ITPO Avoidance

What is an ITPO

A very typical thing that can happen during processing mods is exporting a record that doesn't have any changes compared to the original. This is known as an ITPO (Identical to Previous Override), or sometimes ITM (Idential to Master).

For example, this code can make some ITPOs:

foreach (var weapon in loadOrder.PriorityOrder.Weapon().WinningOverrides())
{
   var weaponOverride = outgoingMod.Weapons.GetOrAddAsOverride(weapon);
   weaponOverride.Weight = 0;
}

If a weapon already has a weight of zero, this will cause an ITPO, as nothing has changed.

Avoiding ITPOs

Typically, the best patterns avoid making ITPOs in the first place, rather than cleaning them up after they exist.

Alternatively, we can adjust the logic to just never make an ITPO in the first place. There are two general patterns for doing this.

Late GetOrAddAsOverride

foreach (var weapon in loadOrder.PriorityOrder.Weapon().WinningOverrides())
{
   // If what we wanted to change is already as we want it, skip
   if (weaponOverride.Weight == 0) continue;

   var weaponOverride = outgoingMod.Weapons.GetOrAddAsOverride(weapon);
   weaponOverride.Weight = 0;
}

As you can see, the call to GetOrAddAsOverride is delayed until we know there's a meaningful modification to do.

DeepCopy With Late Add

foreach (var weapon in loadOrder.PriorityOrder.Weapon().WinningOverrides())
{
   // Make a mutable copy, but don't add it to the mod yet
   var weaponOverride = weapon.DeepCopy();

   bool madeModification = false;

   if (weaponOverride.Weight != 0)
   {
       weaponOverride.Weight = 0;
       madeModification = true;
   }

   if (weaponOverride.Value != 100)
   {
       weaponOverride.Weight = 100;
       madeModification = true;
   }

   // If we didn't do anything, skip
   if (!madeModification) continue;

   outgoingMod.Weapons.Set(weaponOverride);
}

There are many ways to achieve the same goal. The important takeaway is that code should first try to not make any ITPOs in the first place by only adding it to the outgoing mod once/if modifications have been made to the record. Depending on the complexity of what you're doing, it may require different patterns than the ones outlined above.

Late ITPO Removal

While it's preferable to avoid making ITPOs in the first place, they theoretically can be removed after the fact.

Mutagen may or may not get ITPO removal tooling in the future. This would be powered by Equality/HashCode concepts, which already exist, but are not heavily tested for accuracy yet. If/when ITPO removal calls do get implemented, it will mostly be for feature completeness, rather than necessity.