fix for #44 (#45)

* fix for #44

* Update tests.mjs

jsbi.mjs

@@ -1194,9 +1194,11 @@ class JSBI extends Array {
     let carry = true;
     for (let i = 0; i < inputLength; i++) {
       let digit = x.__digit(i);
-      const newCarry = digit === (0xFFFFFFFF | 0);
-      if (carry) digit = (digit + 1) | 0;
-      carry = newCarry;
+      if (carry) {
+        const newCarry = digit === (0xFFFFFFFF | 0);
+        digit = (digit + 1) | 0;
+        carry = newCarry;
+      }
       result.__setDigit(i, digit);
     }
     if (carry) {
@@ -1212,11 +1214,14 @@ class JSBI extends Array {
     let borrow = true;
     for (let i = 0; i < length; i++) {
       let digit = x.__digit(i);
-      const newBorrow = digit === 0;
-      if (borrow) digit = (digit - 1) | 0;
-      borrow = newBorrow;
+      if (borrow) {
+        const newBorrow = digit === 0;
+        digit = (digit - 1) | 0;
+        borrow = newBorrow;
+      }
       result.__setDigit(i, digit);
     }
+    if (borrow) throw new Error('implementation bug');
     for (let i = length; i < resultLength; i++) {
       result.__setDigit(i, 0);
     }

tests/tests.mjs

@@ -44,6 +44,18 @@ const TESTS = [
     b: '0x100000001',
     expected: '0x1',
   },
+  { // https://github.com/GoogleChromeLabs/jsbi/issues/44#issue-630518844
+    operation: 'bitwiseAnd',
+    a: '0b10000010001000100010001000100010001000100010001000100010001000100',
+    b: '-0b10000000000000000000000000000000000000000000000000000000000000001',
+    expected: '0b10001000100010001000100010001000100010001000100010001000100',
+  },
+  { // https://github.com/GoogleChromeLabs/jsbi/issues/44#issue-630518844
+    operation: 'bitwiseXor',
+    a: '0',
+    b: '-0b1111111111111111111111111111111111111111111111111111111111111111',
+    expected: '-0b1111111111111111111111111111111111111111111111111111111111111111',
+  },
 ];
 
 // https://github.com/GoogleChromeLabs/jsbi/issues/36