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.
tc39 · Sep 21, 2022 · d450be1 · d450be1
1 parent b60cb83
commit d450be1
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Showing 2 changed files with 61 additions and 3 deletions.
diff --git a/polyfill/lib/ecmascript.mjs b/polyfill/lib/ecmascript.mjs
@@ -688,14 +688,23 @@ export const ES = ObjectAssign({}, ES2020, {
     return ES.GetOption(options, 'disambiguation', ['compatible', 'earlier', 'later', 'reject'], 'compatible');
   },
   ToTemporalRoundingMode: (options, fallback) => {
-    return ES.GetOption(options, 'roundingMode', ['ceil', 'floor', 'trunc', 'halfExpand'], fallback);
+    return ES.GetOption(
+      options,
+      'roundingMode',
+      ['ceil', 'floor', 'expand', 'trunc', 'halfCeil', 'halfFloor', 'halfExpand', 'halfTrunc', 'halfEven'],
+      fallback
+    );
   },
   NegateTemporalRoundingMode: (roundingMode) => {
     switch (roundingMode) {
       case 'ceil':
         return 'floor';
       case 'floor':
         return 'ceil';
+      case 'halfCeil':
+        return 'halfFloor';
+      case 'halfFloor':
+        return 'halfCeil';
       default:
         return roundingMode;
     }
@@ -4197,20 +4206,40 @@ export const ES = ObjectAssign({}, ES2020, {
     let { quotient, remainder } = quantity.divmod(increment);
     if (remainder.equals(bigInt.zero)) return quantity;
     const sign = remainder.lt(bigInt.zero) ? -1 : 1;
+    const tiebreaker = remainder.multiply(2).abs();
+    const tie = tiebreaker.equals(increment);
+    const expandIsNearer = tiebreaker.gt(increment);
     switch (mode) {
       case 'ceil':
         if (sign > 0) quotient = quotient.add(sign);
         break;
       case 'floor':
         if (sign < 0) quotient = quotient.add(sign);
         break;
+      case 'expand':
+        // always expand if there is a remainder
+        quotient = quotient.add(sign);
+        break;
       case 'trunc':
         // no change needed, because divmod is a truncation
         break;
+      case 'halfCeil':
+        if (expandIsNearer || (tie && sign > 0)) quotient = quotient.add(sign);
+        break;
+      case 'halfFloor':
+        if (expandIsNearer || (tie && sign < 0)) quotient = quotient.add(sign);
+        break;
       case 'halfExpand':
         // "half up away from zero"
-        if (remainder.multiply(2).abs() >= increment) quotient = quotient.add(sign);
+        if (expandIsNearer || tie) quotient = quotient.add(sign);
         break;
+      case 'halfTrunc':
+        if (expandIsNearer) quotient = quotient.add(sign);
+        break;
+      case 'halfEven': {
+        if (expandIsNearer || (tie && quotient.isOdd())) quotient = quotient.add(sign);
+        break;
+      }
     }
     return quotient.multiply(increment);
   },

diff --git a/polyfill/test/ecmascript.mjs b/polyfill/test/ecmascript.mjs
@@ -5,7 +5,9 @@ import Pretty from '@pipobscure/demitasse-pretty';
 const { reporter } = Pretty;
 
 import { strict as assert } from 'assert';
-const { deepEqual, throws } = assert;
+const { deepEqual, equal, throws } = assert;
+
+import bigInt from 'big-integer';
 
 import { ES } from '../lib/ecmascript.mjs';
 import { GetSlot, TIMEZONE_ID } from '../lib/slots.mjs';
@@ -35,6 +37,33 @@ describe('ECMAScript', () => {
       );
     });
   });
+
+  describe('RoundNumberToIncrement', () => {
+    const increment = bigInt(100);
+    const testValues = [-150, -100, -80, -50, -30, 0, 30, 50, 80, 100, 150];
+    const expectations = {
+      ceil: [-100, -100, 0, 0, 0, 0, 100, 100, 100, 100, 200],
+      floor: [-200, -100, -100, -100, -100, 0, 0, 0, 0, 100, 100],
+      trunc: [-100, -100, 0, 0, 0, 0, 0, 0, 0, 100, 100],
+      expand: [-200, -100, -100, -100, -100, 0, 100, 100, 100, 100, 200],
+      halfCeil: [-100, -100, -100, 0, 0, 0, 0, 100, 100, 100, 200],
+      halfFloor: [-200, -100, -100, -100, 0, 0, 0, 0, 100, 100, 100],
+      halfTrunc: [-100, -100, -100, 0, 0, 0, 0, 0, 100, 100, 100],
+      halfExpand: [-200, -100, -100, -100, 0, 0, 0, 100, 100, 100, 200],
+      halfEven: [-200, -100, -100, 0, 0, 0, 0, 0, 100, 100, 200]
+    };
+    for (const roundingMode of Object.keys(expectations)) {
+      describe(roundingMode, () => {
+        testValues.forEach((value, ix) => {
+          const expected = expectations[roundingMode][ix];
+          it(`rounds ${value} to ${expected}`, () => {
+            const result = ES.RoundNumberToIncrement(bigInt(value), increment, roundingMode);
+            equal(result.toJSNumber(), expected);
+          });
+        });
+      });
+    }
+  });
 });
 
 import { normalize } from 'path';