Validate hiring request under role constraints

Problem

You are building a simple HR assignment validator for a platform with companies, employees, and job roles.

When the system receives a request to assign an employee to a role in a company, it must enforce the following rules:

1. Role capacity per company: In a given company, each role can be held by at most 5 employees at the same time.
2. Max roles per employee per company: An employee can hold at most 2 roles in the same company at the same time.

If a request violates any rule, the system must return invalid and also provide a reason. Otherwise, it should apply the assignment and return valid.

Input

• An initial set of existing assignments.
• A sequence of add requests, each of the form:
  • employee_id , company_id , role_name

Output

For each add request, output either:

• valid (and the assignment is applied), or
• invalid: <reason> where <reason> clearly states which rule was violated.

Notes / Clarifications

• If an employee is already assigned to the same role in the same company, treat it as valid (no-op), unless you choose to define it as an error—state your assumption.
• If both rules would be violated, you may return either a single reason or multiple reasons, but be consistent.

Constraints (you may assume)

• Up to 10^5 total assignments and requests.
• IDs/role names are strings.

Example (illustrative)

Given existing assignments where role SWE at company C1 already has 5 employees, adding another SWE to C1 should return:

• invalid: role capacity exceeded (max 5 per role per company)