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CanBuild Is Not a Boolean

· 2 min read
The reTS project
reverse-engineering → modernization

“Can this house build this unit?” looks like a yes-or-no question. In Yuri's Revenge, the answer is a three-state decision produced by an order-sensitive control-flow graph.

Reconstructing that graph took a sequence of small, testable pieces. The final composition is one of the clearest examples yet of why faithful behavior lives in the route to an answer, not only in the answer itself.

Last verified against the project oracle: 2026-07-15.

Three answers, several routes​

The engine distinguishes:

  • buildable;
  • unbuildable; and
  • temporarily unbuildable, such as a currently reached positive build limit.

The full decision considers an explicit unbuildable flag, technology levels, ownership and side restrictions, stolen-technology requirements, the skirmish superweapon option, prerequisites, and per-type build limits. It also contains shortcuts for special ownership cases.

Those checks are not a flat list. One ownership shortcut skips later technology and prerequisite gates but still routes through the build-limit decision. By contrast, once the decision reaches the prerequisite stage, the non-human-controlled path returns buildable before the scan and final limit check; earlier hard gates still apply. Moving either branch to a more “logical” position changes game behavior.

Testing the path, not just the result​

Many rejection branches return the same numeric value. A test that checks only “unbuildable” would still pass if two failures were accidentally reordered.

The reTS breakdown therefore records the stages that actually ran and the exact terminal reason. The oracle cases deliberately combine multiple possible failures to prove which one wins. They also pin shortcut-to-limit behavior and the early non-human return.

One version complete, differences kept honest​

This composition is the complete Yuri's Revenge path. Red Alert 2 preserves the broad shape but omits or changes several pieces. Tiberian Sun has a more substantially different decision, including its own prerequisite vocabulary and build-limit behavior.

Those versions will get complete composers of their own. reTS does not model an older game by running the later path with a handful of fields switched off; that would make the interface look finished before the behavior really is.