[provocating]

spdat.cpp I think was the name of the file, under common.

bool IsBeneficialSpell(uint16 spell_id)
{
    if (!IsValidSpell(spell_id))
        return false;

    // You'd think just checking goodEffect flag would be enough?
    if (spells[spell_id].goodEffect == 1) {
        // If the target type is ST_Self or ST_Pet and is a SE_CancleMagic spell
        // it is not Beneficial
        SpellTargetType tt = spells[spell_id].targettype;
        if (tt != ST_Self && tt != ST_Pet &&
                IsEffectInSpell(spell_id, SE_CancelMagic))
            return false;

        // When our targettype is ST_Target, ST_AETarget, ST_Aniaml, ST_Undead, or ST_Pet
        // We need to check more things!
        if (tt == ST_Target || tt == ST_AETarget || tt == ST_Animal ||
                tt == ST_Undead || tt == ST_Pet) {
            uint16 sai = spells[spell_id].SpellAffectIndex;

            // If the resisttype is magic and SpellAffectIndex is Calm/memblur/dispell sight
            // it's not beneficial.
            if (spells[spell_id].resisttype == RESIST_MAGIC) {
                if (sai == SAI_Calm || sai == SAI_Dispell_Sight ||
                        sai == SAI_Memory_Blur || sai == SAI_Calm_Song)
                    return false;
            } else {
                // If the resisttype is not magic and spell is Bind Sight or Cast Sight or Harmony
                // It's not beneficial
                if ((sai == SAI_Calm && IsEffectInSpell(spell_id, SE_Harmony))
                    || (sai == SAI_Calm_Song && IsEffectInSpell(spell_id, SE_BindSight))
                    || (sai == SAI_Dispell_Sight && spells[spell_id].skill == 18 &&
                        !IsEffectInSpell(spell_id, SE_VoiceGraft)))
                    return false;
            }
        }
    }

    // And finally, if goodEffect is not 0 or if it's a group spell it's beneficial
    return spells[spell_id].goodEffect != 0 || IsGroupSpell(spell_id);
}