simplify相关代码分析

 早期对SAT求解器代码解读最重要的文献是：

[MZ01] M.W. Moskewicz, C.F. Madigan, Y. Zhao, L. Zhang, S. Malik. “Chaff:
Engineering an Efficient SAT Solver” in Proc. of the 38th Design
Automation Conference, 2001.

对各个功能模块进行的介绍，并重点介绍了它们在CDCL框架中的位置与协作关系。

其中首先讲到了simplify。化简操作在solve函数调用之前，simplifyDB()运行不能有unit assumptions。不过可以改进，增加第三个输出：“在假设下不可满足”：

	The simplifyDB() method can be used before calling solve() to simplify the set of problem constraints (often called the constraint database).  译文：simplifyDB()方法可以在调用solve()之前使用，以简化问题约束集(通常称为约束数据库)。
	In my implementation, simplifyDB() will first propagate all unit information, then remove all satisfied  constraints. 译文：在我的实现中，simplifyDB()将首先传播所有单元信息，然后删除所有满足的约束。
	As for addClause(), the simplifier can sometimes detect a conflict, in which case False is returned and the solver state is, again, undefined and must not be used further. 译文：对于addClause()，简化器有时可以检测到冲突，在这种情况下返回False，并且解决程序状态也是未定义的，不能再使用。
	If the solver returns satisfiable, new constraints can be added repeatedly to the existing database and solve() run again. 译文：如果求解器返回satisfiable，则可以向现有数据库重复添加新的约束，并再次运行solve()。
	However, more interesting sequences of SAT-problems can be solved by the use of unit assumptions.译文：然而，更有趣的sat问题序列可以通过使用单位假设来解决
	When passing a non-empty list of assumptions to solve(), the solver temporarily assumes the literals to be true. 译文：当传递一个非空的假设列表给solve()时，求解器会暂时假设字面量为真。 By the use of unit assumptions，calling simplifyDB() before solve() is no longer optional.译文：通过使用单元假设，在solve()之前调用simplifyDB()不再是可选的。
	It is the mechanism for detecting conflicts independent of the assumptions – referred to as a top-level conflict from now on – which puts the solver in an undefined state. For an example of the use if unit assumptions, see [ES03]. 译文：它是检测独立于假设的冲突的机制——从现在开始称为顶级冲突——它将解决程序置于一个未定义的状态。如果使用单元假设，请参见[ES03]。
	An alternative interface would be for solve() to return one of three values: satisfiable, unsatisfiable, or unsatisfiable under assumptions. This is indeed a less error-prone interface as there is no longer a pre-condition on the use of solve().  译文：另一种接口是让solve()返回三个值中的一个:可满足的、不可满足的或在假设下不可满足的。这确实是一个不太容易出错的接口，因为在使用solve()时不再有前置条件。 The current interface, however, represents the smallest modification of a nonincremental SAT-solver.

随后讲到了增量求解器中化简函数的作用

	Problem constraints can also be removed if they are satisfied at the toplevel. The API method simplifyDB() is responsible for this. The procedure is particularly important for incremental SAT-problems. 译文：如果在顶层满足问题约束，也可以删除它们。API方法simplifyDB()负责这一点。该程序对于增量sat问题尤其重要。
	Top-level solver.
	The pseudo-code for the search procedure presented above suffices for a simple conflict-driven SAT-solver, but a solver strategy can improve the performance.
	A typical strategy applied by modern conflict-driven SATsolvers is the use of restarts to prevent from getting stuck in a futile part of the search tree.
	In MINISAT we also vary the number of learnt clauses kept at a given time.
	Furthermore, the solve() method of the API supports incremental assumptions, not handled by the above pseudo-code. 译文：此外，API的solve()方法支持增量假设，而不是由上面的伪代码处理

 

 

 

未完待续