bn: introduce in-place pow, .ipow() [WIP]

README.md

@@ -88,7 +88,7 @@ either `le` (little-endian) or `be` (big-endian).
 * `a.sub(b)` - subtraction (`i`, `n`, `in`)
 * `a.mul(b)` - multiply (`i`, `n`, `in`)
 * `a.sqr()` - square (`i`)
-* `a.pow(b)` - raise `a` to the power of `b`
+* `a.pow(b)` - raise `a` to the power of `b` (`i`)
 * `a.div(b)` - divide (`divn`, `idivn`)
 * `a.mod(b)` - reduct (`u`, `n`) (but no `umodn`)
 * `a.divRound(b)` - rounded division

lib/bn.js

@@ -1919,25 +1919,32 @@
   };
 
   // Math.pow(`this`, `num`)
-  BN.prototype.pow = function pow (num) {
+  BN.prototype.ipow = function ipow (num) {
     var w = toBitArray(num);
-    if (w.length === 0) return new BN(1);
+    if (w.length === 0) {
+      this.words[0] = 1;
+      this.length = 1;
+      return this;
+    }
 
     // Skip leading zeroes
-    var res = this;
-    for (var i = 0; i < w.length; i++, res = res.sqr()) {
+    for (var i = 0; i < w.length; i++, this.isqr()) {
       if (w[i] !== 0) break;
     }
 
     if (++i < w.length) {
-      for (var q = res.sqr(); i < w.length; i++, q = q.sqr()) {
+      for (var q = this.sqr(); i < w.length; i++, q.isqr()) {
         if (w[i] === 0) continue;
 
-        res = res.mul(q);
+        this.imul(q);
       }
     }
 
-    return res;
+    return this;
+  };
+
+  BN.prototype.pow = function pow (num) {
+    return this.clone().ipow(num);
   };
 
   // Shift-left in-place

test/arithmetic-test.js

@@ -282,6 +282,22 @@ describe('BN.js/Arithmetic', function () {
 
       assert.equal(c.toString(16), '15963da06977df51909c9ba5b');
     });
+    it('0 power should equal to 1', function () {
+      assert.equal(new BN(123).pow(new BN(0)).toNumber(), 1);
+    });
+  });
+
+  describe('.ipow()', function () {
+    it('should raise number to the power', function () {
+      var a = new BN('ab', 16);
+      var b = new BN('13', 10);
+      a.ipow(b);
+
+      assert.equal(a.toString(16), '15963da06977df51909c9ba5b');
+    });
+    it('0 power should equal to 1', function () {
+      assert.equal(new BN(123).ipow(new BN(0)).toNumber(), 1);
+    });
   });
 
   describe('.div()', function () {