Field maps, the per-field divergence-diff engine, and the resolver they feed — shared by both mirror sides: Company ↔ an account_type: "organization" User, Contact ↔ an account_type: "person" User (owner decisions Q1-Q6, L006).

Pure functions over structs: no DB access, no context calls. Every writing decision ("actually link/create/copy") lives in the Companies / Contacts contexts (Tasks D/E); this module only computes what would change, whether that change is safe to make silently, and — once a human has picked a side per field — exactly what to write.

The rule this module exists to enforce

"Master = the side the form you're on transfers FROM." When both sides already carry a NON-BLANK value and those values genuinely differ, that is a conflict — diff/2 surfaces it and the caller must ask (the conflict modal, Task F), never silently overwrite. When one side is blank, the copy just fills it — not a conflict. Values are compared trimmed (" Anna Kask " and "Anna Kask" are the same value, on either side), so incidental whitespace never manufactures a false conflict.

Field maps

field_map/1 returns one entry per mirrored field:

• %{crm: atom(), user: atom(), label: String.t()} — a direct 1:1 copy (:email for both kinds; :name ↔ :organization_name for :company).
• %{crm: :name, user: {:split, :first_name, :last_name}, label: ...} — :contact only. User has no single "name" column; Contact.name is the JOIN of first_name + last_name (see below), so there is no single user-side atom this field's value lives in.

Only fields that exist on BOTH Contact/Company and User participate — Company.website/.phone/.address/.industry and Contact.phone/ .locale have no User counterpart (User has no phone, no locale) and are deliberately absent from both maps (Q4).

The :field key is symbolic, not a raw column

diff/2 and resolve/4 both key on the CRM-side atom (:name, :email) for BOTH kinds, uniformly — never a raw User column like :organization_name or :first_name, even for the direct mappings where crm and user atoms happen to share a name (:email). This is deliberate: a caller must never Ecto.Changeset.put_change(cs, field, value) a diff/2 entry directly (that would silently break the contact name mapping, which needs two columns written from one value). Callers pick a side per symbolic field and hand the choices to resolve/4, which owns all split/join + direction logic centrally — LiveViews (Tasks F/G/H) never see first_name/last_name/ organization_name at all.

Name join / split

User has no combined name field; Contact conflates the mirror's "name" concept differently (:company's name maps straight to organization_name, no join/split involved):

• join (User → CRM, attrs_to_crm/2 and resolve/4, contact only): String.trim("#{first_name} #{last_name}"), collapsing to nil when both sides are blank.
• split (CRM → User, attrs_from/2 and resolve/4, contact only): tokenize on whitespace (any run) — the LAST token becomes last_name, every token before it rejoins with a single space to become first_name. This collapses internal multi-space/tab runs to a single space; it does not preserve original in-between spacing. A single token sets first_name and leaves last_name nil. A blank name yields nil for both.