Reimplement five VIC-II blocks independently of VICE's code

[highbyte]

Why

The cycle-exact VIC-II learned several behaviours from VICE's emulator source. That is fine for behaviour, which is a fact about the chip, but five blocks ended up following VICE's GPL C code closely: the same logic and expressions under other names. This project is MIT licensed, so those blocks are replaced by code written independently.

What was replaced

Block	Where
Graphics fetch address, with bitmap mode's one-cycle delay on the NMOS chips and the address mix in the cycle BMM changes	Vic2RasterizerSequencerPixelGenerator (GraphicsFetchAddress)
Graphics data sequencer per pixel: loading at XSCROLL, pairs, mode changes at pixels 4/6/7, the MCM switch flash, colour selection per mode, fast paths	Vic2RasterizerSequencerPixelGenerator (DrawGraphicsCycle and helpers)
Sprite data sequencer per pixel: X expansion, multicolour pairs, per-pixel priority, the halt at the sprite's own fetch	Vic2SpriteManager.DecodeSpriteRun
Sprite data counters: the cycle 58 display decision, the cycle 16 update, rule 1 on $D017 writes, the sprite crunch	Vic2
Vertical border flip-flop	Vic2

How

• Specification first: a behaviour-only specification of each block was written from the VIC-II article, VICE's test programs and the existing tests, with no code and no VICE identifiers.
• Implementation from the specification only: the old bodies were removed, keeping the signatures the tests use. The replacements were written by an implementer who had the specification, the article, the test programs and the rest of this repository, but not VICE's source, the removed code or its history.
• Similarity review: a structural comparison against VICE's code found no transliteration. The remaining resemblances are where the behaviour allows essentially one way to write it, such as the sprite crunch's bit merge or the bitmap-mode address rule.
• Comments: comments that named VICE's source files or internal identifiers now describe the behaviour. Credit to VICE's test programs stays.

Verification

• Tests: unchanged, and all pass.
• VICE VIC-II test programs (274 runs, PAL and NTSC): identical results to before, including every pixel count (258 match their references, as before).
• VICE CIA test programs: identical.
• Games and demos: Commando and Giana Sisters (PAL, NTSC, snapshots) and fourteen demos at three frame counts render byte-identically.
• Performance (MacBook Air M1, For Your Sprites Only): the sequencer with per-line sprites went from 1,314/1,321 to 1,270/1,262 µs/frame, and without them from 1,047/949 to 985/914. The other configurations are unchanged.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud