Adds a new tab-completion helper function

Adds a new tab-completion helper function which is able to tab-complete comma-separated inputs.

src/main/java/net/knarcraft/knarlib/util/TabCompletionHelper.java

@@ -1,9 +1,11 @@
 package net.knarcraft.knarlib.util;
 
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.function.BiFunction;
 
 /**
  * Helper class for getting string lists required for auto-completion
@@ -14,6 +16,51 @@ public final class TabCompletionHelper {
     private TabCompletionHelper() {
     }
 
+    /**
+     * Finds available tab-completions for a list where several comma-separated values are allowed
+     *
+     * <p>If, for example, the available values are [fish,potatoes,stew,strawberries], and the player has already typed
+     * "fish,potatoes,", then available tab-completions become fish,potatoes,stew and fish,potatoes,strawberries.</p>
+     *
+     * @param values    <p>The available tab-completion values</p>
+     * @param typedText <p>The text the player has typed</p>
+     * @param filterer  <p>The filterer (filterMatching) to use to filter by typed text</p>
+     * @return <p>Available tab-completions</p>
+     */
+    public static @NotNull List<String> getStringList(@NotNull List<String> values, @NotNull String typedText,
+                                                      @Nullable BiFunction<List<String>, String, List<String>> filterer) {
+        if (typedText.isBlank() || !typedText.contains(",")) {
+            if (filterer != null) {
+                return filterer.apply(values, typedText);
+            } else {
+                return values;
+            }
+        } else {
+            // The argument base is everything the player has typed, until the last comma
+            List<String> arguments = List.of(typedText.split(","));
+            String base = getBase(arguments);
+            String lastArgument = arguments.get(arguments.size() - 1);
+
+            // Filter available values by the text after the last comma only
+            List<String> filtered;
+            if (filterer != null) {
+                filtered = filterer.apply(new ArrayList<>(values), lastArgument);
+            } else {
+                filtered = values;
+            }
+
+            // Remove any already used values
+            List<String> unused = new ArrayList<>();
+            for (String string : filtered) {
+                if (!arguments.contains(string)) {
+                    unused.add(base + string);
+                }
+            }
+
+            return unused;
+        }
+    }
+
     /**
      * Finds tab complete values that contain the typed text
      *
@@ -48,4 +95,15 @@ public final class TabCompletionHelper {
         return configValues;
     }
 
+    /**
+     * Gets the "base" string from the given arguments, not including the incomplete last argument
+     *
+     * @param arguments <p>The string arguments to get the base of</p>
+     * @return <p>The base string</p>
+     */
+    private static @NotNull String getBase(@NotNull List<String> arguments) {
+        List<String> baseArray = arguments.subList(0, arguments.size() - 1);
+        return String.join(",", baseArray) + ",";
+    }
+
 }