Implement full set of rounding modes in polyfill

This brings the reference polyfill up to date with the new rounding modes.

It adds a quick set of tests that only test the internal implementation,
in order to make sure that the RoundNumberToIncrement operation behaves as
intended. Full tests should test the observable outcome in test262.

UPSTREAM_COMMIT=d450be195a81228d284777969bedd9e0475400bd

Author: ptomato

index.d.ts

@@ -1,6 +1,15 @@
 export namespace Temporal {
   export type ComparisonResult = -1 | 0 | 1;
-  export type RoundingMode = 'halfExpand' | 'ceil' | 'trunc' | 'floor';
+  export type RoundingMode =
+    | 'ceil'
+    | 'floor'
+    | 'expand'
+    | 'trunc'
+    | 'halfCeil'
+    | 'halfFloor'
+    | 'halfExpand'
+    | 'halfTrunc'
+    | 'halfEven';
 
   /**
    * Options for assigning fields using `with()` or entire objects with

lib/ecmascript.ts

@@ -842,7 +842,12 @@ export function ToTemporalRoundingMode(
   options: { roundingMode?: Temporal.RoundingMode },
   fallback: Temporal.RoundingMode
 ) {
-  return GetOption(options, 'roundingMode', ['ceil', 'floor', 'trunc', 'halfExpand'], fallback);
+  return GetOption(
+    options,
+    'roundingMode',
+    ['ceil', 'floor', 'expand', 'trunc', 'halfCeil', 'halfFloor', 'halfExpand', 'halfTrunc', 'halfEven'],
+    fallback
+  );
 }
 
 function NegateTemporalRoundingMode(roundingMode: Temporal.RoundingMode) {
@@ -851,6 +856,10 @@ function NegateTemporalRoundingMode(roundingMode: Temporal.RoundingMode) {
       return 'floor';
     case 'floor':
       return 'ceil';
+    case 'halfCeil':
+      return 'halfFloor';
+    case 'halfFloor':
+      return 'halfCeil';
     default:
       return roundingMode;
   }
@@ -5317,19 +5326,46 @@ function RoundNumberToIncrement(quantity: JSBI, increment: number, mode: Tempora
   let { quotient, remainder } = divmod(quantity, JSBI.BigInt(increment));
   if (JSBI.equal(remainder, ZERO)) return quantity;
   const sign = JSBI.lessThan(remainder, ZERO) ? -1 : 1;
+  const tiebreaker = abs(JSBI.multiply(remainder, JSBI.BigInt(2)));
+  const tie = JSBI.equal(tiebreaker, JSBI.BigInt(increment));
+  const expandIsNearer = JSBI.greaterThan(tiebreaker, JSBI.BigInt(increment));
   switch (mode) {
     case 'ceil':
       if (sign > 0) quotient = JSBI.add(quotient, JSBI.BigInt(sign));
       break;
     case 'floor':
       if (sign < 0) quotient = JSBI.add(quotient, JSBI.BigInt(sign));
       break;
+    case 'expand':
+      // always expand if there is a remainder
+      quotient = JSBI.add(quotient, JSBI.BigInt(sign));
+      break;
     case 'trunc':
       // no change needed, because divmod is a truncation
       break;
+    case 'halfCeil':
+      if (expandIsNearer || (tie && sign > 0)) {
+        quotient = JSBI.add(quotient, JSBI.BigInt(sign));
+      }
+      break;
+    case 'halfFloor':
+      if (expandIsNearer || (tie && sign < 0)) {
+        quotient = JSBI.add(quotient, JSBI.BigInt(sign));
+      }
+      break;
     case 'halfExpand':
       // "half up away from zero"
-      if (JSBI.toNumber(abs(JSBI.multiply(remainder, JSBI.BigInt(2)))) >= increment) {
+      if (expandIsNearer || tie) {
+        quotient = JSBI.add(quotient, JSBI.BigInt(sign));
+      }
+      break;
+    case 'halfTrunc':
+      if (expandIsNearer) {
+        quotient = JSBI.add(quotient, JSBI.BigInt(sign));
+      }
+      break;
+    case 'halfEven':
+      if (expandIsNearer || (tie && JSBI.toNumber(JSBI.remainder(abs(quotient), JSBI.BigInt(2))) === 1)) {
         quotient = JSBI.add(quotient, JSBI.BigInt(sign));
       }
       break;