Add/edit exercises

EXERCISE_CONTRIBUTION_GUIDE.md

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+# Guide to Adding or Editing Starklings Exercises
+
+This guide will help you add new exercises or edit existing ones in the Starklings project, an interactive platform to learn Cairo and Starknet.
+
+## 📁 Project Structure
+
+Starklings exercises are organized as follows:
+
+```
+starklings/
+├── exercises/                 # 📂 Main exercises directory
+│   ├── arrays/               # 📂 Array exercises
+│   ├── variables/            # 📂 Variable exercises
+│   ├── functions/            # 📂 Function exercises
+│   ├── starknet/             # 📂 Starknet-specific exercises
+│   └── ...                   # 📂 Other categories
+├── info.toml                 # ⚙️ Exercises and hints configuration
+```
+
+## 🔧 Exercise Components
+
+Each exercise in Starklings has **3 main components**:
+
+### 1. 📄 Exercise File (`.cairo`)
+- **Location**: `exercises/<category>/<exercise_name>.cairo`
+- **Contains**: Cairo code with descriptive comments and the marker `// I AM NOT DONE`
+
+### 2. 📝 Configuration in `info.toml`
+- **Location**: `info.toml` (project root)
+- **Contains**: Exercise metadata, execution mode, and hints
+
+📋 **Prerequisites:**
+• You must have the project forked in your GitHub account
+• Your fork must be synced with the main repository
+
+## ✏️ Editing an Existing Exercise
+
+### Edit the Exercise Description
+
+To change the description of an exercise (for example, `arrays1.cairo`):
+
+1. **Open the exercise file:**
+   ```bash
+   exercises/arrays/arrays1.cairo
+   ```
+
+2. **Edit the comments at the top of the file:**
+   ```cairo
+   // Your new exercise description here
+   // Explain what the student should do
+   // You can use multiple comment lines
+
+   // I AM NOT DONE  ← This marker must stay
+
+   fn create_array() -> Array<felt252> {
+       // ... exercise code
+   }
+   ```
+
+**⚠️ Important:**
+- Only edit the comments at the top of the file
+- **Do NOT remove** the `// I AM NOT DONE` line (it's needed for the system)
+- Use `//` for all descriptions
+
+### Edit the Exercise Hint
+
+To change the hint shown to students:
+
+1. **Open the configuration file:**
+   ```bash
+   info.toml
+   ```
+
+2. **Find the exercise section:**
+   ```toml
+   [[exercises]]
+   name = "arrays1"
+   path = "exercises/arrays/arrays1.cairo"
+   mode = "test"
+   hint = """
+   Your new hint here.
+   You can use multiple lines.
+   Give key concepts or subtle tips.
+   """
+   ```
+
+## ➕ Adding a New Exercise
+
+### Step 1: Create the Exercise File
+
+1. **Go to the right category** (or create a new one):
+   ```bash
+   cd exercises/<category>/
+   ```
+
+2. **Create the `.cairo` file:**
+   ```cairo
+   // Clear and concise exercise description
+   // Explain what concept it teaches
+   // Give specific instructions
+
+   // I AM NOT DONE
+
+   fn example_exercise() {
+       // Starter code with blanks or errors
+       // for the student to complete
+   }
+
+   // Test or verification code
+   #[test]
+   fn test_example() {
+       // Tests to validate the solution
+   }
+   ```
+
+### Step 2: Configure in `info.toml`
+
+1. **Open `info.toml`** and find your category section
+
+2. **Add the exercise configuration:**
+   ```toml
+   [[exercises]]
+   name = "new_exercise"                    # Unique exercise name
+   path = "exercises/category/new_exercise.cairo"  # Path to the file
+   mode = "test"                            # "test", "run", or "build"
+   hint = """
+   Helpful hint for the student.
+   Can include:
+   - Links to docs: https://book.cairo-lang.org/...
+   - Key concepts to remember
+   - Tips about the solution (not the full answer)
+   """
+   ```
+
+## 🔄 Exercise Modes
+
+In `info.toml`, each exercise has a `mode` field that determines how it runs:
+
+- **`"test"`**: Runs the exercise tests
+- **`"run"`**: Runs the main program
+
+## ✅ Best Practices
+
+### For Exercise Descriptions:
+- 📝 Be clear and concise
+- 🎯 Focus on one concept per exercise
+- 📚 Mention previous concepts if needed
+- 🔍 Give enough context without revealing the solution
+
+### For Hints:
+- 💡 Give tips, not full solutions
+- 🔗 Include links to relevant docs
+- 📖 Explain key concepts if needed
+- 🎯 Be specific about what to look for
+
+### For Code:
+- 🏗️ Include useful starter code
+- ✅ Add tests to validate the solution
+- 🚫 Keep the `// I AM NOT DONE` marker
+- 📦 Import needed dependencies
+
+With this guide and the quick edit buttons, you can now easily add and edit Starklings exercises! 🚀