/**
* 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;
}