RG Live WotLK 3.3.5a

float Unit::GetEffectiveResistChance(Unit const* owner, SpellSchoolMask schoolMask, Unit const* victim, SpellInfo const* spellInfo)
{
    float victimResistance = float(victim->GetResistance(schoolMask));
    if (owner)
    {
        // Xinef: pets inherit 100% of masters penetration
        // Xinef: excluding traps
        Player const* player = owner->GetSpellModOwner();
        if (player && owner->GetEntry() != WORLD_TRIGGER)
        {
            victimResistance += float(player->GetTotalAuraModifierByMiscMask(SPELL_AURA_MOD_TARGET_RESISTANCE, schoolMask));
            victimResistance -= float(player->GetSpellPenetrationItemMod());
        }
        else
            victimResistance += float(owner->GetTotalAuraModifierByMiscMask(SPELL_AURA_MOD_TARGET_RESISTANCE, schoolMask));
    }

    // Chaos Bolt exception, ignore all target resistances (unknown attribute?)
    if (spellInfo && spellInfo->SpellFamilyName == SPELLFAMILY_WARLOCK && spellInfo->SpellIconID == 3178)
        victimResistance = 0;

    victimResistance = std::max(victimResistance, 0.0f);
    if (owner)
        victimResistance += std::max((float(victim->getLevel())-float(owner->getLevel()))*5.0f, 0.0f);

    static uint32 const BOSS_LEVEL = 83;
    static float const BOSS_RESISTANCE_CONSTANT = 510.0f;
    uint32 level = victim->getLevel();
    float resistanceConstant = 0.0f;

    if (level == BOSS_LEVEL)
        resistanceConstant = BOSS_RESISTANCE_CONSTANT;
    else
        resistanceConstant = level * 5.0f;

    return victimResistance / (victimResistance + resistanceConstant);
}