How can I make complicated form logic more intuitive?

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I'm building a search feature that allows users to find people based on providing a combination of their full name and SSN. The part of the business requirements that I am struggling with is this:

  • If no name is provided, the entire SSN is required.
  • When the LAST NAME is provided, you can enter the full SSN or only the last 4 digits - either way the SSN becomes optional.
  • Providing any other combination that isn't the full SSN or last four digits of the SSN is considered an invalid entry.

How can I present this logic in an intuitive way? I'd like to display this logic visually through the use of a clever layout, making it so that the user would follow these rules automatically. I do not want to rely on including text that the user will likely not read.

My thoughts are this: Maybe present the form as two options: name or SSN search. When a Last Name is entered, grey out the first two fields for an SSN. When an SSN is entered, grey out the Name fields.

Default: enter image description here

Last Named Entered: enter image description here

SSN Entered: enter image description here

However, I'm still curious as to what could be the most elegant solution. Is there somewhere that this fails? Is it unintuitive?

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