RangeUtils.java

package com.kickstarter.libs.utils;

import android.support.annotation.NonNull;

import com.kickstarter.libs.Range;

import java.util.ArrayList;
import java.util.List;

public final class RangeUtils {
  private RangeUtils() {}

  /**
   * Given a list of integers computes a list of consecutive, monotonically non-descreasing ranges.
   */
  public static @NonNull List<Range> consecutiveRanges(final @NonNull List<Integer> xs) {
    final List<Range> result = new ArrayList<>();

    for (int idx = 0; idx < xs.size(); idx++) {
      final Integer previous;
      final Integer current;
      if (idx == 0) {
        previous = null;
        current = xs.get(idx);
      } else {
        previous = xs.get(idx - 1);
        current = xs.get(idx);
      }

      if (previous == null) {
        result.add(Range.create(0, 1));
      } else if (current == previous + 1 || current.equals(previous)) {
        final int lastIdx = result.size() - 1;
        final Range lastRange = result.get(lastIdx);
        result.set(lastIdx, Range.create(lastRange.start, lastRange.length + 1));
      } else {
        result.add(Range.create(idx, 1));
      }
    }

    return result;
  }

  public static @NonNull List<Range> positionalRanges(final @NonNull List<Integer> positions) {
    final Integer firstPosition;
    if (positions.size() == 0) {
      return new ArrayList<>();
    }
    firstPosition = positions.get(0);

    final List<Range> ranges = RangeUtils.consecutiveRanges(positions);
    final List<Range> result = new ArrayList<>();
    for (final Range range : ranges) {
      result.add(Range.create(range.start + firstPosition, range.length));
    }

    return result;
  }
}