Add a pluggable ChooserStrategy to the LibGDX ResolutionFileResolver, to allow the choosing algorithm to be changed by the user.

ResolutionFileResolver.java

/*******************************************************************************
 * Copyright 2011 See AUTHORS file.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 *   http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 ******************************************************************************/

package com.badlogic.gdx.assets.loaders.resolvers;

import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.assets.loaders.FileHandleResolver;
import com.badlogic.gdx.assets.loaders.resolvers.ResolutionFileResolver.Resolution;
import com.badlogic.gdx.files.FileHandle;

public class ResolutionFileResolver implements FileHandleResolver {
  public static class Resolution {
  	public final int portraitWidth;
		public final int portraitHeight;
		public final String suffix;

		public Resolution (int portraitWidth, int portraitHeight, String suffix) {
			this.portraitWidth = portraitWidth;
			this.portraitHeight = portraitHeight;
			this.suffix = suffix;
		}
	}

	protected final FileHandleResolver baseResolver;
	protected final ChooserStrategy chooserStrategy;
	protected final Resolution[] descriptors;

	
	public ResolutionFileResolver (FileHandleResolver baseResolver, Resolution... descriptors) {
		this.baseResolver = baseResolver;
		this.chooserStrategy = ChooserStrategy.PreferLowerChooserStrategy;
		this.descriptors = descriptors;
	}

	public ResolutionFileResolver (FileHandleResolver baseResolver, ChooserStrategy chooserStrategy, Resolution... descriptors) {
		this.baseResolver = baseResolver;
		this.chooserStrategy = chooserStrategy;
		this.descriptors = descriptors;
	}
	
	@Override
	public FileHandle resolve (String fileName) {
		Resolution bestDesc = chooserStrategy.choose(Gdx.graphics.getWidth(), Gdx.graphics.getHeight(), descriptors);
		FileHandle originalHandle = new FileHandle(fileName);
		FileHandle handle = baseResolver.resolve(resolve(originalHandle, bestDesc.suffix));
		if (!handle.exists()) handle = baseResolver.resolve(fileName);
		return handle;
	}

	protected String resolve (FileHandle originalHandle, String suffix) {
		return originalHandle.parent() + "/" + suffix + "/" + originalHandle.name();
	}
	
	public interface ChooserStrategy {
		/**
		 * Chooses the Resolution which best matches the given dimensions.
		 * @param w the width
		 * @param h the height
		 * @param descriptors the resolutions to test against
		 */
		public Resolution choose(int w, int h, Resolution... descriptors);
		
		/**
		 * Implementation of ChooserStrategy which chooses the Resolution,
		 * which most closely matches the given dimensions,
		 * without going above the given dimensions.
		 */
		public static ChooserStrategy PreferLowerChooserStrategy = new ChooserStrategy() {
			@Override
			public Resolution choose (int w, int h, Resolution... descriptors) {
				if (descriptors == null) throw new IllegalArgumentException("descriptors cannot be null.");

				Resolution best = descriptors[0];
				if (w < h) {
					for (int i = 0, n = descriptors.length; i < n; i++) {
						Resolution other = descriptors[i];
						if (w >= other.portraitWidth && other.portraitWidth >= best.portraitWidth && h >= other.portraitHeight
							&& other.portraitHeight >= best.portraitHeight) best = descriptors[i];
					}
				} else {
					for (int i = 0, n = descriptors.length; i < n; i++) {
						Resolution other = descriptors[i];
						if (w >= other.portraitHeight && other.portraitHeight >= best.portraitHeight && h >= other.portraitWidth
							&& other.portraitWidth >= best.portraitWidth) best = descriptors[i];
					}
				}
				return best;
			}
		};

		/**
		 * Implementation of ChooserStrategy which chooses the Resolution,
		 * which most closely matches the given dimensions,
		 * without going below the given dimensions.
		 */
		public static ChooserStrategy PreferHigherChooserStrategy = new ChooserStrategy() {
			@Override
			public Resolution choose (int w, int h, Resolution... descriptors) {
				if (descriptors == null) throw new IllegalArgumentException("descriptors cannot be null.");

				Resolution best = descriptors[descriptors.length - 1];
				if (w < h) {
					for (int i = 0, n = descriptors.length; i < n; i++) {
						Resolution other = descriptors[i];
						if (w <= other.portraitWidth  && other.portraitWidth  <= best.portraitWidth &&
							 h <= other.portraitHeight && other.portraitHeight <= best.portraitHeight)
							 best = descriptors[i];
					}
				} else {
					for (int i = 0, n = descriptors.length; i < n; i++) {
						Resolution other = descriptors[i];
						if (w <= other.portraitHeight && other.portraitHeight <= best.portraitHeight &&
							 h <= other.portraitWidth  && other.portraitWidth  <= best.portraitWidth)
                      best = descriptors[i];
					}
				}
				return best;
			}
		};
		
		/**
		 * Implementation of ChooserStrategy which chooses the Resolution,
		 * which most closely matches the given dimensions.
		 */
		public ChooserStrategy PreferClosestChooserStrategy = new ChooserStrategy() {

			@Override
			public Resolution choose (int w, int h, Resolution... descriptors) {
				if (descriptors == null) throw new IllegalArgumentException("descriptors cannot be null.");

				Resolution best = descriptors[0];
				if (w < h) {
					for (int i = 0, n = descriptors.length; i < n; i++) {
						Resolution other = descriptors[i];						
						int dstWtoBPW = Math.abs(w - best.portraitWidth);
						int dstWtoRPW = Math.abs(w - other.portraitWidth);						
						if (dstWtoRPW <= dstWtoBPW) {
							int dstHtoBPH = Math.abs(h - best.portraitHeight);
							int dstHtoRPH = Math.abs(h - other.portraitHeight);
							if (dstHtoRPH <= dstHtoBPH) {
								best = descriptors[i];	
							}
						}
					}
				} else {
					for (int i = 0, n = descriptors.length; i < n; i++) {
						Resolution other = descriptors[i];
						int dstHtoBPW = Math.abs(h - best.portraitWidth);
						int dstHtoRPW = Math.abs(h - other.portraitWidth);
						if (dstHtoRPW <= dstHtoBPW) {
							int dstWtoBPH = Math.abs(w - best.portraitHeight);
							int dstWtoRPH = Math.abs(w - other.portraitHeight);
							if (dstWtoRPH <= dstWtoBPH) {
									best = descriptors[i];	
							}					
						}
					}
				}
				return best;
			}
		};
	}
	
	// Tester
	public static void main(String[] args) {
		Resolution[] resolutions = new Resolution[] {
			new Resolution(320, 480, "320"),
			new Resolution(480, 800, "480"),
			new Resolution(768, 1280, "768"),
		};
		Resolution resolution1 = ChooserStrategy.PreferLowerChooserStrategy.choose(1920, 1080, resolutions);
		Resolution resolution2 = ChooserStrategy.PreferHigherChooserStrategy.choose(1920, 1080, resolutions);
		Resolution resolution3 = ChooserStrategy.PreferClosestChooserStrategy.choose(1920, 1080, resolutions);
		System.out.println(resolution1.suffix);
		System.out.println(resolution2.suffix);
		System.out.println(resolution3.suffix);
	}
}