0 } }

collisionHandler(spriteType) {
	if (spriteType < SQUIRT_GUN) {
		return false
	}
	this.collidedSpriteType = spriteType

	weaponType = spriteType & 0x7FFF
	if (weaponType == MARTIAN_BUBBLE_GUN) {
		return bubbleMonster()
	} else if (weaponType == FIRE_EXTINGUISHER) {
		return freezeMonster()
	} else {
		newHealth = this.health - weaponDamage[weaponType - SQUIRT_GUN]
		if (newHealth < 0) {
			this.health = newHealth
			this.freezeTimer = 0
			this.killed()
			return true
		} else if (newHealth != this.health) {
			this.health = newHealth
			return showDamageAnimation()
		}
		return false
	}
}