Part 12 - assembling the pieces together into "game"

The final stretch including:

Refactoring code to put some repeating things into sub-routines.

End of level logic with stars-animation over Santa's bag while adding
level-bonus to score.

Logic and init of state when player gets killed, finishes level or
starts a new game (in other words does lose last life), 50k score bonus
life (it's there, you will have to believe me).

Snowballs launching, mostly random with increasing difficulty for each
level (probably unhuman around level 8, hopefully completely impossible
to pass to level 99, as the game is not ready for level 100).

Tiny gameplay changes (making again possible to jump at top of the
ladder and making the player collision radius even one pixel less).

Part 11 - jump-over-ball bonus score counting

When the player jumps, the game does note the coordinates of player's
sprite (advances them +-4px in direction of jump) and keeps invisible
collision detector in that place for the whole duration of jump.

When some ball collides with this detector, it's counted as
"jump-evasion bonus", added to the score - each jump resets the
jump-bonus to zero, then first ball adds 100 and every following one
doubles the bonus, so the total sum per jump is: 100, 300, 700, 1500,
3100, ...
(probably 3 balls with single jump are maximum any way).

Such collision will also emit new "star" sprite visually communicating
to player that some bonus is being added to the score, and animates the
star (going up slowly and changing mirror/rotate flags to flicker
"randomly").

Part 10 - game UI and helper functions (random, add-score, level-init, etc)

Now most of the elements (to assemble simple "game" out of this) are in
place, the only major item missing is scoring the "jump over snowball".

In this part 10 we did add UI elements like score/bonus/lives, they are
displayed in classic ULA layer, on "white" paper which is in palette set
to be transparent, and layers priorities were modified to "USL", so
these UI texts are above anything else (including sprites).

We added also simple random generator from Baze, and added some debug
features like random score increments to test new routines.

And finally the code will switch to 3.5MHz at beginning, to give us some
fun perspective on how much of frame time this little of machine code
can take, and how difficult it was to produce more complex games on
classic ZX Spectrum (where you would have to add all the manual drawing
of sprites and background restoring).

Part 9 - ladder mechanics

Player can now grab ladder and climb up/down on it.

Part 8 - player controls: left+right and jump+fall mechanics

Walking on platforms, falling down off the ledge, jumping, staying
within play-field, detecting when falling from too high (will just
disable controls for 1s in this version).

No ladder mechanics yet (will be part 9)

Part 7 - collision detection against platforms, snowball AI using them

The screen is divided into 16px wide columns (0..15, 16..31, ...), each column has list of platforms (their Y coordinates) and there's new routine GetPlatformPosUnder which will return the platform under the current sprite.

And the snowball AI was modified to use the new function and make the snowballs roll down through the level until they get out of screen completely.

Part 6 - collision detection "player vs snowball"

calculating the collision based on distance between the centre point player sprite and snowball (not pixel-level collision masking individual pixels of sprites, just two "virtual circles", when overlap = collision)

When collision happens, the code will add extra "sparks" sprite over the snowball, and change palette offset of the main character.

(the effects are for debug/educative purposes, probably not to remain for final version)

Part 5 - adding controls to main character - just DEBUG free move around

The main character reacts to Kempston/MD joystick, or OPQA space

Just moves around PAPER area by 2px per frame, and stays there.

Part 4C - adding performance "measuring" by color stripes in border

This is very tiny change to the source, but very handy and commonly used by ZX developers... :)

Part 4B - introducing EQU constants instead of magic numbers

no NEX binary, because binary is identical to Part 4.