refactor(pure): extract MS_PER_DAY constant and document computeEstimate precondition

src/pure.js

@@ -1,4 +1,5 @@
 const SUFFIXES = { k: 1e3, m: 1e6, b: 1e9, t: 1e12 };
+const MS_PER_DAY = 86_400_000;
 
 export function parseTarget(input) {
   if (input === null || input === undefined || input === '') return null;
@@ -24,6 +25,19 @@ export function parseTarget(input) {
   return Math.floor(num * multiplier);
 }
 
+/**
+ * Estimate remaining work and ETA.
+ *
+ * Preconditions: `current` and `target` are expected to be non-negative
+ * integers. This function does not validate its inputs — callers are
+ * responsible for ensuring sane values.
+ *
+ * @param {number} current   current stat value (non-negative integer)
+ * @param {number} target    target stat value  (non-negative integer)
+ * @param {number} perTrain  stat gain per training session
+ * @param {number} perDay    stat gain per day
+ * @returns {{remaining:number, trainsToGo:number, days:number, eta:Date|null}}
+ */
 export function computeEstimate(current, target, perTrain, perDay) {
   const remaining = Math.max(0, target - current);
   if (remaining === 0) {
@@ -32,7 +46,7 @@ export function computeEstimate(current, target, perTrain, perDay) {
 
   const trainsToGo = perTrain > 0 ? Math.ceil(remaining / perTrain) : 0;
   const days = perDay > 0 ? Math.ceil(remaining / perDay) : 0;
-  const eta = days > 0 ? new Date(Date.now() + days * 86_400_000) : null;
+  const eta = days > 0 ? new Date(Date.now() + days * MS_PER_DAY) : null;
 
   return { remaining, trainsToGo, days, eta };
 }