A simple Vesting Smart Contract in Ink! for Polkadot

This Ink! smart contract enables an entity (e.g., an employer) to lock funds for a beneficiary (e.g., an employee) until a specified timestamp is reached. Funds can be deposited into multiple schedules and later withdrawn by the beneficiary once the unlock time occurs. Key features include:

• Time-Locked Deposits: Funds locked until specified timestamps
• Multi-Beneficiary Support: Manage multiple payment schedules
• Transparent Tracking: On-chain proof of committed funds
• Secure Withdrawals: Automated time-based release

Time Calculation

When funds are deposited, an unlock_time (a UNIX timestamp) is recorded. During withdrawal, the contract reads the current block timestamp with self.env().block_timestamp(). If the current time equals or exceeds the unlock time, the funds are eligible for withdrawal.

• Block Timestamp: Each block contains creation time (milliseconds since UNIX epoch)
• Deterministic Execution: Contracts only access current block's timestamp
• Reliability:: Relying on the blockchain’s timestamp ensures that the schedule is enforced transparently and securely.

Data Structures

The contract uses two primary storage mappings:

• Schedules Mapping (ID to Schedule): Each VestingSchedule holds the owner, beneficiary, deposited amount, and unlock time.
• Beneficiary Lookup: A mapping from beneficiary to a list of schedule IDs allows quick access to all vesting schedules associated with a given account.

An auto-incrementing id guarantees unique identifiers for each new schedule, with proper overflow checks.

Component	Type	Purpose
id	u64	Auto-incrementing schedule ID
schedules	Mapping<u64, VestingSchedule>	ID → Schedule lookup
beneficiary_to_ids	Mapping<AccountId, Vec<u64>>	Beneficiary → Schedule IDs

Core Functionality

Deposit and Withdrawal Mechanics

Deposit Process (deposit_fund)

1. Validate Deposit:
   • The caller provides funds (non-zero) along with the beneficiary and unlock time.
2. Generate and Store Schedule:
   • A unique schedule ID is generated. A new vesting schedule is created and stored.
3. Update Beneficiary Data:
   • The schedule ID is appended to the beneficiary’s list for future reference.

sequenceDiagram
    participant Depositer
    participant Contract
    participant Blockchain
    participant Storage

    Depositer->>Blockchain: Initiate Transaction (value)
    Blockchain->>Contract: env.caller() & env.transferred_value()
    
    Contract->>Contract: Verify amount > 0
    Contract->>Contract: Generate new ID (overflow check)
    
    Contract->>Storage: Store VestingSchedule
    Contract->>Storage: Update beneficiary→IDs mapping
    
    Contract-->>Blockchain: Confirm Storage Update
    Blockchain-->>Depositer: Transaction Receipt

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Withdrawal Process (withdraw_fund)

1. Fetch Schedules:
   • The beneficiary calls the function, which retrieves all their schedule IDs.
2. Check Unlock Conditions:
   • For each schedule, if unlock_time ≤ current block timestamp, its funds are aggregated and the schedule is removed.
3. Transfer Funds:
   • If any funds are unlocked, they are transferred to the beneficiary; otherwise, an error is returned.

sequenceDiagram
    participant User as Beneficiary
    participant Contract
    participant Blockchain
    participant Storage

    User->>Contract: withdraw_fund()
    Contract->>Blockchain: get_block_timestamp()
    Blockchain-->>Contract: current_time
    
    Contract->>Storage: get_vesting_schedules()
    Storage-->>Contract: schedule_list
    
    loop Process Schedules
        Contract->>Storage: read_schedule()
        Storage-->>Contract: schedule_data
        
        alt Unlock Time Reached
            Contract->>Contract: accumulate_funds()
            Contract->>Storage: remove_schedule()
        else Still Locked
            Contract->>Contract: retain_schedule()
        end
    end
    
    Contract->>Blockchain: transfer_funds()
    Contract->>Storage: update_remaining_schedules()
    Blockchain-->>User: funds_transferred

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