c: ex3: add solution

c/ex3.c

@@ -0,0 +1,32 @@
+#include <limits.h>
+#include <stddef.h>
+
+static unsigned char reverse_byte(unsigned char c) {
+    unsigned char res = 0;
+    for (size_t i = 0; i < CHAR_BIT; ++i) {
+        unsigned char bit = (c & (1 << i));
+        res |= bit << (CHAR_BIT - i - 1);
+    }
+    return res;
+}
+
+static void swap_bytes(unsigned char* lhs, unsigned char* rhs) {
+    unsigned char tmp = *lhs;
+    *lhs = *rhs;
+    *rhs = tmp;
+}
+
+void reverse_bytes(unsigned char* buf, size_t n) {
+    if (!buf || !n)
+        return;
+
+    // No need to worry about applying reverse_byte twice in the middle by
+    // mistake if done in a preliminary pass
+    for (size_t i = 0; i < n; ++i) {
+        buf[i] = reverse_byte(buf[i]);
+    }
+
+    for (size_t i = 0; i < ((n + 1) / 2); ++i) {
+        swap_bytes(&buf[i], &buf[n - 1 - i]);
+    }
+}