reorganize module

_archive/rhai_engine/rhaibook/safety/max-operations.md

@@ -0,0 +1,51 @@
+Maximum Number of Operations
+============================
+
+{{#include ../links.md}}
+
+
+In Rhai, it is trivial to construct _infinite loops_, or scripts that run for a very long time.
+
+```rust
+loop { ... }                        // infinite loop
+
+while 1 < 2 { ... }                 // loop with always-true condition
+```
+
+Rhai by default does not limit how much time or CPU a script consumes.
+
+This can be changed via the [`Engine::set_max_operations`][options] method, with zero being
+unlimited (the default).
+
+The _operations count_ is intended to be a very course-grained measurement of the amount of CPU that
+a script has consumed, allowing the system to impose a hard upper limit on computing resources.
+
+A script exceeding the maximum operations count terminates with an error result. This can be
+disabled via the [`unchecked`] feature for higher performance (but higher risks as well).
+
+```rust
+let mut engine = Engine::new();
+
+engine.set_max_operations(500);     // allow only up to 500 operations for this script
+
+engine.set_max_operations(0);       // allow unlimited operations
+```
+
+
+```admonish question "TL;DR &ndash; What does one _operation_ mean?"
+
+The concept of one single _operation_ in Rhai is volatile &ndash; it roughly equals one expression
+node, loading one [variable]/[constant], one [operator] call, one iteration of a loop, or one
+[function] call etc. with sub-expressions, statements and [function] calls executed inside these
+contexts accumulated on top.
+
+A good rule-of-thumb is that one simple non-trivial expression consumes on average 5-10 operations.
+
+One _operation_ can take an unspecified amount of time and real CPU cycles, depending on the particulars.
+For example, loading a [constant] consumes very few CPU cycles, while calling an external Rust function,
+though also counted as only one operation, may consume much more computing resources.
+
+To help visualize, think of an _operation_ as roughly equals to one _instruction_ of a hypothetical CPU
+which includes _specialized_ instructions, such as _function call_, _load module_ etc., each taking up
+one CPU cycle to execute.
+```