Add a first version of release notes.

Author: niklasso

doc/ReleaseNotes-2.2.0.txt

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+Release Notes for MiniSat 2.2.0
+===============================
+
+Changes since version 2.0:
+
+ * Started using a more standard release numbering.
+
+ * Includes some now well-known heuristics: phase-saving and luby
+   restarts. The old heuristics are still present and can be activated
+   if needed.
+
+ * Detection/Handling of out-of-memory and vector capacity
+   overflow. This is fairly new and relatively untested.
+
+ * Simple resource controls: CPU-time, memory, number of
+   conflicts/decisions.
+
+ * CPU-time limiting is implemented by a more general, but simple,
+   asynchronous interruption feature. This means that the solving
+   procedure can be interrupted from another thread or in a signal
+   handler.
+
+ * Improved portability with respect to building on Solaris and with
+   Visual Studio. This is not regularly tested and chances are that
+   this have been broken since, but should be fairly easy to fix if
+   so.
+
+ * Changed C++ file-extention to the less problematic ".cc".
+
+ * Source code is now namespace-protected
+
+ * Introducing a new Clause Memory Allocator that brings reduced
+   memory consumption on 64-bit architechtures and improved
+   performance (to some extent). The allocator uses a region-based
+   approach were all references to clauses are represented as a 32-bit
+   index into a global memory region that contains all clauses. To
+   free up and compact memory it uses a simple copying garbage
+   collector.
+
+ * Improved unit-propagation by Blocking Literals. For each entry in
+   the watcher lists, pair the pointer to a clause with some
+   (arbitrary) literal from the clause. The idea is that if the
+   literal is currently true (i.e. the clause is satisfied) the
+   watchers of the clause does not need to be altered. This can thus
+   be detected without touching the clause's memory at all. As often
+   as can be done cheaply, the blocking literal for entries to the
+   watcher list of a literal 'p' is set to the other literal watched
+   in the corresponding clause.
+
+ * Basic command-line/option handling system. Makes it easy to specify
+   options in the class that they affect, and whenever that class is
+   used in an executable, parsing of options and help messages are
+   brought in automatically.
+
+ * General clean-up and various minor bug-fixes.
+
+ * Changed implementation of variable-elimination/model-extension:
+    
+     - The interface is changed so that arbitrary remembering is no longer
+       possible. If you need to mention some variable again in the future,
+       this variable has to be frozen.
+    
+     - When eliminating a variable, only clauses that contain the variable
+       with one sign is necessary to store. Thereby making the other sign
+       a "default" value when extending models.
+    
+     - The memory consumption for eliminated clauses is further improved
+       by storing all eliminated clauses in a single contiguous vector.
+
+  * Some common utility code (I/O, Parsing, CPU-time, etc) is ripped
+    out and placed in a separate "utils" directory.
+
+  * The DIMACS parse is refactored so that it can be reused in other
+    applications (not very elegant, but at least possible).
+
+  * Some simple improvements to scalability of preprocessing, using
+    more lazy clause removal from data-structures and a couple of
+    ad-hoc limits (the longest clause that can be produced in variable
+    elimination, and the longest clause used in backward subsumption).