From 8fcf25523124008a56515dc0b3de0a32f3283e18 Mon Sep 17 00:00:00 2001
From: Adam Carpenter <gitlab@53hor.net>
Date: Tue, 23 Jul 2019 10:18:39 -0400
Subject: Super simplified 2-6 2-7, added 2-8

---
 ch2/2-6.c | 15 +++++++++------
 ch2/2-7.c | 19 +++++++++++++++++++
 ch2/2-8.c | 21 +++++++++++++++++++++
 3 files changed, 49 insertions(+), 6 deletions(-)
 create mode 100644 ch2/2-7.c
 create mode 100644 ch2/2-8.c

(limited to 'ch2')

diff --git a/ch2/2-6.c b/ch2/2-6.c
index 0932206..918f583 100644
--- a/ch2/2-6.c
+++ b/ch2/2-6.c
@@ -5,15 +5,18 @@
  */
 
 unsigned setbits(unsigned x, unsigned p, unsigned n, unsigned y) {
-    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.
+    // Create a basa from x that zeroes out all of the bits to be set.
+    unsigned a = x & ~(~(~0 << n) << (p + 1 - n));
+
+    // Create a mask from y that zeroes out all of the bits except those to set.
+    unsigned b = (y & ~(~0 << n)) << (p + 1 - n);
+
+    // Apply the mask to the base.
+    return a | b;
 }
 
 int main() {
     printf("%x\n", setbits(0xaa, 3, 3, 0x33));
     printf("%x\n", setbits(0x00, 3, 3, 0xff));
+    return 0;
 }
diff --git a/ch2/2-7.c b/ch2/2-7.c
new file mode 100644
index 0000000..bf9fed1
--- /dev/null
+++ b/ch2/2-7.c
@@ -0,0 +1,19 @@
+#include<stdio.h>
+
+/*
+ * Returns x with the n bits that begin at position p inverted (i.e., 1 changed
+ * into 0 and vice versa), leaving the others unchanged.
+ */
+
+unsigned invert(unsigned x, unsigned p, unsigned n) {
+    // Create mask where bits should be flipped.
+    unsigned mask = ~(~0 << n) << (p + 1 - n);
+
+    // Flip bits with mask.
+    return x ^ mask;
+}
+
+int main() {
+    printf("%x\n", invert(0xaa, 3, 3));
+    printf("%x\n", invert(0xff, 3, 3));
+}
diff --git a/ch2/2-8.c b/ch2/2-8.c
new file mode 100644
index 0000000..9f7adbd
--- /dev/null
+++ b/ch2/2-8.c
@@ -0,0 +1,21 @@
+#include<stdio.h>
+
+/*
+ * Returns the value of the integer x rotated to the right by n bit positions.
+ */
+
+unsigned rightrot(unsigned x, int n) {
+    unsigned mask = ~(~0 >> 1);
+
+    while (n-- > 0)
+        if (x & 1)
+            x = (x >> 1) | mask;
+        else
+            x >>= 1;
+
+    return x;
+}
+
+int main() {
+    printf("%x\n", rightrot(0xaa, 3));
+}
-- 
cgit v1.2.3