Implement modInverse for an even modulus.

sdk/lib/_internal/compiler/js_lib/js_number.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of _interceptors;
 
 /**
  * The super interceptor class for [JSInt] and [JSDouble]. The compiler
  * recognizes this class as an interceptor, and changes references to
  * [:this:] to actually use the receiver of the method, which is
@@ skipping 397 matching lines @@
       e ~/= 2;
       b = (b * b) % m;
     }
     return r;
   }
 
   // Returns 1/this % m, with m > 0.
   int modInverse(int m) {
     if (m is! int) throw new ArgumentError(m);
     if (m <= 0) throw new RangeError(m);
-    if (this == 0) return 0;
-    bool ac = m.isEven;
+    if (m == 1) return 0;
+    int t = this;
+    if ((t < 0) || (t >= m)) t %= m;
+    if (t == 1) return 1;
+    final bool ac = m.isEven;
+    if ((t == 0) || (ac && t.isEven)) throw new RangeError("Not coprime");
     int u = m;
-    int v = this;
+    int v = t;
     int a = 1,
         b = 0,
         c = 0,
         d = 1;
-    while (u != 0) {
+    do {
       while (u.isEven) {
         u ~/= 2;
         if (ac) {
           if (!a.isEven || !b.isEven) {
-            a += this;
+            a += t;
             b -= m;
           }
           a ~/= 2;
         } else if (!b.isEven) {
           b -= m;
         }
         b ~/= 2;
       }
       while (v.isEven) {
         v ~/= 2;
         if (ac) {
           if (!c.isEven || !d.isEven) {
-            c += this;
+            c += t;
             d -= m;
           }
           c ~/= 2;
         } else if (!d.isEven) {
           d -= m;
         }
         d ~/= 2;
       }
       if (u >= v) {
         u -= v;
         if (ac) a -= c;
         b -= d;
       } else {
         v -= u;
         if (ac) c -= a;
         d -= b;
       }
+    } while (u != 0);
+    if (v != 1) throw new RangeError("Not coprime");
+    if (d < 0) {
+      d += m;
+      if (d < 0) d += m;
+    } else if (d > m) {
+      d -= m;
+      if (d > m) d -= m;
     }
-    if (v != 1) return 0;
-    if (d > m) return d - m;
-    if (d < 0) d += m;
-    if (d < 0) d += m;
     return d;
   }
 
   // Assumes i is <= 32-bit and unsigned.
   static int _bitCount(int i) {
     // See "Hacker's Delight", section 5-1, "Counting 1-Bits".
 
     // The basic strategy is to use "divide and conquer" to
     // add pairs (then quads, etc.) of bits together to obtain
     // sub-counts.
@@ skipping 37 matching lines @@
 }
 
 class JSDouble extends JSNumber implements double {
   const JSDouble();
   Type get runtimeType => double;
 }
 
 class JSPositiveInt extends JSInt {}
 class JSUInt32 extends JSPositiveInt {}
 class JSUInt31 extends JSUInt32 {}