重构了mymod.

multimod/multimod.c

@@ -6,15 +6,15 @@
 uint64_t mymod(uint64_t a, uint64_t m){
   //先实现基于倍增的取模
   uint64_t nowa = a;
-  for(int i = 63; i; i--){
-    if(m < (1ull << (64-i))){
-      if( (m << i) < nowa)
-        nowa = nowa - (m << i);
-    }
+  uint64_t nowm = m;
+  while (nowm <= a >> 1){
+    nowm = nowm << 1;
+  }//m扩大到不超过a的最大。
+  while (nowm >= m){
+    if(nowa >= nowm)
+      nowa = nowa - nowm;
+    nowm = nowm >> 1;
   }
-  //最后还要判定一次二的零次方
-  if(nowa > m)
-    nowa = nowa - m;
   return nowa;
 }
 
@@ -38,7 +38,7 @@ uint64_t multimod(uint64_t a, uint64_t b, uint64_t m) {
   uint64_t nowa = mymod(a, m);
   uint64_t res = 0; 
   for(int i = 1; i <= 64; i++){
-    if(nowa & 1){
+    if(nowa & 1ull){
         res = domod(nowb, res, m);
     }
     nowb = domod(nowb, nowb, m);