fix 2.63 2.66

2.63 remove the use of if and <
2.66 fix case 0x80000000

chapter2/code/leftmost-one.c

@@ -21,15 +21,17 @@ int leftmost_one(unsigned x) {
   x |= x >> 8;
   x |= x >> 16;
   /*
-   * then, do  mask & (~mask >> 1), reserve leftmost bit one
+   * then, do (mask >> 1) + (mask && 1), in which mask && 1 deals with case x = 0, reserve leftmost bit one
    * that's we want
    */
-  return x & (~x >> 1);
+  return (x >> 1) + (x && 1);
 }
 
 int main(int argc, char* argv[]) {
   assert(leftmost_one(0xFF00) == 0x8000);
   assert(leftmost_one(0x6000) == 0x4000);
+  assert(leftmost_one(0x0) == 0x0);
+  assert(leftmost_one(0x80000000) == 0x80000000);
   return 0;
 }
 

chapter2/code/srl-sra.c

@@ -18,10 +18,10 @@ int sra(int x, int k) {
 
   int w = sizeof(int) << 3;
   int mask = (int) -1 << (w - k);
-  if (x < 0) {
-    return xsrl | mask;
-  }
-  return xsrl;
+  //let mask remain unchanged when the first bit of x is 1, otherwise 0.
+  int m = 1 << (w - 1);
+  mask &= ! (x & m) - 1;
+  return xsrl | mask;
 }
 
 int main(int argc, char* argv[]) {
@@ -31,5 +31,11 @@ int main(int argc, char* argv[]) {
   assert(srl(test_unsigned, 4) == test_unsigned >> 4);
   assert(sra(test_int, 4) == test_int >> 4);
 
+  test_unsigned = 0x87654321;
+	test_int = 0x87654321;
+
+	assert (srl (test_unsigned, 4) == test_unsigned >> 4);
+	assert (sra (test_int, 4) == test_int >> 4);
+
   return 0;
 }