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'''Guaranteed damage reduction (GDR)''' gives players a layer of protection in melee and ranged combat. Normally, each attack that hits you is reduced by a random amount up to your AC. Against regular melee and weapon attacks, your minimum AC reduction is equal to GDR% of the attack's damage, unless this would be higher than <code>1/2 * AC</code>. In essence, GDR is a form of insurance against poor AC rolls.

==What GDR Affects==
GDR only works against physical damage, either in melee, with [[reaching]], or with a ranged/throwing weapon.

It does ''not'' apply against any sort of spell or ability, unless that ability specifically uses the weapon/melee damage formula (e.g., [[Manifold Assault]]). Therefore, GDR does not apply to a [[Conjurations]] spell that does physical damage. Neither does it apply against "physical" abilities like [[Slug Dart]] or [[Harpoon Shot]]. It also doesn't work against elemental melee damage, such as an [[ice beast]]'s additional cold damage.

==Calculating GDR==
Your GDR is calculated by the following formula: <code>GDR% = AC^(1/4) * 16</code>. Any means of obtaining AC counts towards your GDR.

===GDR of Various AC Values===
{| class="wikitable"
! Base AC || GDR
|-
| 2 ([[Robe]]) || 19.0%
|-
| 6 ([[Scale mail]]) || 25.0%
|-
| 10 ([[Plate armour]]) || 28.5%
|-
| 16 (+2 [[Robe]], aux) || 32%
|-
| 32 (+10 [[Plate armour]], aux) || 38.1%
|-
| 40 (+14 [[Crystal plate armour|Crystal plate]], aux) ||40.2%
|-
| 50 || 42.5%
|-
| 100 || 50.6%
|-
|}
Auxiliary slots include a +2 [[Helmet]], +2 [[Cloak]], +2 [[Gloves]], and +2 [[Boots]]. Percents on this table are rounded to the nearest tenth.

==Damage Reduction==
GDR is '''not''' a form of damage reduction independent from AC. If you could have excellent GDR and terrible AC, it would do you little good. It is effectively a minimum AC roll, or a ''guaranteed'' AC roll. It is capped at <code>GDR% * damage taken</code> or <code>1/2 * AC</code>, whichever is lower.

Let's say you take a 50 damage attack, but have 48% GDR. First, [[AC]] rolls. If AC roll is a 1, then GDR will raise the reduction to (50 * .48) = 24, meaning you actually lose 26 HP. But if your AC roll is 36, then GDR will not do anything, as you are above the guaranteed reduction of 24.

If you took said 50 damage attack with 16 AC and 32% GDR, then the AC/2 limit applies instead: you'll always reduce damage by at least (16 / 2) = 8 over the expected (50 * .32) = 16.

==Strategy==
GDR is tied to your [[AC]]; any form of extra AC will also increase GDR. This comes with the same costs and benefits; are you willing to sacrifice [[EV]] and spellcasting potential in order to become more bulky? Characters who want to avoid melee will still want to avoid melee as much as possible. However, GDR is a large reason why AC is more consistent in stopping damage.

For most characters, GDR will slightly reduce the maximum physical damage enemies can deal.

==History==
*Prior to [[0.27]], GDR was more complex. Each set of body [[armour]] had a defined GDR, calculated by <code>(14 * (Body Armour Base AC - 2)^(1/2))%</code>, no matter what your actual AC was. Certain [[transmutation]]s also had a set GDR. [[Gargoyle]]s increased base GDR, while [[Draconian]]s (being unable to wear body armour) didn't have any. For more information, see [http://crawl.chaosforge.org/index.php?title=Guaranteed_damage_reduction&oldid=52698 this past revision].
*Prior to [[0.8]], GDR was equal to <code>(Body Armour Base AC) * (13 + Armour) / 17</code>.
*Prior to [[0.5]], GDR was higher.

[[Category:Defence]][[Category:Game mechanics]]

Guaranteed damage reduction - Revision history

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