Finished 2-6.

1 files changed, 11 insertions, 7 deletions

diff --git a/ch2/2-6.c b/ch2/2-6.c
index ee6770d..0932206 100644
--- a/ch2/2-6.c
+++ b/ch2/2-6.c
@@ -1,15 +1,19 @@
 #include<stdio.h>
 
+/*
+ * Set n bits in x starting at position p to rightmost n bits of y.
+ */
+
 unsigned setbits(unsigned x, unsigned p, unsigned n, unsigned y) {
-    // set n bits in x starting at p to rightmost n bits of y
-    unsigned offset = (p + 1 - n);
-    unsigned y_mask = y & ~(~0 << n);
-    unsigned bot = x & ~(~0 << offset);
-    unsigned top = (x >> offset << offset) & y_mask;
-    unsigned result = top & bot;
-    return result;
+    p++;                                            // We're actually interested in p+1.
+    unsigned offset = (p - n);                      // Calculate offset of y bits being inserted into x.
+    unsigned lower_x = x & ~(~0 << offset);         // Grab LSBs of x to save.
+    unsigned y_mask = (y & ~(~0 << n)) << offset;   // Grab pattern from LSBs of y.
+    x = x >> p << p;                                // Just keep MSBs of x.
+    return x | y_mask | lower_x;                    // Concatenate MSBs of x with LSB pattern from y with LSBs of x.
 }
 
 int main() {
     printf("%x\n", setbits(0xaa, 3, 3, 0x33));
+    printf("%x\n", setbits(0x00, 3, 3, 0xff));
 }