Fix top digit handling in “even” case of __inplaceSub (#14)

GoogleChromeLabs · Nov 20, 2018 · e77f163 · e77f163
1 parent ca255a2
commit e77f163
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Showing 2 changed files with 36 additions and 13 deletions.
diff --git a/jsbi.mjs b/jsbi.mjs
@@ -56,7 +56,7 @@ class JSBI extends Array {
   toDebugString() {
     const result = ['BigInt['];
     for (const digit of this) {
-      result.push((digit ? digit.toString(16) : digit) + ', ');
+      result.push((digit ? (digit >>> 0).toString(16) : digit) + ', ');
     }
     result.push(']');
     return result.join('');
@@ -1454,9 +1454,9 @@ class JSBI extends Array {
       }
       // D4.
       JSBI.__internalMultiplyAdd(divisor, qhat, 0, n2, qhatv);
-      let c = u.__inplaceSub(qhatv, j);
+      let c = u.__inplaceSub(qhatv, j, n + 1);
       if (c !== 0) {
-        c = u.__inplaceAdd(divisor, j);
+        c = u.__inplaceAdd(divisor, j, n);
         u.__setHalfDigit(j + n, u.__halfDigit(j + n) + c);
         qhat--;
       }
@@ -1483,10 +1483,9 @@ class JSBI extends Array {
   }
 
   // TODO: work on full digits, like __inplaceSub?
-  __inplaceAdd(summand, startIndex) {
+  __inplaceAdd(summand, startIndex, halfDigits) {
     let carry = 0;
-    const n = summand.__halfDigitLength();
-    for (let i = 0; i < n; i++) {
+    for (let i = 0; i < halfDigits; i++) {
       const sum = this.__halfDigit(startIndex + i) +
                 summand.__halfDigit(i) +
                 carry;
@@ -1496,7 +1495,8 @@ class JSBI extends Array {
     return carry;
   }
 
-  __inplaceSub(subtrahend, startIndex) {
+  __inplaceSub(subtrahend, startIndex, halfDigits) {
+    let fullSteps = (halfDigits - 1) >>> 1;
     let borrow = 0;
     if (startIndex & 1) {
       // this:   [..][..][..]
@@ -1505,7 +1505,7 @@ class JSBI extends Array {
       let current = this.__digit(startIndex);
       let r0 = current & 0xFFFF;
       let i = 0;
-      for (; i < subtrahend.length - 1; i++) {
+      for (; i < fullSteps; i++) {
         const sub = subtrahend.__digit(i);
         const r16 = (current >>> 16) - (sub & 0xFFFF) - borrow;
         borrow = (r16 >>> 16) & 1;
@@ -1521,18 +1521,19 @@ class JSBI extends Array {
       this.__setDigit(startIndex + i, (r16 << 16) | (r0 & 0xFFFF));
       const subTop = sub >>> 16;
       if (startIndex + i + 1 >= this.length) {
-throw new RangeError('out of bounds');
-}
-      current = this.__digit(startIndex + i + 1);
-      if (subTop !== 0 || current !== 0) {
+        throw new RangeError('out of bounds');
+      }
+      if ((halfDigits & 1) === 0) {
+        current = this.__digit(startIndex + i + 1);
         r0 = (current & 0xFFFF) - subTop - borrow;
         borrow = (r0 >>> 16) & 1;
         this.__setDigit(startIndex + subtrahend.length,
                         (current & 0xFFFF0000) | (r0 & 0xFFFF));
       }
     } else {
       startIndex >>= 1;
-      for (let i = 0; i < subtrahend.length; i++) {
+      let i = 0;
+      for (; i < subtrahend.length - 1; i++) {
         const current = this.__digit(startIndex + i);
         const sub = subtrahend.__digit(i);
         const r0 = (current & 0xFFFF) - (sub & 0xFFFF) - borrow;
@@ -1541,6 +1542,16 @@ throw new RangeError('out of bounds');
         borrow = (r16 >>> 16) & 1;
         this.__setDigit(startIndex + i, (r16 << 16) | (r0 & 0xFFFF));
       }
+      const current = this.__digit(startIndex + i);
+      const sub = subtrahend.__digit(i);
+      const r0 = (current & 0xFFFF) - (sub & 0xFFFF) - borrow;
+      borrow = (r0 >>> 16) & 1;
+      let r16 = 0;
+      if ((halfDigits & 1) === 0) {
+        r16 = (current >>> 16) - (sub >>> 16) - borrow;
+        borrow = (r16 >>> 16) & 1;
+      }
+      this.__setDigit(startIndex + i, (r16 << 16) | (r0 & 0xFFFF));
     }
     return borrow;
   }

diff --git a/tests/tests.mjs b/tests/tests.mjs
@@ -30,6 +30,18 @@ const TESTS = [
     b: '-0xF3CF5EDD759DBCC7449962CDB52AE0295BE7306D51555C70',
     expected: '-0x1034209C3C2F251E3F2EF7068FA5CAE964C0B57661EB577E7',
   },
+  { // https://github.com/GoogleChromeLabs/jsbi/pull/14
+    operation: 'remainder',
+    a: '0x62A49213A5CD1793CB4518A12CA4FB5F3AB6DBD8B465D0D86975CEBDA6B6093',
+    b: '0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF',
+    expected: '0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFE',
+  },
+  { // https://github.com/GoogleChromeLabs/jsbi/pull/14#issuecomment-439484605
+    operation: 'remainder',
+    a: '0x10000000000000000',
+    b: '0x100000001',
+    expected: '0x1',
+  },
 ];
 
 function parse(string) {