fix(js/ts): Fix Decimal.GetBits returning incorrect mantissa after round arithmetic result (#4561)

Co-authored-by: Mangel Maxime <me@mangelmaxime.fr>

Author: NaserAnjum21

src/fable-library-ts/Decimal.ts

@@ -248,14 +248,16 @@ export function fromParts(low: number, mid: number, high: number, isNegative: bo
 
 export function getBits(d: Decimal) {
   const bitSize = 96;
-  const decDigits = Uint8Array.from(d.c);
+  const decStr = d.toString();
+  const absStr = decStr[0] === "-" ? decStr.slice(1) : decStr;
+  const dotPos = absStr.indexOf(".");
+  const scale = dotPos < 0 ? 0 : absStr.length - dotPos - 1;
+  const mantissaStr = absStr.replace(".", "");
+  const decDigits = Uint8Array.from(mantissaStr, ch => ch.charCodeAt(0) - 48);
   const hexDigits = decimalToHex(decDigits, bitSize);
   const low = getInt32Bits(hexDigits, 0);
   const mid = getInt32Bits(hexDigits, 8);
   const high = getInt32Bits(hexDigits, 16);
-  const decStr = d.toString();
-  const dotPos = decStr.indexOf(".");
-  const scale = dotPos < 0 ? 0 : decStr.length - dotPos - 1;
   const signExp = ((scale & 0x7F) << 16) | (d.s < 0 ? 0x80000000 : 0);
   return [low, mid, high, signExp];
 }

tests/Js/Main/ArithmeticTests.fs

@@ -253,6 +253,18 @@ let tests =
         d2 |> equal d
         d3 |> equal -d
 
+    testCase "Decimal GetBits works with arithmetic operations" <| fun () ->
+        let check (d: decimal) =
+            let bits = Decimal.GetBits(d)
+            Decimal(bits) |> equal d
+
+        check (10M + 10M)
+        check (100.5M - 0.5M)
+        check (5M * 200M)
+        check (-0.1M * 100M)
+        check (2000M / 10M)
+        check (1.5M + 0.5M)
+
     testCase "Decimal abs works" <| fun () ->
         abs -4M |> equal 4M