[Bug]: Downranking spell coefficient penalty unmodeled (TBC level penalty formula)

[Xerrion]

Problem

PhDamage applies the full configured spell-power coefficient regardless of spell rank vs player level. TBC''s level penalty formula multiplies the SP bonus by a downrank factor for spells whose MaxLevel is below the player''s level. PhDamage''s calculation overstates the SP contribution of low-rank spells - most acutely on Frostbolt R3 at L70 (~58% overstatement), and progressively on all sub-max-rank spells.

Canonical TBC formula

From cmangos/mangos-tbc Unit.cpp:2920-2936 CalculateLevelPenalty:

uint32 spellLevel = spellProto->spellLevel;
uint32 maxSpellLevel = spellProto->maxLevel;
if (spellLevel <= 0 || spellLevel >= maxSpellLevel) return 1.0f;
float LvlPenalty = 0.0f;
if (spellLevel < 20)
    LvlPenalty = (20.0f - spellLevel) * 3.75f;
float LvlFactor = (float(maxSpellLevel) + 6.0f) / float(GetLevel());
if (LvlFactor > 1.0f) LvlFactor = 1.0f;
return (100.0f - LvlPenalty) * LvlFactor / 100.0f;

Critical field semantics:

• + 6 term uses MaxLevel (DBC Spell.MaxLevel, the level base points stop scaling, generally nextRankLevel - 1).
• < 20 sub-penalty uses SpellLevel (rank-acquired level, DBC Spell.SpellLevel).
• Top-rank exemption: spellLevel >= maxLevel returns 1.0 outright.
• Sub-20 multiplier stacks multiplicatively with downrank factor.

Note on AzerothCore divergence: AzerothCore (Unit.cpp:3208-3225) and cMaNGOS-WotLK use SpellLevel + 6. For TBC 2.5.4, use MaxLevel + 6 per cMaNGOS-TBC and the original Blizzard 2005 blue post.

Worked examples (player L70)

Spell	SpellLevel	MaxLevel	LvlFactor	Sub-20	Penalty	Notes
Frostbolt R11 (top)	68	68	1.0 (exempt)	1.0	1.000	No penalty
Frostbolt R3	12	~19	25/70 = 0.357	(20-12)x0.0375 = 0.30 -> x0.70	0.250	75% reduction
Greater Heal R1	40	~45	51/70 = 0.729	n/a	0.729	27% reduction
Shadow Bolt R5	30	~35	41/70 = 0.586	n/a	0.586	41% reduction

Proposed solution

1. New module Engine/LevelPenalty.lua exporting CalculateLevelPenalty(spellLevel, maxLevel, playerLevel) matching cMaNGOS-TBC exactly.
2. Plumb a maxLevel field into per-rank entries in every Data/SpellData_*.lua file. Convention: maxLevel = nextRankLevel - 1 for non-top ranks; maxLevel = spellLevel for top ranks (triggers exemption via spellLevel >= maxLevel).
3. In Engine/ModifierCalc.lua::BuildModifiedResult, multiply the SP bonus (not base damage) by the level penalty before adding to base.
4. New test file tests/test_levelpenalty.lua covering: top-rank exemption, sub-20 stacking, MaxLevel+6 formula, healing spells, channels, DoTs.
5. Update Engine/Pipeline.lua::CalculateAll to iterate per-rank for headline diagnostics (so action-bar UI exercises lower ranks). This also resolves N1 from the audit.

Engine integration

• Apply uniformly to direct damage, DoTs, HoTs, channels, shield absorbs (no exemptions in cMaNGOS-TBC).
• Apply ONLY to the SP bonus, not base damage.
• Order of operations: final = (base + sp * coeff * levelPenalty) * modMultiplier.

References

• cMaNGOS-TBC Unit.cpp:2920-2936: https://github.com/cmangos/mangos-tbc/blob/master/src/game/Entities/Unit.cpp#L2920-L2936
• Blizzard 2005 blue post (BlueTracker): https://www.bluetracker.gg/wow/topic/us-en/45384575-change-to-coefficient-bonuses-on-spells/
• Vanilla wiki Downranking article: https://vanilla-wow-archive.fandom.com/wiki/Downranking
• Schlemiel-10753 EU forum 2026-01-14 confirming TBC Anniversary 2.5.4 uses 2.2 formula: https://eu.forums.blizzard.com/en/wow/t/priest-spells-and-spell-multipliers/601483

Acceptance criteria

• Engine/LevelPenalty.lua exists and matches cMaNGOS-TBC formula exactly.
• All SpellData_*.lua per-rank entries have a maxLevel field.
• BuildModifiedResult applies the penalty to SP bonus only.
• Test suite covers top-rank exemption, sub-20 stacking, healing, channels.
• Pipeline.CalculateAll iterates per-rank for diagnostics.
• Frostbolt R3 @ L70 yields effective coefficient ~0.2035 (was ~0.814).
• busted --verbose passes.