fix: optimize dayOfWeek & dateFromEpoch routines in date (#3033)

Author: MaxGraey

std/assembly/date.ts

@@ -3,15 +3,14 @@ import { Date as Date_binding } from "./bindings/dom";
 
 // @ts-ignore: decorator
 @inline const
+  MILLIS_LIMIT      = 8640000000000000,
   MILLIS_PER_DAY    = 1000 * 60 * 60 * 24,
   MILLIS_PER_HOUR   = 1000 * 60 * 60,
   MILLIS_PER_MINUTE = 1000 * 60,
   MILLIS_PER_SECOND = 1000,
-
-  YEARS_PER_EPOCH = 400,
-  DAYS_PER_EPOCH = 146097,
-  EPOCH_OFFSET = 719468, // Jan 1, 1970
-  MILLIS_LIMIT = 8640000000000000;
+  YEARS_PER_EPOCH   = 400,
+  DAYS_PER_EPOCH    = 146097, // 400 * 365 + 400 / 4 - 400 / 100 + 1
+  EPOCH_OFFSET      = 719468; // Jan 1, 1970
 
 // ymdFromEpochDays returns values via globals to avoid allocations
 // @ts-ignore: decorator
@@ -62,7 +61,7 @@ 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 +81,8 @@ 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;
@@ -333,25 +332,23 @@ function invalidDate(millis: i64): bool {
 // Paper: https://arxiv.org/pdf/2102.06959.pdf
 function dateFromEpoch(ms: i64): i32 {
   let da = (<i32>floorDiv(ms, MILLIS_PER_DAY) * 4 + EPOCH_OFFSET * 4) | 3;
-  let q0 = floorDiv(da, DAYS_PER_EPOCH); // [0, 146096]
+  let q0 = floorDiv(da, DAYS_PER_EPOCH);  // [0, 146096]
   let r1 = <u32>da - q0 * DAYS_PER_EPOCH;
   let u1 = u64(r1 | 3) * 2939745;
   let dm1 = <u32>u1 / 11758980;
   let n1 = 2141 * dm1 + 197913;
   let year = 100 * q0 + i32(u1 >>> 32);
-  let mo = n1 >>> 16;
-  _day = (n1 & 0xFFFF) / 2141 + 1; // [1, 31]
-  if (dm1 >= 306) { mo -= 12; ++year; }
-  _month = mo; // [1, 12]
-  return year;
+  _day = (n1 & 0xFFFF) / 2141 + 1;              // [1, 31]
+  _month = (n1 >>> 16) - (dm1 >= 306 ? 12 : 0); // [1, 12]
+  return year + (dm1 >= 306 ?  1 : 0);
 }
 
 // http://howardhinnant.github.io/date_algorithms.html#days_from_civil
-function daysSinceEpoch(y: i32, m: i32, d: i32): i64 {
-  y -= i32(m <= 2);
-  let era = <u32>floorDiv(y, YEARS_PER_EPOCH);
-  let yoe = <u32>y - era * YEARS_PER_EPOCH; // [0, 399]
-  let doy = <u32>(153 * (m + (m > 2 ? -3 : 9)) + 2) / 5 + d - 1; // [0, 365]
+function daysSinceEpoch(year: i32, month: i32, day: i32): i64 {
+  year -= i32(month <= 2);
+  let era = <u32>floorDiv(year, YEARS_PER_EPOCH);
+  let yoe = <u32>year - era * YEARS_PER_EPOCH; // [0, 399]
+  let doy = <u32>(153 * (month + (month > 2 ? -3 : 9)) + 2) / 5 + day - 1; // [0, 365]
   let doe = yoe * 365 + yoe / 4 - yoe / 100 + doy; // [0, 146096]
   return <i64><i32>(era * 146097 + doe - EPOCH_OFFSET);
 }
@@ -360,8 +357,9 @@ function daysSinceEpoch(y: i32, m: i32, d: i32): i64 {
 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);
+  year -= i32(month <= 2);
+  const cent = floorDiv(year >> 2, 100 >> 2);
+  year += (year >> 2) - cent + (cent >> 2);
   month = <i32>load<u8>(tab + month - 1);
   return euclidRem(year + month + day, 7);
 }