QTemporaryFileName: use the random data more wisely

First, instead of getting 32 bits of random for every character, we can
get reasonable randomness with just 10 bits, so we can replace 3
characters per get32() call. A typical template with six X will only
need 64 bits of random data.

Second, using the PID is not random at all. The reason why mktemp used
to use it is irrelevant (probably because applications failed to seed
rand()). Either way, we don't need nor want the PID anymore. And because
the code is completely rewritten, the UCB copyright no longer applies.

Change-Id: I84e45059a888497fb55ffffd14d2d4cfe159abca
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qtemporaryfile.cpp

@@ -133,36 +133,6 @@ QTemporaryFileName::QTemporaryFileName(const QString &templateName)
     length = phLength;
 }
 
-// The following code is a heavily modified version, originally copyright:
-/*
- * Copyright (c) 1987, 1993
- *      The Regents of the University of California.  All rights reserved.
- *
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions
- * are met:
- * 1. Redistributions of source code must retain the above copyright
- *    notice, this list of conditions and the following disclaimer.
- * 2. Redistributions in binary form must reproduce the above copyright
- *    notice, this list of conditions and the following disclaimer in the
- *    documentation and/or other materials provided with the distribution.
- * 3. Neither the name of the University nor the names of its contributors
- *    may be used to endorse or promote products derived from this software
- *    without specific prior written permission.
- *
- * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
- * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
- * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
- * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
- * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
- * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
- * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
- * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
- * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
- * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
- * SUCH DAMAGE.
- */
-
 /*!
     \internal
 
@@ -178,26 +148,42 @@ QFileSystemEntry::NativePath QTemporaryFileName::generateNext()
     Char *const placeholderStart = (Char *)path.data() + pos;
     Char *const placeholderEnd = placeholderStart + length;
 
-    // Initialize placeholder with random chars + PID.
+    // Replace placeholder with random chars.
     {
+        // Since our dictionary is 26+26 characters, it would seem we only need
+        // a random number from 0 to 63 to select a character. However, due to
+        // the limited range, that would mean 12 (64-52) characters have double
+        // the probability of the others: 1 in 32 instead of 1 in 64.
+        //
+        // To overcome this limitation, we use more bits per character. With 10
+        // bits, there are 16 characters with probability 19/1024 and the rest
+        // at 20/1024 (i.e, less than .1% difference). This allows us to do 3
+        // characters per 32-bit random number, which is also half the typical
+        // placeholder length.
+        enum { BitsPerCharacter = 10 };
+
         Char *rIter = placeholderEnd;
-
-#if defined(QT_BUILD_CORE_LIB)
-        // don't consume more than half of the template with the PID
-        Char *pidStart = placeholderEnd - (placeholderEnd - placeholderStart) / 2;
-        quint64 pid = quint64(QCoreApplication::applicationPid());
-        do {
-            *--rIter = Latin1Char((pid % 10) + '0');
-            pid /= 10;
-        } while (rIter != pidStart && pid != 0);
-#endif
-
         while (rIter != placeholderStart) {
-            char ch = char(QRandomGenerator::bounded(26 + 26));
+            quint32 rnd = QRandomGenerator::get32();
-            if (ch < 26)
+            auto applyOne = [&]() {
-                *--rIter = Latin1Char(ch + 'A');
+                quint32 v = rnd & ((1 << BitsPerCharacter) - 1);
-            else
+                rnd >>= BitsPerCharacter;
-                *--rIter = Latin1Char(ch - 26 + 'a');
+                char ch = char((26 + 26) * v / (1 << BitsPerCharacter));
+                if (ch < 26)
+                    *--rIter = Latin1Char(ch + 'A');
+                else
+                    *--rIter = Latin1Char(ch - 26 + 'a');
+            };
+
+            applyOne();
+            if (rIter == placeholderStart)
+                break;
+
+            applyOne();
+            if (rIter == placeholderStart)
+                break;
+
+            applyOne();
         }
     }