Elite units small discrepancy between maxHitpoints and hitpoints

[Langbart]

to reproduce

• start the units_demo map

pyrogenesis -autostart="scenarios/units_demo" -autostart-victory="endless"

• seek the Mauryan Hero Healer Acharya Chanakya, somewhere in the back rows. You will find him, because he is the only one walking around .

• the Healer is going to some Elite units to heal them, for example the Cretan Mercenary Archer

• select the Cretan Mercenary Elite Archer, toggle the Developer Overlay Alt+D and the option Display selection state
  • the difference between hitpoints and maxHitpoints is only 0.00000000000001 == 1E-14

• the Mauryan Hero Healer is the only walking around, because of its large vision range he inherits from the ​simulation/templates/template_unit_hero.xml template

Line
66	</TrainingRestrictions>
67	<Vision>
68	<Range>100</Range>
69	</Vision>
70	</Entity>

• other healers have only 30 vision - ​simulation/templates/template_unit_support_healer.xml

Line
53	</Sound>
54	<Vision>
55	<Range>30</Range>
56	</Vision>
57	</Entity>

minimal amount of units to reproduce the issue

• change the units_demo.js to only show three units. the combo is enough to create the discrepancy in health points.

  binaries/data/mods/public/maps/scenarios/units_demo.js

  @@ -31 +31 @@
   let gap = 14;
   
   let cmpTemplateManager = Engine.QueryInterface(SYSTEM_ENTITY, IID_TemplateManager);
  -for (let template of cmpTemplateManager.FindAllTemplates(actors))
  +for (let template of ["units/mace/hero_pyrrhus_i", "units/iber/catafalque", "units/maur/elephant_archer_e"])
   {
       print(template + "...\n");

intended behavior

• the hitpoints for all elite units should be the same as the maxHitpoints when the game starts

cause of the issue

See the comment by Stan suggesting floating point math

❯ node
Welcome to Node.js v19.1.0.
> 0.1+0.2
0.30000000000000004

possible solution

one-line change, there is a calculation being made in the Health.js file.

example units/maur/elephant_archer_e

# this.hitpoints * newMaxHitpoints / oldMaxHitpoints
❯ node
Welcome to Node.js v19.1.0.
> 233.75000000000003*292.18750000000006/233.75000000000003
# 292.1875

binaries/data/mods/public/simulation/components/Health.js

@@ -465 +465 @@
     let newMaxHitpoints = ApplyValueModificationsToEntity("Health/Max", +this.template.Max, this.entity);
     if (oldMaxHitpoints != newMaxHitpoints)
     {
-        let newHitpoints = this.hitpoints * newMaxHitpoints/oldMaxHitpoints;
+        let newHitpoints = (this.hitpoints === oldMaxHitpoints) ? newMaxHitpoints : this.hitpoints * newMaxHitpoints / oldMaxHitpoints;
         this.maxHitpoints = newMaxHitpoints;
         this.SetHitpoints(newHitpoints);
     }

[Stan]

Might be float imprecision

[phosit]

Yes moast propably it is float imprecision: Bigger values are (absolute) more imprecise. Elite Vachii Gaia (Elephant Archer) have 292.18750000000006.

The imprecision alone is not a problem the difference between hitpoints and maxHitpoints is. For Advanced Vachii Gaia (Elephant Archer) maxHitpoints does round to 233.75000000000003 but hitpoints does that to. There is no difference and they need no heal.

Typeing 187 * 1.25 in to the in game console results in 233.75 an not 233.75000000000003

[Freagarach]

It was reported by @wowgetoffyourcellphone before as well. Maybe even this: ​https://github.com/JustusAvramenko/delenda_est/commit/cd88bed66168a0366ac816dcdd8a841de8e536fc.

[Langbart]

Replying to phosit:

Typeing 187 * 1.25 in to the in game console results in 233.75 an not 233.75000000000003

---

Source: ​ESLINT rule: no-loss-of-precision

In JS, Numbers are stored as double-precision floating-point numbers according to the IEEE 754 standard. Because of this, numbers can only retain accuracy up to a certain amount of digits. If the programmer enters additional digits, those digits will be lost in the conversion to the Number type and will result in unexpected behavior.

example - node.js

the second calculation has one more zero

node
Welcome to Node.js v19.1.0.
> 58.43750000000001 * 1.25
# 73.04687500000001
> 58.437500000000001 * 1.25
# 73.046875

[Langbart]

adding possible solution section

[Freagarach]

In 27425:

Fix discrepancy between max hitpoints and actual hitpoints due to floats in JS.

Patch by: @Langbart
Fixes #6657