zenno: stamina calculator first pass

zenno/src/lib/race.ts

@@ -354,6 +354,17 @@ export function maxHP(raceLen: number, style: RunningStyle, stamStat: number): n
 	return 0.8 * hpStrategyCoeff[style] * stamStat + raceLen;
 }
 
+/**
+ * Compute the stamina stat that would produce a given effective HP
+ * @param raceLen Length of the race in meters
+ * @param style Horse's running style
+ * @param hp Effective HP, i.e. max HP plus recoveries minus debuffs
+ * @returns Stamina
+ */
+export function effectiveStam(raceLen: number, style: RunningStyle, hp: number): number {
+	return (hp - raceLen) / (0.8 * hpStrategyCoeff[style]);
+}
+
 const groundHPRateMod = {
 	[Surface.Turf]: {
 		[GroundConditions.Firm]: 1,