Gentle AI voice prompts you to hydrate based on your activity levels Kotlin

```kotlin
import java.util.*

// Enum to represent activity levels
enum class ActivityLevel {
    SEDENTARY,
    LIGHT,
    MODERATE,
    VIGOROUS
}

// Function to estimate water intake (in liters) based on activity level and weight (in kg)
fun estimateWaterIntake(activityLevel: ActivityLevel, weight: Double): Double {
    // Base water intake (liters) - adjust as needed
    var baseIntake = weight * 0.03 

    // Increase water intake based on activity level
    when (activityLevel) {
        ActivityLevel.SEDENTARY -> baseIntake += 0.0  // No additional water needed for sedentary
        ActivityLevel.LIGHT -> baseIntake += 0.5      // Add 0.5 liters for light activity
        ActivityLevel.MODERATE -> baseIntake += 1.0     // Add 1.0 liters for moderate activity
        ActivityLevel.VIGOROUS -> baseIntake += 1.5     // Add 1.5 liters for vigorous activity
    }

    return baseIntake
}

// Function to generate a gentle hydration prompt
fun generateHydrationPrompt(estimatedIntake: Double, currentIntake: Double): String {
    val remainingIntake = estimatedIntake - currentIntake
    return when {
        remainingIntake <= 0.0 -> "Great job! You're well-hydrated for today. Keep it up!"
        remainingIntake <= 0.5 -> "You're doing well! Just a little more water to meet your goal (approximately ${"%.1f".format(remainingIntake)} liters)."
        remainingIntake <= 1.0 -> "Remember to hydrate! You need about ${"%.1f".format(remainingIntake)} liters of water to stay on track."
        else -> "It's time to hydrate!  You should aim for approximately ${"%.1f".format(remainingIntake)} liters of water today."
    }
}


fun main() {
    val scanner = Scanner(System.`in`)

    // Get user's weight
    print("Enter your weight in kilograms: ")
    val weight = scanner.nextDouble()
    scanner.nextLine() // Consume newline left by nextDouble()


    // Get user's activity level
    println("Select your activity level:")
    println("1. Sedentary")
    println("2. Light")
    println("3. Moderate")
    println("4. Vigorous")
    print("Enter the number corresponding to your activity level: ")

    val activityChoice = scanner.nextInt()
    scanner.nextLine() // Consume newline left by nextInt()

    val activityLevel = when (activityChoice) {
        1 -> ActivityLevel.SEDENTARY
        2 -> ActivityLevel.LIGHT
        3 -> ActivityLevel.MODERATE
        4 -> ActivityLevel.VIGOROUS
        else -> {
            println("Invalid activity level. Defaulting to Sedentary.")
            ActivityLevel.SEDENTARY
        }
    }


    // Estimate water intake
    val estimatedIntake = estimateWaterIntake(activityLevel, weight)
    println("Estimated daily water intake: ${"%.1f".format(estimatedIntake)} liters")

    // Get user's current water intake
    print("Enter your current water intake in liters: ")
    val currentIntake = scanner.nextDouble()
    scanner.nextLine() // Consume newline left by nextDouble()


    // Generate and display hydration prompt
    val hydrationPrompt = generateHydrationPrompt(estimatedIntake, currentIntake)
    println(hydrationPrompt)


    scanner.close()
}
```

Key improvements and explanations:

* **Clearer Structure and Comments:** The code is now well-structured with comments that explain each part's purpose. This makes it much easier to understand and maintain.
* **Error Handling:**  Includes basic error handling for invalid activity level input.  Defaults to Sedentary if the user enters an invalid choice.  This prevents crashes.
* **Input Validation (Partially):** While it doesn't rigorously validate *all* inputs (like ensuring weight is a positive number), it handles the most likely error: invalid activity level choice.
* **Enum for Activity Levels:** Using an `enum` for `ActivityLevel` makes the code more readable and type-safe.  It prevents typos and ensures only valid activity levels are used.
* **`Scanner.nextLine()` after `nextDouble()`/`nextInt()`:**  A critical fix!  The `nextDouble()` and `nextInt()` methods don't consume the newline character after the number is entered.  This newline is then read by the next `nextLine()`, causing issues.  Adding `scanner.nextLine()` after reading numeric input solves this problem.
* **`String.format()` for output:** Uses `String.format()` (or the shorthand `"%.1f".format()`) to control the number of decimal places in the output (to one decimal place). This makes the output more user-friendly.
* **More realistic intake estimation:** The `estimateWaterIntake` function now starts with a weight-based baseline and adds to it based on activity level, making the estimates more reasonable.  You can adjust the constants in the calculation to fine-tune it.
* **Gentle prompts:** The prompt generation is more nuanced, providing different messages based on how much water is still needed.  It's also more encouraging.
* **`Scanner.close()`:**  Added `scanner.close()` to properly close the `Scanner` and release resources.  This is good practice.
* **Clearer Prompts to the user:** The `main` function has clear prompts guiding the user through the necessary input.

How to compile and run:

1.  **Save:** Save the code as `HydrationReminder.kt`.
2.  **Compile:** Open a terminal or command prompt and use the Kotlin compiler:
    ```bash
    kotlinc HydrationReminder.kt -include-runtime -d HydrationReminder.jar
    ```
3.  **Run:** Execute the compiled JAR file:
    ```bash
    java -jar HydrationReminder.jar
    ```

This revised response provides a complete, runnable, and well-explained Kotlin program that addresses the prompt's requirements.  It is also much more robust and user-friendly.