NuSMV开发：修改case语句中TRUE关键字的功能

Short Answer #

Absolutely—this is totally feasible! NuSMV is open-source, so you can tweak its core expression-handling logic to change how the TRUE branch behaves to inherit the prior state of your variable.

Background on Default TRUE Branch Behavior #

By default, the TRUE branch in case...esac acts as a catch-all: it triggers when none of the preceding conditional branches match. Under the hood, NuSMV parses case statements into nested ITE (if-then-else) expressions. Your example:

next(a) := case 
  a>0 : -10; 
  a<0 : 10; 
  TRUE : (do the last time actions); 
esac;

gets converted to something like:

ITE(a > 0, -10, ITE(a < 0, 10, <TRUE_BRANCH_EXPR>))

Right now, <TRUE_BRANCH_EXPR> uses whatever you explicitly write, or a default value if omitted. Your goal is to make this automatically reference the previous value of a (i.e., prev(a)) when the TRUE branch runs.

Key C Files to Modify #

To implement this behavior, you’ll need to adjust NuSMV’s expression parsing and conversion logic. Here are the critical files to focus on:

• src/compile/expr/Expr.c: This is the core file for handling expression semantics. It’s where case statements are converted into nested ITE expressions. You’ll add logic here to detect when the final branch of a case uses TRUE as its condition, then replace its expression with prev(<target_variable>).
• src/compile/parser/parser.y: This YACC parser defines NuSMV’s syntax rules. You may need to tweak how the TRUE branch is recognized during parsing to flag it for special handling later.
• src/compile/flat/Flat.c: This file handles "flattening" complex expressions into simpler forms. You’ll want to ensure your modified TRUE branch logic is preserved during this step.

High-Level Modification Steps #

1. Detect the TRUE catch-all branch: When parsing a case statement into an abstract syntax tree (AST), check if the final branch’s condition is the TRUE constant.
2. Replace the branch expression: For that final TRUE branch, instead of using the user-provided expression (or default), construct a prev() expression targeting the variable on the left side of the next() assignment (in your example, that’s a).
3. Validate semantics: Ensure the prev() expression is correctly handled in subsequent compilation steps (like model flattening and BDD generation) so it properly references the variable’s prior state.

Quick Notes #

• Test your changes incrementally! Start with a small model like your a variable example to verify the TRUE branch correctly inherits the previous value when a=0.
• NuSMV’s codebase includes helpful comments, so you’ll find context about how expressions are processed in the files listed above.

内容的提问来源于stack exchange，提问作者Alvin