# Creating Custom Balance Format Rule

## Adding Custom Currency Formatting Rules for TheNewEconomy

The currency formatting system in **TheNewEconomy** allows developers to define custom rules for displaying currencies. These rules are used in the `Format` configuration of a currency YAML file (e.g., `USD.yml`) and control how currency balances are formatted.

***

### Overview of Format Rules

A **Format Rule** is a component that defines how a placeholder in the currency format is replaced with dynamic content. For example, `<symbol>` can be replaced with `$`, and `<decimal>` can include the decimal separator.

#### Example Default Format in YAML

```yaml
Format: "<symbol><major><decimal><minor>"
```

***

### Creating a Custom Format Rule

#### 1. **Implement the `FormatRule` Interface**

To create a custom formatting rule, implement the `FormatRule` interface. Below is an example of a rule that adds a custom placeholder `<custom>`, which displays a fixed string.

```java
import net.tnemc.core.account.Account;
import net.tnemc.core.account.holdings.HoldingsEntry;
import net.tnemc.core.currency.format.FormatRule;
import org.jetbrains.annotations.Nullable;

public class CustomRule implements FormatRule {

  @Override
  public String name() {
    return "<custom>";
  }

  @Override
  public String format(@Nullable Account account, HoldingsEntry entry, String format) {
    return format.replace("<custom>", "CustomValue");
  }
}
```

#### 2. **Register the Rule**

Register your custom rule with the `CurrencyFormatter` class to make it available in TNE:

```java
CurrencyFormatter.addRule(new CustomRule());
```

This should be done during your plugin or integration's initialization phase.

***

### Examples of Default Rules

#### **SymbolRule**

Replaces `<symbol>` with the currency symbol:

```java
public class SymbolRule implements FormatRule {
  @Override
  public String name() {
    return "<symbol>";
  }

  @Override
  public String format(@Nullable Account account, HoldingsEntry entry, String format) {
    final Optional<Currency> currency = entry.currency();
    return currency.map(value -> format.replace("<symbol>", value.getSymbol())).orElse(format);
  }
}
```

#### **DecimalRule**

Handles the `<decimal>` placeholder, adding or omitting the decimal separator based on the amount:

```java
public class DecimalRule implements FormatRule {
  @Override
  public String name() {
    return "<decimal>";
  }

  @Override
  public String format(@Nullable Account account, HoldingsEntry entry, String format) {
    if (entry.currency().isPresent()) {
      if (entry.asMonetary().scale() == 0) {
        return format.replace("<decimal>", "");
      }
      return format.replace("<decimal>", entry.currency().get().getDecimal());
    }
    return format;
  }
}
```

***

### Example Custom Formatting Rule Usage

#### YAML Configuration

After adding your custom rule, it can be used in the `Format` configuration of a currency YAML file:

```yaml
Format: "<symbol><major><decimal><minor><custom>"
```

When applied, the `<custom>` placeholder will be replaced with the value defined in your `CustomRule`.

***

### Notes

1. **Registration**: Always register custom rules using `CurrencyFormatter.addRule()`.
2. **Compatibility**: Ensure your custom rules align with TNE's currency formatting system.
3. **Dynamic Content**: Use the `Account` and `HoldingsEntry` parameters in your rule to provide dynamic formatting based on player or balance data.

***

By following these steps, you can expand TNE's currency formatting system to suit your server's unique needs. For further reference, review default rules such as `SymbolRule`, `DecimalRule`, and `ShortenRule`.