Multiplayer Counter contract (clarity)

;; Multiplayer Counter contract

(define-map counters principal uint)

(define-read-only (get-count (who principal))
    (default-to u0 (map-get? counters who))
)

(define-public (count-up) 
    (ok (map-set counters tx-sender (+ (get-count tx-sender) u1))) 
)

🧠 Brief Overview

• Purpose: Each user (principal) gets their own unique counter.
• Data storage: A key-value map that stores each user’s counter.
• Functions:
  • get-count: Read-only function to check a user’s count.
  • count-up: Public function to increase the sender’s count by 1

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🔍 Code Breakdown

;; Multiplayer Counter contract

(define-map counters principal uint)

• Declares a map named counters.
• Key: principal (the user’s blockchain address).
• Value: uint (unsigned integer — the counter value).

---

(define-read-only (get-count (who principal))
    (default-to u0 (map-get? counters who))
)

• This is a read-only function. It does not modify blockchain state.
• Input: who — a user address.
• Action: Checks if the user has a counter.
  • If yes: returns the value.
  • If no: returns u0 (default value 0).

---

(define-public (count-up)
    (ok (map-set counters tx-sender (+ (get-count tx-sender) u1)))
)

• A public function (anyone can call it).
• tx-sender: The address of the user calling this function.
• Gets the sender’s current count using get-count.
• Adds 1 to it.
• Updates the counters map with the new value.
• Wraps the result in an ok response.

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🧭 Step-by-Step: How This Contract Works

Let’s walk through an example interaction:

1. 🧍 User A checks their counter:

(get-count tx-sender)

• If User A is new, this returns 0 because there’s no entry yet.

2. 🆙 User A calls count-up

(count-up)

• Contract fetches User A’s current count (e.g., 0).
• Adds 1 → new value = 1.
• Stores it in the counters map: counters[User A] = 1.

3. 🔁 User A calls count-up again

• Now it fetches 1, adds 1 → value becomes 2, and updates the map.

4. 🧑‍🤝‍🧑 User B joins and starts at 0

• Their get-count will return 0.
• They can now increment their own counter without affecting User A’s.

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📘 Summary

Function	Type	Description
counters	Map	Stores each user’s counter.
get-count	Read-only	Returns current count or 0 if not found.
count-up	Public	Increments the caller’s counter by 1.

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🧩 Real-World Analogy

Imagine a scoreboard where every player has their own slot. When a player hits a button, only their score increases. No one can touch your slot but you.