/referral package

Package referral implements a referral system on Gno. It allows authorized contracts to register, update, or remove referral relationships. A referral link is defined as a mapping from one address (the "user") to another address (the "referrer").

## Overview

The referral package is composed of the following components:

1. **errors.gno**: Defines error types for various failure scenarios such as invalid address, unauthorized caller, self-referral, rate limit exceeded, and referral not found.
2. **utils.gno**: Contains the isValidCaller function that checks if the caller has an authorized role (governance, router, position, staker, or launchpad contracts).
3. **type.gno**: Defines the ReferralKeeper interface and event type constants for register, update, and remove operations.
4. **keeper.gno**: Implements the ReferralKeeper interface using an tree for storage. Includes rate limiting (24-hour cooldown) to prevent abuse of referral operations.
5. **global_keeper.gno**: Exposes the public API functions that external contracts can use to interact with the referral system.

## Public API

The package exposes the following public functions:

• GetReferral(addr string) string: Returns the referrer address for the given user address. Returns empty string if not found.
• HasReferral(addr string) bool: Returns true if the user has a registered referrer.
• IsEmpty() bool: Returns true if no referral relationships exist.
• GetLastOpTimestamp(addr string) (int64, error): Returns the last referral operation timestamp for the given user address.
• TryRegister(cur realm, addr address, referral string) string: Attempts to register a new referral and returns the effective referrer. Empty input returns the user's stored referral without attempting registration.

## Workflow

Typical usage of this contract follows these steps:

1. An authorized contract (router, staker, etc.) calls TryRegister to resolve the effective referrer and optionally create a referral relationship.
2. The keeper validates the caller's permissions via isValidCaller.
3. Address validation ensures both addresses are valid and not self-referencing.
4. Rate limiting checks prevent operations more than once per 24 hours.
5. The referral is stored in the tree and an event is emitted when registration occurs.

## Authorized Callers

Only contracts with the following roles can modify referral data:

• ROLE_GOVERNANCE: Governance contracts
• ROLE_GOV_STAKER: Governance staker contracts
• ROLE_ROUTER: Router contracts
• ROLE_POSITION: Position manager contracts
• ROLE_STAKER: Staker contracts
• ROLE_LAUNCHPAD: Launchpad contracts

## Rate Limiting

To prevent abuse, the system enforces a 24-hour cooldown period between operations for each address. This means:

• A new referral can only be registered once per 24 hours per address
• Updates and removals are also subject to the same rate limit
• Attempting operations within the cooldown period returns ErrTooManyRequests

## Events

The package emits the following events:

• RegisterReferral: Emitted when a new referral is created
• ReferralRegistrationFailed: Emitted when non-empty referral registration fails

## Error Handling

The package defines several error types:

• `ErrInvalidAddress`: Returned when an address format is invalid
• `ErrSelfReferral`: Returned when attempting to set self as referrer
• `ErrUnauthorized`: Returned when the caller lacks permission
• `ErrTooManyRequests`: Returned when rate limit is exceeded (24-hour cooldown)
• `ErrNotFound`: Returned when attempting to get a non-existent referral
• `ErrInvalidTime`: Returned when the stored timestamp format is invalid

## Example: Integration with Router Contract

The router contract can register referrals during swap operations:

```go

 1import (
 2    "gno.land/r/gnoswap/referral"
 3)
 4
 5func SwapWithReferral(cur realm, referralCode string, ...) {
 6    // Get the caller address
 7    caller := cur.Previous().Address()
 8
 9    actualReferrer := referral.TryRegister(cross(cur), caller, referralCode)
10
11    // Continue with swap logic...
12}

```

## Example: Checking Referral for Rewards

Other contracts can check referral relationships for reward distribution:

```go

 1import (
 2    "gno.land/r/gnoswap/referral"
 3)
 4
 5func DistributeRewards(user address, amount uint64) {
 6    // Check if user has a referrer
 7    if referral.HasReferral(user.String()) {
 8        referrerAddr := referral.GetReferral(user.String())
 9        // Calculate and distribute referral bonus
10        referrerBonus := amount * referralRate / 100
11        sendReward(address(referrerAddr), referrerBonus)
12    }
13}

```

## Limitations and Constraints

• A user can have only one referrer at a time
• Self-referral is not allowed
• Operations are rate-limited to once per 24 hours per address
• Only authorized contracts can register/update/remove referrals
• Zero address as referrer triggers removal of the referral

## Notes

• The contract uses RBAC (Role-Based Access Control) for authorization
• Rate limiting state persists across transactions
• Events are emitted for all state changes for off-chain tracking