ps: auto-derive years of service from join year + retirement age

site/engine/pspension.js

@@ -87,6 +87,25 @@
     return y <= GROUP1_CUTOFF_YEAR ? 1 : 2;
   }
 
+  /* Auto-derive years of pensionable service (PS-AUTO-SERVICE):
+   *   yearsWorkedSoFar = currentYear − joinYear
+   *   ageAtJoin        = age − yearsWorkedSoFar
+   *   serviceYears     = retireAge − ageAtJoin, clamped to [0, 60] (whole years;
+   *                      the pension formula applies its own 35-yr MAX_SERVICE cap).
+   * currentYear defaults to the calendar year. Returns null when age, joinYear or
+   * retireAge is missing/invalid — the UI then falls back to manual entry. */
+  function autoServiceYears(age, joinYear, retireAge, currentYear) {
+    var y = +currentYear;
+    if (!(y > 0)) y = new Date().getFullYear();
+    var a = +age, j = +joinYear, r = +retireAge;
+    if (!(a > 0) || !(r > 0) || !(y > 0)) return null;
+    if (!(j >= 1900 && j <= 2999)) return null;
+    if (!isFinite(a) || !isFinite(j) || !isFinite(r) || !isFinite(y)) return null;
+    var joinAge = a - (y - j);
+    var s = r - joinAge;
+    return Math.round(Math.max(0, Math.min(60, s)));
+  }
+
   function itLevel(id) {
     for (var i = 0; i < IT_LEVELS.length; i++) if (IT_LEVELS[i].id === id) return IT_LEVELS[i];
     return null;
@@ -194,7 +213,7 @@
     SOURCES: SOURCES,
     psGroup: psGroup, psLifetimePension: psLifetimePension,
     psBridgeBenefit: psBridgeBenefit, psEarlyReduction: psEarlyReduction,
-    computePSPension: computePSPension,
+    computePSPension: computePSPension, autoServiceYears: autoServiceYears,
     itLevel: itLevel, itRange: itRange
   };
 });