/**
 * This library modifies the diff-patch-match library by Neil Fraser
 * by removing the patch and match functionality and certain advanced
 * options in the diff function. The original license is as follows:
 *
 * ===
 *
 * Diff Match and Patch
 *
 * Copyright 2006 Google Inc.
 * http://code.google.com/p/google-diff-match-patch/
 *
 * 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.
 */


/**
 * The data structure representing a diff is an array of tuples:
 * [[DIFF_DELETE, 'Hello'], [DIFF_INSERT, 'Goodbye'], [DIFF_EQUAL, ' world.']]
 * which means: delete 'Hello', add 'Goodbye' and keep ' world.'
 */
var DIFF_DELETE = -1;
var DIFF_INSERT = 1;
var DIFF_EQUAL = 0;


/**
 * Find the differences between two texts.  Simplifies the problem by stripping
 * any common prefix or suffix off the texts before diffing.
 * @param {string} text1 Old string to be diffed.
 * @param {string} text2 New string to be diffed.
 * @param {Int} cursor_pos Expected edit position in text1 (optional)
 * @return {Array} Array of diff tuples.
 */
function diff_main(text1, text2, cursor_pos) {
  // Check for equality (speedup).
  if (text1 == text2) {
    if (text1) {
      return [[DIFF_EQUAL, text1]];
    }
    return [];
  }

  // Check cursor_pos within bounds
  if (cursor_pos < 0 || text1.length < cursor_pos) {
    cursor_pos = null;
  }

  // Trim off common prefix (speedup).
  var commonlength = diff_commonPrefix(text1, text2);
  var commonprefix = text1.substring(0, commonlength);
  text1 = text1.substring(commonlength);
  text2 = text2.substring(commonlength);

  // Trim off common suffix (speedup).
  commonlength = diff_commonSuffix(text1, text2);
  var commonsuffix = text1.substring(text1.length - commonlength);
  text1 = text1.substring(0, text1.length - commonlength);
  text2 = text2.substring(0, text2.length - commonlength);

  // Compute the diff on the middle block.
  var diffs = diff_compute_(text1, text2);

  // Restore the prefix and suffix.
  if (commonprefix) {
    diffs.unshift([DIFF_EQUAL, commonprefix]);
  }
  if (commonsuffix) {
    diffs.push([DIFF_EQUAL, commonsuffix]);
  }
  diff_cleanupMerge(diffs);
  if (cursor_pos != null) {
    diffs = fix_cursor(diffs, cursor_pos);
  }
  diffs = fix_emoji(diffs);
  return diffs;
};


/**
 * Find the differences between two texts.  Assumes that the texts do not
 * have any common prefix or suffix.
 * @param {string} text1 Old string to be diffed.
 * @param {string} text2 New string to be diffed.
 * @return {Array} Array of diff tuples.
 */
function diff_compute_(text1, text2) {
  var diffs;

  if (!text1) {
    // Just add some text (speedup).
    return [[DIFF_INSERT, text2]];
  }

  if (!text2) {
    // Just delete some text (speedup).
    return [[DIFF_DELETE, text1]];
  }

  var longtext = text1.length > text2.length ? text1 : text2;
  var shorttext = text1.length > text2.length ? text2 : text1;
  var i = longtext.indexOf(shorttext);
  if (i != -1) {
    // Shorter text is inside the longer text (speedup).
    diffs = [[DIFF_INSERT, longtext.substring(0, i)],
             [DIFF_EQUAL, shorttext],
             [DIFF_INSERT, longtext.substring(i + shorttext.length)]];
    // Swap insertions for deletions if diff is reversed.
    if (text1.length > text2.length) {
      diffs[0][0] = diffs[2][0] = DIFF_DELETE;
    }
    return diffs;
  }

  if (shorttext.length == 1) {
    // Single character string.
    // After the previous speedup, the character can't be an equality.
    return [[DIFF_DELETE, text1], [DIFF_INSERT, text2]];
  }

  // Check to see if the problem can be split in two.
  var hm = diff_halfMatch_(text1, text2);
  if (hm) {
    // A half-match was found, sort out the return data.
    var text1_a = hm[0];
    var text1_b = hm[1];
    var text2_a = hm[2];
    var text2_b = hm[3];
    var mid_common = hm[4];
    // Send both pairs off for separate processing.
    var diffs_a = diff_main(text1_a, text2_a);
    var diffs_b = diff_main(text1_b, text2_b);
    // Merge the results.
    return diffs_a.concat([[DIFF_EQUAL, mid_common]], diffs_b);
  }

  return diff_bisect_(text1, text2);
};


/**
 * Find the 'middle snake' of a diff, split the problem in two
 * and return the recursively constructed diff.
 * See Myers 1986 paper: An O(ND) Difference Algorithm and Its Variations.
 * @param {string} text1 Old string to be diffed.
 * @param {string} text2 New string to be diffed.
 * @return {Array} Array of diff tuples.
 * @private
 */
function diff_bisect_(text1, text2) {
  // Cache the text lengths to prevent multiple calls.
  var text1_length = text1.length;
  var text2_length = text2.length;
  var max_d = Math.ceil((text1_length + text2_length) / 2);
  var v_offset = max_d;
  var v_length = 2 * max_d;
  var v1 = new Array(v_length);
  var v2 = new Array(v_length);
  // Setting all elements to -1 is faster in Chrome & Firefox than mixing
  // integers and undefined.
  for (var x = 0; x < v_length; x++) {
    v1[x] = -1;
    v2[x] = -1;
  }
  v1[v_offset + 1] = 0;
  v2[v_offset + 1] = 0;
  var delta = text1_length - text2_length;
  // If the total number of characters is odd, then the front path will collide
  // with the reverse path.
  var front = (delta % 2 != 0);
  // Offsets for start and end of k loop.
  // Prevents mapping of space beyond the grid.
  var k1start = 0;
  var k1end = 0;
  var k2start = 0;
  var k2end = 0;
  for (var d = 0; d < max_d; d++) {
    // Walk the front path one step.
    for (var k1 = -d + k1start; k1 <= d - k1end; k1 += 2) {
      var k1_offset = v_offset + k1;
      var x1;
      if (k1 == -d || (k1 != d && v1[k1_offset - 1] < v1[k1_offset + 1])) {
        x1 = v1[k1_offset + 1];
      } else {
        x1 = v1[k1_offset - 1] + 1;
      }
      var y1 = x1 - k1;
      while (x1 < text1_length && y1 < text2_length &&
             text1.charAt(x1) == text2.charAt(y1)) {
        x1++;
        y1++;
      }
      v1[k1_offset] = x1;
      if (x1 > text1_length) {
        // Ran off the right of the graph.
        k1end += 2;
      } else if (y1 > text2_length) {
        // Ran off the bottom of the graph.
        k1start += 2;
      } else if (front) {
        var k2_offset = v_offset + delta - k1;
        if (k2_offset >= 0 && k2_offset < v_length && v2[k2_offset] != -1) {
          // Mirror x2 onto top-left coordinate system.
          var x2 = text1_length - v2[k2_offset];
          if (x1 >= x2) {
            // Overlap detected.
            return diff_bisectSplit_(text1, text2, x1, y1);
          }
        }
      }
    }

    // Walk the reverse path one step.
    for (var k2 = -d + k2start; k2 <= d - k2end; k2 += 2) {
      var k2_offset = v_offset + k2;
      var x2;
      if (k2 == -d || (k2 != d && v2[k2_offset - 1] < v2[k2_offset + 1])) {
        x2 = v2[k2_offset + 1];
      } else {
        x2 = v2[k2_offset - 1] + 1;
      }
      var y2 = x2 - k2;
      while (x2 < text1_length && y2 < text2_length &&
             text1.charAt(text1_length - x2 - 1) ==
             text2.charAt(text2_length - y2 - 1)) {
        x2++;
        y2++;
      }
      v2[k2_offset] = x2;
      if (x2 > text1_length) {
        // Ran off the left of the graph.
        k2end += 2;
      } else if (y2 > text2_length) {
        // Ran off the top of the graph.
        k2start += 2;
      } else if (!front) {
        var k1_offset = v_offset + delta - k2;
        if (k1_offset >= 0 && k1_offset < v_length && v1[k1_offset] != -1) {
          var x1 = v1[k1_offset];
          var y1 = v_offset + x1 - k1_offset;
          // Mirror x2 onto top-left coordinate system.
          x2 = text1_length - x2;
          if (x1 >= x2) {
            // Overlap detected.
            return diff_bisectSplit_(text1, text2, x1, y1);
          }
        }
      }
    }
  }
  // Diff took too long and hit the deadline or
  // number of diffs equals number of characters, no commonality at all.
  return [[DIFF_DELETE, text1], [DIFF_INSERT, text2]];
};


/**
 * Given the location of the 'middle snake', split the diff in two parts
 * and recurse.
 * @param {string} text1 Old string to be diffed.
 * @param {string} text2 New string to be diffed.
 * @param {number} x Index of split point in text1.
 * @param {number} y Index of split point in text2.
 * @return {Array} Array of diff tuples.
 */
function diff_bisectSplit_(text1, text2, x, y) {
  var text1a = text1.substring(0, x);
  var text2a = text2.substring(0, y);
  var text1b = text1.substring(x);
  var text2b = text2.substring(y);

  // Compute both diffs serially.
  var diffs = diff_main(text1a, text2a);
  var diffsb = diff_main(text1b, text2b);

  return diffs.concat(diffsb);
};


/**
 * Determine the common prefix of two strings.
 * @param {string} text1 First string.
 * @param {string} text2 Second string.
 * @return {number} The number of characters common to the start of each
 *     string.
 */
function diff_commonPrefix(text1, text2) {
  // Quick check for common null cases.
  if (!text1 || !text2 || text1.charAt(0) != text2.charAt(0)) {
    return 0;
  }
  // Binary search.
  // Performance analysis: http://neil.fraser.name/news/2007/10/09/
  var pointermin = 0;
  var pointermax = Math.min(text1.length, text2.length);
  var pointermid = pointermax;
  var pointerstart = 0;
  while (pointermin < pointermid) {
    if (text1.substring(pointerstart, pointermid) ==
        text2.substring(pointerstart, pointermid)) {
      pointermin = pointermid;
      pointerstart = pointermin;
    } else {
      pointermax = pointermid;
    }
    pointermid = Math.floor((pointermax - pointermin) / 2 + pointermin);
  }
  return pointermid;
};


/**
 * Determine the common suffix of two strings.
 * @param {string} text1 First string.
 * @param {string} text2 Second string.
 * @return {number} The number of characters common to the end of each string.
 */
function diff_commonSuffix(text1, text2) {
  // Quick check for common null cases.
  if (!text1 || !text2 ||
      text1.charAt(text1.length - 1) != text2.charAt(text2.length - 1)) {
    return 0;
  }
  // Binary search.
  // Performance analysis: http://neil.fraser.name/news/2007/10/09/
  var pointermin = 0;
  var pointermax = Math.min(text1.length, text2.length);
  var pointermid = pointermax;
  var pointerend = 0;
  while (pointermin < pointermid) {
    if (text1.substring(text1.length - pointermid, text1.length - pointerend) ==
        text2.substring(text2.length - pointermid, text2.length - pointerend)) {
      pointermin = pointermid;
      pointerend = pointermin;
    } else {
      pointermax = pointermid;
    }
    pointermid = Math.floor((pointermax - pointermin) / 2 + pointermin);
  }
  return pointermid;
};


/**
 * Do the two texts share a substring which is at least half the length of the
 * longer text?
 * This speedup can produce non-minimal diffs.
 * @param {string} text1 First string.
 * @param {string} text2 Second string.
 * @return {Array.<string>} Five element Array, containing the prefix of
 *     text1, the suffix of text1, the prefix of text2, the suffix of
 *     text2 and the common middle.  Or null if there was no match.
 */
function diff_halfMatch_(text1, text2) {
  var longtext = text1.length > text2.length ? text1 : text2;
  var shorttext = text1.length > text2.length ? text2 : text1;
  if (longtext.length < 4 || shorttext.length * 2 < longtext.length) {
    return null;  // Pointless.
  }

  /**
   * Does a substring of shorttext exist within longtext such that the substring
   * is at least half the length of longtext?
   * Closure, but does not reference any external variables.
   * @param {string} longtext Longer string.
   * @param {string} shorttext Shorter string.
   * @param {number} i Start index of quarter length substring within longtext.
   * @return {Array.<string>} Five element Array, containing the prefix of
   *     longtext, the suffix of longtext, the prefix of shorttext, the suffix
   *     of shorttext and the common middle.  Or null if there was no match.
   * @private
   */
  function diff_halfMatchI_(longtext, shorttext, i) {
    // Start with a 1/4 length substring at position i as a seed.
    var seed = longtext.substring(i, i + Math.floor(longtext.length / 4));
    var j = -1;
    var best_common = '';
    var best_longtext_a, best_longtext_b, best_shorttext_a, best_shorttext_b;
    while ((j = shorttext.indexOf(seed, j + 1)) != -1) {
      var prefixLength = diff_commonPrefix(longtext.substring(i),
                                           shorttext.substring(j));
      var suffixLength = diff_commonSuffix(longtext.substring(0, i),
                                           shorttext.substring(0, j));
      if (best_common.length < suffixLength + prefixLength) {
        best_common = shorttext.substring(j - suffixLength, j) +
            shorttext.substring(j, j + prefixLength);
        best_longtext_a = longtext.substring(0, i - suffixLength);
        best_longtext_b = longtext.substring(i + prefixLength);
        best_shorttext_a = shorttext.substring(0, j - suffixLength);
        best_shorttext_b = shorttext.substring(j + prefixLength);
      }
    }
    if (best_common.length * 2 >= longtext.length) {
      return [best_longtext_a, best_longtext_b,
              best_shorttext_a, best_shorttext_b, best_common];
    } else {
      return null;
    }
  }

  // First check if the second quarter is the seed for a half-match.
  var hm1 = diff_halfMatchI_(longtext, shorttext,
                             Math.ceil(longtext.length / 4));
  // Check again based on the third quarter.
  var hm2 = diff_halfMatchI_(longtext, shorttext,
                             Math.ceil(longtext.length / 2));
  var hm;
  if (!hm1 && !hm2) {
    return null;
  } else if (!hm2) {
    hm = hm1;
  } else if (!hm1) {
    hm = hm2;
  } else {
    // Both matched.  Select the longest.
    hm = hm1[4].length > hm2[4].length ? hm1 : hm2;
  }

  // A half-match was found, sort out the return data.
  var text1_a, text1_b, text2_a, text2_b;
  if (text1.length > text2.length) {
    text1_a = hm[0];
    text1_b = hm[1];
    text2_a = hm[2];
    text2_b = hm[3];
  } else {
    text2_a = hm[0];
    text2_b = hm[1];
    text1_a = hm[2];
    text1_b = hm[3];
  }
  var mid_common = hm[4];
  return [text1_a, text1_b, text2_a, text2_b, mid_common];
};


/**
 * Reorder and merge like edit sections.  Merge equalities.
 * Any edit section can move as long as it doesn't cross an equality.
 * @param {Array} diffs Array of diff tuples.
 */
function diff_cleanupMerge(diffs) {
  diffs.push([DIFF_EQUAL, '']);  // Add a dummy entry at the end.
  var pointer = 0;
  var count_delete = 0;
  var count_insert = 0;
  var text_delete = '';
  var text_insert = '';
  var commonlength;
  while (pointer < diffs.length) {
    switch (diffs[pointer][0]) {
      case DIFF_INSERT:
        count_insert++;
        text_insert += diffs[pointer][1];
        pointer++;
        break;
      case DIFF_DELETE:
        count_delete++;
        text_delete += diffs[pointer][1];
        pointer++;
        break;
      case DIFF_EQUAL:
        // Upon reaching an equality, check for prior redundancies.
        if (count_delete + count_insert > 1) {
          if (count_delete !== 0 && count_insert !== 0) {
            // Factor out any common prefixies.
            commonlength = diff_commonPrefix(text_insert, text_delete);
            if (commonlength !== 0) {
              if ((pointer - count_delete - count_insert) > 0 &&
                  diffs[pointer - count_delete - count_insert - 1][0] ==
                  DIFF_EQUAL) {
                diffs[pointer - count_delete - count_insert - 1][1] +=
                    text_insert.substring(0, commonlength);
              } else {
                diffs.splice(0, 0, [DIFF_EQUAL,
                                    text_insert.substring(0, commonlength)]);
                pointer++;
              }
              text_insert = text_insert.substring(commonlength);
              text_delete = text_delete.substring(commonlength);
            }
            // Factor out any common suffixies.
            commonlength = diff_commonSuffix(text_insert, text_delete);
            if (commonlength !== 0) {
              diffs[pointer][1] = text_insert.substring(text_insert.length -
                  commonlength) + diffs[pointer][1];
              text_insert = text_insert.substring(0, text_insert.length -
                  commonlength);
              text_delete = text_delete.substring(0, text_delete.length -
                  commonlength);
            }
          }
          // Delete the offending records and add the merged ones.
          if (count_delete === 0) {
            diffs.splice(pointer - count_insert,
                count_delete + count_insert, [DIFF_INSERT, text_insert]);
          } else if (count_insert === 0) {
            diffs.splice(pointer - count_delete,
                count_delete + count_insert, [DIFF_DELETE, text_delete]);
          } else {
            diffs.splice(pointer - count_delete - count_insert,
                count_delete + count_insert, [DIFF_DELETE, text_delete],
                [DIFF_INSERT, text_insert]);
          }
          pointer = pointer - count_delete - count_insert +
                    (count_delete ? 1 : 0) + (count_insert ? 1 : 0) + 1;
        } else if (pointer !== 0 && diffs[pointer - 1][0] == DIFF_EQUAL) {
          // Merge this equality with the previous one.
          diffs[pointer - 1][1] += diffs[pointer][1];
          diffs.splice(pointer, 1);
        } else {
          pointer++;
        }
        count_insert = 0;
        count_delete = 0;
        text_delete = '';
        text_insert = '';
        break;
    }
  }
  if (diffs[diffs.length - 1][1] === '') {
    diffs.pop();  // Remove the dummy entry at the end.
  }

  // Second pass: look for single edits surrounded on both sides by equalities
  // which can be shifted sideways to eliminate an equality.
  // e.g: A<ins>BA</ins>C -> <ins>AB</ins>AC
  var changes = false;
  pointer = 1;
  // Intentionally ignore the first and last element (don't need checking).
  while (pointer < diffs.length - 1) {
    if (diffs[pointer - 1][0] == DIFF_EQUAL &&
        diffs[pointer + 1][0] == DIFF_EQUAL) {
      // This is a single edit surrounded by equalities.
      if (diffs[pointer][1].substring(diffs[pointer][1].length -
          diffs[pointer - 1][1].length) == diffs[pointer - 1][1]) {
        // Shift the edit over the previous equality.
        diffs[pointer][1] = diffs[pointer - 1][1] +
            diffs[pointer][1].substring(0, diffs[pointer][1].length -
                                        diffs[pointer - 1][1].length);
        diffs[pointer + 1][1] = diffs[pointer - 1][1] + diffs[pointer + 1][1];
        diffs.splice(pointer - 1, 1);
        changes = true;
      } else if (diffs[pointer][1].substring(0, diffs[pointer + 1][1].length) ==
          diffs[pointer + 1][1]) {
        // Shift the edit over the next equality.
        diffs[pointer - 1][1] += diffs[pointer + 1][1];
        diffs[pointer][1] =
            diffs[pointer][1].substring(diffs[pointer + 1][1].length) +
            diffs[pointer + 1][1];
        diffs.splice(pointer + 1, 1);
        changes = true;
      }
    }
    pointer++;
  }
  // If shifts were made, the diff needs reordering and another shift sweep.
  if (changes) {
    diff_cleanupMerge(diffs);
  }
};


var diff = diff_main;
diff.INSERT = DIFF_INSERT;
diff.DELETE = DIFF_DELETE;
diff.EQUAL = DIFF_EQUAL;

module.exports = diff;

/*
 * Modify a diff such that the cursor position points to the start of a change:
 * E.g.
 *   cursor_normalize_diff([[DIFF_EQUAL, 'abc']], 1)
 *     => [1, [[DIFF_EQUAL, 'a'], [DIFF_EQUAL, 'bc']]]
 *   cursor_normalize_diff([[DIFF_INSERT, 'new'], [DIFF_DELETE, 'xyz']], 2)
 *     => [2, [[DIFF_INSERT, 'new'], [DIFF_DELETE, 'xy'], [DIFF_DELETE, 'z']]]
 *
 * @param {Array} diffs Array of diff tuples
 * @param {Int} cursor_pos Suggested edit position. Must not be out of bounds!
 * @return {Array} A tuple [cursor location in the modified diff, modified diff]
 */
function cursor_normalize_diff (diffs, cursor_pos) {
  if (cursor_pos === 0) {
    return [DIFF_EQUAL, diffs];
  }
  for (var current_pos = 0, i = 0; i < diffs.length; i++) {
    var d = diffs[i];
    if (d[0] === DIFF_DELETE || d[0] === DIFF_EQUAL) {
      var next_pos = current_pos + d[1].length;
      if (cursor_pos === next_pos) {
        return [i + 1, diffs];
      } else if (cursor_pos < next_pos) {
        // copy to prevent side effects
        diffs = diffs.slice();
        // split d into two diff changes
        var split_pos = cursor_pos - current_pos;
        var d_left = [d[0], d[1].slice(0, split_pos)];
        var d_right = [d[0], d[1].slice(split_pos)];
        diffs.splice(i, 1, d_left, d_right);
        return [i + 1, diffs];
      } else {
        current_pos = next_pos;
      }
    }
  }
  throw new Error('cursor_pos is out of bounds!')
}

/*
 * Modify a diff such that the edit position is "shifted" to the proposed edit location (cursor_position).
 *
 * Case 1)
 *   Check if a naive shift is possible:
 *     [0, X], [ 1, Y] -> [ 1, Y], [0, X]    (if X + Y === Y + X)
 *     [0, X], [-1, Y] -> [-1, Y], [0, X]    (if X + Y === Y + X) - holds same result
 * Case 2)
 *   Check if the following shifts are possible:
 *     [0, 'pre'], [ 1, 'prefix'] -> [ 1, 'pre'], [0, 'pre'], [ 1, 'fix']
 *     [0, 'pre'], [-1, 'prefix'] -> [-1, 'pre'], [0, 'pre'], [-1, 'fix']
 *         ^            ^
 *         d          d_next
 *
 * @param {Array} diffs Array of diff tuples
 * @param {Int} cursor_pos Suggested edit position. Must not be out of bounds!
 * @return {Array} Array of diff tuples
 */
function fix_cursor (diffs, cursor_pos) {
  var norm = cursor_normalize_diff(diffs, cursor_pos);
  var ndiffs = norm[1];
  var cursor_pointer = norm[0];
  var d = ndiffs[cursor_pointer];
  var d_next = ndiffs[cursor_pointer + 1];

  if (d == null) {
    // Text was deleted from end of original string,
    // cursor is now out of bounds in new string
    return diffs;
  } else if (d[0] !== DIFF_EQUAL) {
    // A modification happened at the cursor location.
    // This is the expected outcome, so we can return the original diff.
    return diffs;
  } else {
    if (d_next != null && d[1] + d_next[1] === d_next[1] + d[1]) {
      // Case 1)
      // It is possible to perform a naive shift
      ndiffs.splice(cursor_pointer, 2, d_next, d)
      return merge_tuples(ndiffs, cursor_pointer, 2)
    } else if (d_next != null && d_next[1].indexOf(d[1]) === 0) {
      // Case 2)
      // d[1] is a prefix of d_next[1]
      // We can assume that d_next[0] !== 0, since d[0] === 0
      // Shift edit locations..
      ndiffs.splice(cursor_pointer, 2, [d_next[0], d[1]], [0, d[1]]);
      var suffix = d_next[1].slice(d[1].length);
      if (suffix.length > 0) {
        ndiffs.splice(cursor_pointer + 2, 0, [d_next[0], suffix]);
      }
      return merge_tuples(ndiffs, cursor_pointer, 3)
    } else {
      // Not possible to perform any modification
      return diffs;
    }
  }
}

/*
 * Check diff did not split surrogate pairs.
 * Ex. [0, '\uD83D'], [-1, '\uDC36'], [1, '\uDC2F'] -> [-1, '\uD83D\uDC36'], [1, '\uD83D\uDC2F']
 *     '\uD83D\uDC36' === '🐶', '\uD83D\uDC2F' === '🐯'
 *
 * @param {Array} diffs Array of diff tuples
 * @return {Array} Array of diff tuples
 */
function fix_emoji (diffs) {
  var compact = false;
  var starts_with_pair_end = function(str) {
    return str.charCodeAt(0) >= 0xDC00 && str.charCodeAt(0) <= 0xDFFF;
  }
  var ends_with_pair_start = function(str) {
    return str.charCodeAt(str.length-1) >= 0xD800 && str.charCodeAt(str.length-1) <= 0xDBFF;
  }
  for (var i = 2; i < diffs.length; i += 1) {
    if (diffs[i-2][0] === DIFF_EQUAL && ends_with_pair_start(diffs[i-2][1]) &&
        diffs[i-1][0] === DIFF_DELETE && starts_with_pair_end(diffs[i-1][1]) &&
        diffs[i][0] === DIFF_INSERT && starts_with_pair_end(diffs[i][1])) {
      compact = true;

      diffs[i-1][1] = diffs[i-2][1].slice(-1) + diffs[i-1][1];
      diffs[i][1] = diffs[i-2][1].slice(-1) + diffs[i][1];

      diffs[i-2][1] = diffs[i-2][1].slice(0, -1);
    }
  }
  if (!compact) {
    return diffs;
  }
  var fixed_diffs = [];
  for (var i = 0; i < diffs.length; i += 1) {
    if (diffs[i][1].length > 0) {
      fixed_diffs.push(diffs[i]);
    }
  }
  return fixed_diffs;
}

/*
 * Try to merge tuples with their neigbors in a given range.
 * E.g. [0, 'a'], [0, 'b'] -> [0, 'ab']
 *
 * @param {Array} diffs Array of diff tuples.
 * @param {Int} start Position of the first element to merge (diffs[start] is also merged with diffs[start - 1]).
 * @param {Int} length Number of consecutive elements to check.
 * @return {Array} Array of merged diff tuples.
 */
function merge_tuples (diffs, start, length) {
  // Check from (start-1) to (start+length).
  for (var i = start + length - 1; i >= 0 && i >= start - 1; i--) {
    if (i + 1 < diffs.length) {
      var left_d = diffs[i];
      var right_d = diffs[i+1];
      if (left_d[0] === right_d[1]) {
        diffs.splice(i, 2, [left_d[0], left_d[1] + right_d[1]]);
      }
    }
  }
  return diffs;
}