fix: speedup dayOfWeek

Author: MaxGraey

std/assembly/date.ts

@@ -62,7 +62,7 @@ export class Date {
       let timeString: string;
       dateString = dateTimeString.substring(0, posT);
       timeString = dateTimeString.substring(posT + 1);
-      
+
       // might end with an offset ("Z", "+05:30", "-08:00", etc.)
       for (let i = timeString.length - 1; i >= 0; i--) {
         let c = timeString.charCodeAt(i);
@@ -82,8 +82,8 @@ export class Date {
           } else {
             let offsetHours = i32.parse(timeString.substring(i + 1));
             offsetMs = offsetHours * MILLIS_PER_HOUR;
-          }    
-    
+          }
+
           if (c == 45) offsetMs = -offsetMs; // negative offset
           timeString = timeString.substring(0, i);
           break;
@@ -361,9 +361,12 @@ function dayOfWeek(year: i32, month: i32, day: i32): i32 {
   const tab = memory.data<u8>([0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4]);
 
   year -= i32(month < 3);
-  year += floorDiv(year, 4) - floorDiv(year, 100) + floorDiv(year, YEARS_PER_EPOCH);
-  month = <i32>load<u8>(tab + month - 1);
-  return euclidRem(year + month + day, 7);
+
+  const century = floorDiv(year >> 2, 25);
+  year += (year >> 2) + (century >> 2) - century;
+
+  const monthOffset = <i32>load<u8>(tab + month - 1);
+  return euclidRem(year + monthOffset + day, 7);
 }
 
 function stringify(value: i32, padding: i32 = 2): string {