fix solution 2.70

chapter2/code/fits-bits.c

@@ -1,31 +1,39 @@
 /*
- * fit-bits.c
+ * fits-bits.c
  */
 #include <stdio.h>
 #include <assert.h>
 
 int fits_bits(int x, int n) {
   /*
-   * Assume w = 8, n = 3
+   * 1 <= n <= w
+   *
+   * assume w = 8, n = 3
    * if x > 0
-   *   0b00000110 is ok, 0b00001010 is not
-   *   first w-n bits must be 0
+   *   0b00000010 is ok, 0b00001010 is not, and 0b00000110 is not yet (thanks itardc@163.com)
    * if x < 0
    *   0b11111100 is ok, 0b10111100 is not, and 0b11111000 is not yet
-   *   first w-n+1 bits must be 1
+   *
+   * the point is
+   *   x << (w-n) >> (w-n) must be equal to x itself.
+   *
    */
   int w = sizeof(int) << 3;
-  x >>= n - 1;
-  /*
-   * !(x >> 1) return 1 only when all bits are 0,
-   * !~x == 1 only when all bits are 1
-   */
-  return !(x >> 1) || !~x;
+  int offset = w - n;
+  return (x << offset >> offset) == x;
 }
 
 int main(int argc, char* argv[]) {
-  assert(fits_bits(0xFF, 8));
-  assert(!fits_bits(0xFFFFFF00, 8));
+  assert(!fits_bits(0xFF, 8));
+  assert(!fits_bits(~0xFF, 8));
+
+  assert(fits_bits(0b0010, 3));
+  assert(!fits_bits(0b1010, 3));
+  assert(!fits_bits(0b0110, 3));
+
+  assert(fits_bits(~0b11, 3));
+  assert(!fits_bits(~0b01000011, 3));
+  assert(!fits_bits(~0b111, 3));
   return 0;
 }