combine switch() and rev() into a single function

This lets me do the mapping in-place and avoids the mallocs and extra
copies.

1 files changed, 7 insertions, 22 deletions

diff --git a/challenge-069/walt-mankowski/c/ch-2.c b/challenge-069/walt-mankowski/c/ch-2.c
index 36f1463e32..21347722ce 100644
--- a/challenge-069/walt-mankowski/c/ch-2.c
+++ b/challenge-069/walt-mankowski/c/ch-2.c
@@ -6,41 +6,26 @@ const size_t min(const size_t a, const size_t b) {
     return a <= b ? a : b;
 }
 
-char *rev(const char *s) {
+void switch_rev(char *s) {
     const size_t len = strlen(s);
-    char *t = malloc(len+1);
-    strcpy(t, s);
     for (size_t i = 0; i < min(len/2+1, len); i++) {
         const size_t j = len-1-i;
-        char tmp = t[i];
-        t[i] = t[j];
-        t[j] = tmp;
+        const char tmp = s[i];
+        s[i] = s[j] == '0' ? '1' : '0';
+        s[j] = tmp  == '0' ? '1' : '0';
     }
-
-    return t;
-}
-
-char *swtch(char *s) {
-    for (size_t i = 0; i < strlen(s); i++)
-        if (s[i] == '0')
-            s[i] = '1';
-        else
-            s[i] = '0';
-
-    return s;
 }
 
-char *S(const int n) {
+const char *S(const int n) {
     const size_t len = (1 << n) - 1;
     char *s = malloc(len);
     s[0] = '\0';
 
     for (int i = 1; i <= n; i++) {
-        char *rs = swtch(rev(s));
         const size_t eos = (1 << (i-1)) - 1;
+        strcpy(s+eos+1, s);
         s[eos] = '0';
-        strcat(s+eos+1, rs);
-        free(rs);
+        switch_rev(s+eos+1);
     }
     return s;
 }