hostlist_find: refine allocation-free case

problem: we still use strlen and parse integers significantly more than
we need to

solution: use the existing lengths in more places, and instead of
re-parsing the suffix find the power of 10 matching the length of the
string, and mod by that to chop off the extra digit in the recursive
case

src/common/libhostlist/hostname.c

@@ -16,6 +16,7 @@
 #include <string.h>
 #include <ctype.h>
 #include <errno.h>
+#include <math.h>
 #include <stdio.h>
 #include <string.h>
 
@@ -46,14 +47,13 @@ static int host_prefix_end (const char *hostname, int len)
 static int hostname_len (const char *hostname)
 {
     int len = strlen (hostname);
-    for (int i = 0; i < len; i++)
-        if (memchr (INVALID_CHARS, sizeof(INVALID_CHARS) - 1, hostname[i]))
-            return -1;
+    if (len != strcspn (hostname, INVALID_CHARS))
+        return -1;
     return len;
 }
 
 struct stack_hostname * hostname_stack_create_with_suffix (struct stack_hostname *hn,
-        const char *hostname, unsigned long len,  int idx) {
+        const char *hostname, int len,  int idx) {
     if (!hostname || !hn || len < 0) {
         errno = EINVAL;
         return NULL;
@@ -74,30 +74,36 @@ struct stack_hostname * hostname_stack_create_with_suffix (struct stack_hostname
         errno = EINVAL;
         return NULL;
     }
-    hn->width = strlen(hn->suffix);
+    hn->width = hn->len - hn->len_prefix;
     return hn;
 }
 
-struct stack_hostname * hostname_stack_copy_with_suffix (struct stack_hostname *dst,
-        struct stack_hostname *src, int idx) {
+struct stack_hostname * hostname_stack_copy_one_less_digit (struct stack_hostname *dst,
+        struct stack_hostname *src) {
     if (!dst || !src) {
         errno = EINVAL;
         return NULL;
     }
     *dst = *src;
-    dst->len_prefix = idx + 1;
-    if (idx == dst->len - 1) {
+    dst->len_prefix = src->len_prefix + 1;
+    if (src->len_prefix == dst->len - 1) {
         return dst;
     }
     dst->suffix = dst->hostname + dst->len_prefix;
 
-    char *p = NULL;
-    dst->num = strtoul (dst->suffix, &p, 10);
-    if (p == dst->suffix && dst->num == 0) {
+    dst->width = dst->len - dst->len_prefix;
+    if (dst->width < 0 || dst->width > 10){
         errno = EINVAL;
         return NULL;
     }
-    dst->width = strlen(dst->suffix);
+
+    // remove the most significant decimal digit without reparsing
+    // lookup table because pow is slow
+    static const int pow10[10] = {
+        1, 10, 100, 1000, 10000,
+        100000, 1000000, 10000000, 100000000, 1000000000
+    };
+    dst->num = src->num % pow10[dst->width];
     return dst;
 }
 

src/common/libhostlist/hostname.h

@@ -41,9 +41,9 @@ struct hostname * hostname_create (const char *s);
 struct hostname * hostname_create_with_suffix (const char *s, int i);
 struct stack_hostname * hostname_stack_create (struct stack_hostname *hn, const char *hostname);
 struct stack_hostname * hostname_stack_create_with_suffix (struct stack_hostname *hn,
-        const char *hostname, unsigned long len, int idx);
-struct stack_hostname * hostname_stack_copy_with_suffix (struct stack_hostname *dst,
-        struct stack_hostname *src, int idx);
+        const char *hostname, int len, int idx);
+struct stack_hostname * hostname_stack_copy_one_less_digit (struct stack_hostname *dst,
+        struct stack_hostname *src);
 void hostname_destroy (struct hostname *hn);
 
 int hostname_suffix_is_valid (struct hostname *hn);

src/common/libhostlist/hostrange.c

@@ -404,8 +404,7 @@ int hostrange_hn_within (struct hostrange * hr, struct stack_hostname * hn)
          *  Create new hostname object with its prefix offset by one
          */
         struct stack_hostname shn;
-        struct stack_hostname * h = hostname_stack_copy_with_suffix (&shn,
-                hn, hn->len_prefix);
+        struct stack_hostname * h = hostname_stack_copy_one_less_digit (&shn, hn);
         /*
          *  Recursive call :-o
          */