Validate a resource lifecycle

Challenge

Validate that a resource's lifecycle is well-formed: you may only use or free something that is currently allocated, you may not double-free, and you may not allocate over a resource that is already live.

Task

Implement int valid_lifecycle(const char *ops) over a trace using 'a' (allocate), 'f' (free), 'u' (use). Return 1 if every operation respects its precondition, else 0. No main — the grader calls it.

Rules for each operation

• 'a' is valid only when the resource is not currently allocated-and-live (after a free, re-allocation is allowed).
• 'f' is valid only when the resource is currently live (no free-before-alloc, no double-free).
• 'u' is valid only when the resource is currently live (no use-before-alloc, no use-after-free).

Input

ops — a NUL-terminated string of 'a', 'f', 'u' characters (may be empty).

Output

Returns 1 if the whole sequence is valid, else 0.

Example

valid_lifecycle("auf")    ->   1
valid_lifecycle("afaf")   ->   1   (allocate again after freeing)
valid_lifecycle("ufa")    ->   0   (use before allocate)
valid_lifecycle("aff")    ->   0   (double free)
valid_lifecycle("afu")    ->   0   (use after free)

Edge cases

• Empty string: valid (returns 1).

Why this matters

Drill: practice one contract before combining it with other skills.

Input format

ops — a NUL-terminated string of 'a', 'f', 'u' characters (may be empty).

Output format

Returns 1 if the whole sequence is valid, else 0.

Constraints

ops is a non-NULL terminated trace. Only a, f, and u are valid. Allocation while live, use/free while not live, or another character returns 0. End-of-trace cleanup is not required by this lifecycle validator.

Starter code

int valid_lifecycle(const char *ops) {
    /* TODO */
    return 0;
}

Common mistakes

Confusing lifecycle validity with leak freedom, or allowing allocation over a live resource.

Edge cases to handle

Empty trace; double-free; use before allocation; reallocation after free; unknown character; a live resource at the end is allowed.

Up next

• Is an index in bounds?