Bidirectional Map

Unique in both directions, demoing the p/moul/x/daily/bidimap library.

Roles

person	role
alice	admin
bob	auditor
carol	treasurer

Both directions are O(1)

• Get("bob") → auditor
• GetKey("treasurer") → carol — the reverse index, not a scan

Displacing a binding

Both sides are unique, so giving alice the auditor role cannot simply be written — bob already holds it, and alice already holds admin. Put replaces, and reports what it displaced:

displaced key	displaced value
alice	admin
bob	auditor

person	role
alice	auditor
carol	treasurer

bob is now unassigned and the admin role is vacant — one call, both indexes still agreeing.

Refusing instead

PutUnique makes the other choice: it declines rather than displace.

• PutUnique("dave", "auditor") → false — the role is taken
• PutUnique("dave", "secretary") → true — both sides free

person	role
alice	auditor
carol	treasurer
dave	secretary

Keys come back sorted, never in map order: gno map iteration order is unspecified, and a page built from one can differ between nodes — a consensus bug, not a cosmetic one.