Add solution for Challenge 7 by nzamulov

[nzamulov]

Challenge 7 Solution

Submitted by: @nzamulov
Challenge: Challenge 7

Description

This PR contains my solution for Challenge 7.

Changes

• Added solution file to challenge-7/submissions/nzamulov/solution-template.go

Testing

• Solution passes all test cases
• Code follows Go best practices

Thank you for reviewing my submission! 🚀

[coderabbitai]

Walkthrough

Introduces a new Go package challenge7 implementing a thread-safe BankAccount type with deposit, withdraw, and transfer operations. Includes custom error types for validation and uses mutex-based concurrency control with deadlock-avoidant locking for multi-account transfers.

Changes

Cohort / File(s)

Summary

BankAccount Implementation challenge-7/submissions/nzamulov/solution-template.go

New file introducing BankAccount struct with ID, Owner, Balance, MinBalance, and mutex. Adds MaxTransactionAmount constant and four custom error types (AccountError, InsufficientFundsError, NegativeAmountError, ExceedsLimitError). Implements NewBankAccount() factory function, Deposit(), Withdraw(), and Transfer() methods with comprehensive input validation and deadlock-avoidant locking for concurrent operations.

Sequence Diagrams

sequenceDiagram
    participant User
    participant Transfer as Transfer(amt, target)
    participant Lock1 as Lock Account 1
    participant Lock2 as Lock Account 2
    participant Validation as Validation

    User->>Transfer: Call Transfer
    Transfer->>Validation: Check non-nil target, no self-transfer
    alt Validation fails
        Validation-->>Transfer: Error
        Transfer-->>User: Error returned
    else Validation passes
        Validation-->>Transfer: OK
        Transfer->>Lock1: Acquire lock (lower ID)
        Transfer->>Lock2: Acquire lock (higher ID)
        Lock1-->>Transfer: Acquired
        Lock2-->>Transfer: Acquired
        Transfer->>Validation: Check amount, balance, limit
        alt Balance check fails
            Validation-->>Transfer: InsufficientFundsError
        else All checks pass
            Transfer->>Transfer: Update balances
            Transfer-->>Lock2: Release
            Transfer-->>Lock1: Release
            Transfer-->>User: nil (success)
        end
    end

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

• Verify the deadlock-avoidant locking strategy based on ID comparison is correctly implemented across all transfer paths
• Ensure all error types are properly validated and returned in appropriate scenarios
• Confirm mutex locks are properly acquired and released in all methods, including error paths
• Validate that the MinBalance constraint is enforced in both Withdraw and Transfer operations

Possibly related PRs

• Add solution for Challenge 7 by Kosench #612: Implements identical BankAccount structure with mutex-based concurrency, custom error types, and deadlock-avoidant locking for the Transfer method.
• Add solution for Challenge 7 by RezaSi #555: Adds nearly identical BankAccount implementation with same error types, MaxTransactionAmount constant, and all core methods (Deposit, Withdraw, Transfer, NewBankAccount).

Pre-merge checks

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Add solution for Challenge 7 by nzamulov' accurately reflects the main change: adding a Challenge 7 solution file by the specified contributor.
Description check	✅ Passed	The description is directly related to the changeset, explaining that it contains the Challenge 7 solution, specifies the file location, and notes testing completion.

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[coderabbitai]

Actionable comments posted: 1

📜 Review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 231bd24 and e9a90af.

📒 Files selected for processing (1)

• challenge-7/submissions/nzamulov/solution-template.go (1 hunks)

[coderabbitai]

⚠️ Potential issue | 🔴 Critical

Deadlock risk when two accounts share the same ID

Lock ordering swaps only when a.ID > target.ID. If two distinct accounts happen to share the same ID, goroutine A transferring from account1→account2 locks in the sequence (account1, account2) while goroutine B transferring from account2→account1 also locks (account2, account1). Both goroutines then block forever waiting on the other mutex. Because ID uniqueness is not enforced anywhere, this deadlock is reachable at runtime. To make the ordering unambiguous, add a deterministic tie-breaker (e.g., pointer address) when IDs compare equal so every transfer uses the same lock order.

-import "sync"
+import (
+	"sync"
+	"unsafe"
+)
@@
-	first, second := a, target
-	if a.ID > target.ID {
-		first, second = target, a
-	}
+	first, second := a, target
+	if a.ID > target.ID ||
+		(a.ID == target.ID && uintptr(unsafe.Pointer(a)) > uintptr(unsafe.Pointer(target))) {
+		first, second = target, a
+	}

Committable suggestion skipped: line range outside the PR's diff.

🤖 Prompt for AI Agents

In challenge-7/submissions/nzamulov/solution-template.go around lines 138 to
146, the current lock-ordering swaps only when a.ID > target.ID which leaves a
deadlock when two distinct accounts share the same ID; change the ordering logic
to include a deterministic tie-breaker when IDs are equal (for example compare
the account object pointer addresses or another unique stable value) so that for
any pair of accounts you always choose the same first/second before locking;
implement the comparison so if a.ID == target.ID you fall back to comparing
uintptr(unsafe.Pointer(a)) vs uintptr(unsafe.Pointer(target)) (or another stable
unique key) and then lock first then second in that determined order.