Avoid the creation of too many strings in 'package'

Both when setting a path and searching for a file ('searchpath'),
this commit reduces the number of intermediate strings created
in Lua.
(For setting a path the change is not relevant, because this is
done only twice when loading the module. Anyway, it is a nice example
of how to use auxlib buffers to manipulate strings in the C API.)

loadlib.c

@@ -290,22 +290,33 @@ static int noenv (lua_State *L) {
 static void setpath (lua_State *L, const char *fieldname,
                                    const char *envname,
                                    const char *dft) {
+  const char *dftmark;
   const char *nver = lua_pushfstring(L, "%s%s", envname, LUA_VERSUFFIX);
-  const char *path = getenv(nver);  /* use versioned name */
-  if (path == NULL)  /* no environment variable? */
+  const char *path = getenv(nver);  /* try versioned name */
+  if (path == NULL)  /* no versioned environment variable? */
     path = getenv(envname);  /* try unversioned name */
   if (path == NULL || noenv(L))  /* no environment variable? */
     lua_pushstring(L, dft);  /* use default */
-  else {
-    /* replace ";;" by ";AUXMARK;" and then AUXMARK by default path */
-    path = luaL_gsub(L, path, LUA_PATH_SEP LUA_PATH_SEP,
-                              LUA_PATH_SEP AUXMARK LUA_PATH_SEP);
-    luaL_gsub(L, path, AUXMARK, dft);
-    lua_remove(L, -2); /* remove result from 1st 'gsub' */
+  else if ((dftmark = strstr(path, LUA_PATH_SEP LUA_PATH_SEP)) == NULL)
+    lua_pushstring(L, path);  /* nothing to change */
+  else {  /* path contains a ";;": insert default path in its place */
+    size_t len = strlen(path);
+    luaL_Buffer b;
+    luaL_buffinit(L, &b);
+    if (path < dftmark) {  /* is there a prefix before ';;'? */
+      luaL_addlstring(&b, path, dftmark - path);  /* add it */
+      luaL_addchar(&b, *LUA_PATH_SEP);
+    }
+    luaL_addstring(&b, dft);  /* add default */
+    if (dftmark < path + len - 2) {  /* is there a sufix after ';;'? */
+      luaL_addchar(&b, *LUA_PATH_SEP);
+      luaL_addlstring(&b, dftmark + 2, (path + len - 2) - dftmark);
+    }
+    luaL_pushresult(&b);
   }
   setprogdir(L);
   lua_setfield(L, -3, fieldname);  /* package[fieldname] = path value */
-  lua_pop(L, 1);  /* pop versioned variable name */
+  lua_pop(L, 1);  /* pop versioned variable name ('nver') */
 }
 
 /* }================================================================== */
@@ -421,17 +432,26 @@ static int readable (const char *filename) {
 }
 
 
-static const char *pushnextfilename (lua_State *L, const char *path) {
-  const char *l;
-  if (*path == *LUA_PATH_SEP)
-    path++;  /* skip separator */
-  if (*path == '\0')
+/*
+** Get the next name in '*path' = 'name1;name2;name3;...', changing
+** the ending ';' to '\0' to create a zero-terminated string. Return
+** NULL when list ends.
+*/
+static const char *getnextfilename (char **path, char *end) {
+  char *sep;
+  char *name = *path;
+  if (name == end)
     return NULL;  /* no more names */
-  l = strchr(path, *LUA_PATH_SEP);  /* find next separator */
-  if (l == NULL)  /* no more separators? */
-    l = path + strlen(path);  /* go until the end */
-  lua_pushlstring(L, path, l - path);  /* file name */
-  return l;  /* rest of the path */
+  else if (*name == '\0') {  /* from previous iteration? */
+    *name = *LUA_PATH_SEP;  /* restore separator */
+    name++;  /* skip it */
+  }
+  sep = strchr(name, *LUA_PATH_SEP);  /* find next separator */
+  if (sep == NULL)  /* separator not found? */
+    sep = end;  /* name goes until the end */
+  *sep = '\0';  /* finish file name */
+  *path = sep;  /* will start next search from here */
+  return name;
 }
 
 
@@ -442,12 +462,12 @@ static const char *pushnextfilename (lua_State *L, const char *path) {
 **	no file 'blublu.so'
 */
 static void pusherrornotfound (lua_State *L, const char *path) {
-  if (*path == *LUA_PATH_SEP)
-    path++;  /* skip separator */
-  lua_pushstring(L, "\n\tno file '");
-  luaL_gsub(L, path, LUA_PATH_SEP, "'\n\tno file '");
-  lua_pushstring(L, "'");
-  lua_concat(L, 3);
+  luaL_Buffer b;
+  luaL_buffinit(L, &b);
+  luaL_addstring(&b, "\n\tno file '");
+  luaL_addgsub(&b, path, LUA_PATH_SEP, "'\n\tno file '");
+  luaL_addstring(&b, "'");
+  luaL_pushresult(&b);
 }
 
 
@@ -455,17 +475,24 @@ static const char *searchpath (lua_State *L, const char *name,
                                              const char *path,
                                              const char *sep,
                                              const char *dirsep) {
+  luaL_Buffer buff;
+  char *pathname;  /* path with name inserted */
+  char *endpathname;  /* its end */
+  const char *filename;
   /* separator is non-empty and appears in 'name'? */
   if (*sep != '\0' && strchr(name, *sep) != NULL)
     name = luaL_gsub(L, name, sep, dirsep);  /* replace it by 'dirsep' */
-  /* replace marks ('?') in 'path' by the file name */
-  path = luaL_gsub(L, path, LUA_PATH_MARK, name);
-  while ((path = pushnextfilename(L, path)) != NULL) {
-    const char *filename = lua_tostring(L, -1);
+  luaL_buffinit(L, &buff);
+  /* add path to the buffer, replacing marks ('?') with the file name */
+  luaL_addgsub(&buff, path, LUA_PATH_MARK, name);
+  luaL_addchar(&buff, '\0');
+  pathname = luaL_buffaddr(&buff);  /* writable list of file names */
+  endpathname = pathname + luaL_bufflen(&buff) - 1;
+  while ((filename = getnextfilename(&pathname, endpathname)) != NULL) {
     if (readable(filename))  /* does file exist and is readable? */
-      return filename;  /* return that file name */
-    lua_pop(L, 1);  /* else remove file name */
+      return lua_pushstring(L, filename);  /* save and return name */
   }
+  luaL_pushresult(&buff);  /* push path to create error message */
   pusherrornotfound(L, lua_tostring(L, -1));  /* create error message */
   return NULL;  /* not found */
 }