win32: code shrink quote_arg()

Alter quote_arg() to perform a single pass over the string in the
case where no change is required.  Based on a proposal by @avih in
GitHub PR #317.

Saves 16 bytes.

1 files changed, 20 insertions, 24 deletions

diff --git a/win32/process.c b/win32/process.c
index a70b9484e..e7c9ca187 100644
--- a/win32/process.c
+++ b/win32/process.c
@@ -109,38 +109,34 @@ parse_interpreter(const char *cmd, interp_t *interp)
 char * FAST_FUNC
 quote_arg(const char *arg)
 {
-        char *r = xmalloc(2 * strlen(arg) + 3);  // max-esc, enclosing DQ, \0
-        char *d = r;
-        int nbs = 0;  // n consecutive BS right before current char
+        char *d, *r = xmalloc(2 * strlen(arg) + 3);     // max-esc, quotes, \0
+        size_t nbs = 0; // consecutive backslashes before current char
+        int quoted = !*arg;
 
-        /* empty arguments and those containing tab/space must be quoted */
-        if (!*arg || strpbrk(arg, " \t")) {
-                *d++ = '"';
-        }
+        for (d = r; *arg; *d++ = *arg++) {
+                if (*arg == ' ' || *arg == '\t')
+                        quoted = 1;
+
+                if (*arg == '\\' || *arg == '"')
+                        *d++ = '\\';
+                else
+                        d -= nbs; // undo nbs escapes, if any (not followed by DQ)
 
-        while (*arg) {
-                switch (*arg) {
-                case '\\':
+                if (*arg == '\\')
                         ++nbs;
-                        break;
-                case '"':  // double consecutive-BS, plus one to escape the DQ
-                        for (++nbs; nbs; --nbs)
-                                *d++ = '\\';
-                        break;
-                default:  // reset count if followed by not-DQ
+                else
                         nbs = 0;
-                }
-                *d++ = *arg++;
         }
 
-        if (*r == '"') {
-                while (nbs--)  // double consecutive-BS before the closing DQ
-                        *d++ = '\\';
-                *d++ = '"';
+        if (quoted) {
+                memmove(r + 1, r, d++ - r);
+                *r = *d++ = '"';
+        } else {
+                d -= nbs;
         }
-        *d++ = '\0';
 
-        return xrealloc(r, d - r);
+        *d = 0;
+        return r;
 }
 
 char * FAST_FUNC