/*
 * Copyright (C) Lightbend Inc. <https://www.lightbend.com>
 */

package play.core;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Stack;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

/**
 * Implementations to work with URL paths. This is a utility class with usages by {@link
 * play.mvc.Call}.
 */
public final class Paths {
  private Paths() {}

  private static String CURRENT_DIR = ".";
  private static String SEPARATOR = "/";
  private static String PARENT_DIR = "..";

  /** Create a path to targetPath that's relative to the given startPath. */
  public static String relative(String startPath, String targetPath) {
    // If the start and target path's are the same then link to the current directory
    if (startPath.equals(targetPath)) {
      return CURRENT_DIR;
    }

    String[] start = toSegments(canonical(startPath));
    String[] target = toSegments(canonical(targetPath));

    // If start path has no trailing separator (a "file" path), then drop file segment
    if (!startPath.endsWith(SEPARATOR)) start = Arrays.copyOfRange(start, 0, start.length - 1);

    // If target path has no trailing separator, then drop file segment, but keep a reference to add
    // it later
    String targetFile = "";
    if (!targetPath.endsWith(SEPARATOR)) {
      targetFile = target[target.length - 1];
      target = Arrays.copyOfRange(target, 0, target.length - 1);
    }

    // Work out how much of the filepath is shared by start and path.
    String[] common = commonPrefix(start, target);
    String[] parents = toParentDirs(start.length - common.length);

    int relativeStartIdx = common.length;
    String[] relativeDirs = Arrays.copyOfRange(target, relativeStartIdx, target.length);
    String[] relativePath = Arrays.copyOf(parents, parents.length + relativeDirs.length);
    System.arraycopy(relativeDirs, 0, relativePath, parents.length, relativeDirs.length);

    // If this is not a sibling reference append a trailing / to path
    String trailingSep = "";
    if (relativePath.length > 0) trailingSep = SEPARATOR;

    return Arrays.stream(relativePath).collect(Collectors.joining(SEPARATOR))
        + trailingSep
        + targetFile;
  }

  /**
   * Create a canonical path that does not contain parent directories, current directories, or
   * superfluous directory separators.
   */
  public static String canonical(String url) {
    String[] urlPath = toSegments(url);
    Stack<String> canonical = new Stack<>();
    for (String comp : urlPath) {
      if (comp.isEmpty() || comp.equals(CURRENT_DIR)) continue;
      if (!comp.equals(PARENT_DIR) || (!canonical.empty() && canonical.peek().equals(PARENT_DIR)))
        canonical.push(comp);
      else canonical.pop();
    }

    String prefixSep = url.startsWith(SEPARATOR) ? SEPARATOR : "";
    String trailingSep = url.endsWith(SEPARATOR) ? SEPARATOR : "";

    return prefixSep + canonical.stream().collect(Collectors.joining(SEPARATOR)) + trailingSep;
  }

  private static String[] toSegments(String url) {
    return Arrays.stream(url.split(SEPARATOR)).filter(s -> !s.isEmpty()).toArray(String[]::new);
  }

  private static String[] toParentDirs(int count) {
    return IntStream.range(0, count).mapToObj(i -> PARENT_DIR).toArray(String[]::new);
  }

  private static String[] commonPrefix(String[] path1, String[] path2) {
    int minLength = path1.length < path2.length ? path1.length : path2.length;

    ArrayList<String> match = new ArrayList<>();
    for (int i = 0; i < minLength; i++)
      if (!path1[i].equals(path2[i])) break;
      else match.add(path1[i]);

    return match.toArray(new String[0]);
  }
}
