

function newSplice(index, removed, addedCount) {
  return {
    index: index,
    removed: removed,
    addedCount: addedCount,
  };
}

const EDIT_LEAVE = 0;
const EDIT_UPDATE = 1;
const EDIT_ADD = 2;
const EDIT_DELETE = 3;

// Note: This function is *based* on the computation of the Levenshtein
// "edit" distance. The one change is that "updates" are treated as two
// edits - not one. With Array splices, an update is really a delete
// followed by an add. By retaining this, we optimize for "keeping" the
// maximum array items in the original array. For example:
//
//   'xxxx123' -> '123yyyy'
//
// With 1-edit updates, the shortest path would be just to update all seven
// characters. With 2-edit updates, we delete 4, leave 3, and add 4. This
// leaves the substring '123' intact.
function calcEditDistances(
  current,
  currentStart,
  currentEnd,
  old,
  oldStart,
  oldEnd
) {
  // "Deletion" columns
  let rowCount = oldEnd - oldStart + 1;
  let columnCount = currentEnd - currentStart + 1;
  let distances = new Array(rowCount);

  // "Addition" rows. Initialize null column.
  for (let i = 0; i < rowCount; i++) {
    distances[i] = new Array(columnCount);
    distances[i][0] = i;
  }

  // Initialize null row
  for (let j = 0; j < columnCount; j++) {
    distances[0][j] = j;
  }

  for (let i = 1; i < rowCount; i++) {
    for (let j = 1; j < columnCount; j++) {
      if (equals(current[currentStart + j - 1], old[oldStart + i - 1])) {
        distances[i][j] = distances[i - 1][j - 1];
      } else {
        let north = distances[i - 1][j] + 1;
        let west = distances[i][j - 1] + 1;
        distances[i][j] = north < west ? north : west;
      }
    }
  }

  return distances;
}

// This starts at the final weight, and walks "backward" by finding
// the minimum previous weight recursively until the origin of the weight
// matrix.
function spliceOperationsFromEditDistances(distances) {
  let i = distances.length - 1;
  let j = distances[0].length - 1;
  let current = distances[i][j];
  let edits = [];
  while (i > 0 || j > 0) {
    if (i == 0) {
      edits.push(EDIT_ADD);
      j--;
      continue;
    }
    if (j == 0) {
      edits.push(EDIT_DELETE);
      i--;
      continue;
    }
    let northWest = distances[i - 1][j - 1];
    let west = distances[i - 1][j];
    let north = distances[i][j - 1];

    let min;
    if (west < north) {
      min = west < northWest ? west : northWest;
    } else {
      min = north < northWest ? north : northWest;
    }

    if (min == northWest) {
      if (northWest == current) {
        edits.push(EDIT_LEAVE);
      } else {
        edits.push(EDIT_UPDATE);
        current = northWest;
      }
      i--;
      j--;
    } else if (min == west) {
      edits.push(EDIT_DELETE);
      i--;
      current = west;
    } else {
      edits.push(EDIT_ADD);
      j--;
      current = north;
    }
  }

  edits.reverse();
  return edits;
}

/**
 * Splice Projection functions:
 *
 * A splice map is a representation of how a previous array of items
 * was transformed into a new array of items. Conceptually it is a list of
 * tuples of
 *
 *   <index, removed, addedCount>
 *
 * which are kept in ascending index order of. The tuple represents that at
 * the |index|, |removed| sequence of items were removed, and counting forward
 * from |index|, |addedCount| items were added.
 */

/**
 * Lacking individual splice mutation information, the minimal set of
 * splices can be synthesized given the previous state and final state of an
 * array. The basic approach is to calculate the edit distance matrix and
 * choose the shortest path through it.
 *
 * Complexity: O(l * p)
 *   l: The length of the current array
 *   p: The length of the old array
 */
function calcSplices(current, currentStart, currentEnd, old, oldStart, oldEnd) {
  let prefixCount = 0;
  let suffixCount = 0;
  let splice;

  let minLength = Math.min(currentEnd - currentStart, oldEnd - oldStart);
  if (currentStart == 0 && oldStart == 0) {
    prefixCount = sharedPrefix(current, old, minLength);
  }

  if (currentEnd == current.length && oldEnd == old.length) {
    suffixCount = sharedSuffix(current, old, minLength - prefixCount);
  }

  currentStart += prefixCount;
  oldStart += prefixCount;
  currentEnd -= suffixCount;
  oldEnd -= suffixCount;

  if (currentEnd - currentStart == 0 && oldEnd - oldStart == 0) {
    return [];
  }

  if (currentStart == currentEnd) {
    splice = newSplice(currentStart, [], 0);
    while (oldStart < oldEnd) {
      splice.removed.push(old[oldStart++]);
    }

    return [splice];
  } else if (oldStart == oldEnd) {
    return [newSplice(currentStart, [], currentEnd - currentStart)];
  }

  let ops = spliceOperationsFromEditDistances(
    calcEditDistances(current, currentStart, currentEnd, old, oldStart, oldEnd)
  );

  splice = undefined;
  let splices = [];
  let index = currentStart;
  let oldIndex = oldStart;
  for (let i = 0; i < ops.length; i++) {
    switch (ops[i]) {
      case EDIT_LEAVE:
        if (splice) {
          splices.push(splice);
          splice = undefined;
        }

        index++;
        oldIndex++;
        break;
      case EDIT_UPDATE:
        if (!splice) {
          splice = newSplice(index, [], 0);
        }

        splice.addedCount++;
        index++;

        splice.removed.push(old[oldIndex]);
        oldIndex++;
        break;
      case EDIT_ADD:
        if (!splice) {
          splice = newSplice(index, [], 0);
        }

        splice.addedCount++;
        index++;
        break;
      case EDIT_DELETE:
        if (!splice) {
          splice = newSplice(index, [], 0);
        }

        splice.removed.push(old[oldIndex]);
        oldIndex++;
        break;
    }
  }

  if (splice) {
    splices.push(splice);
  }
  return splices;
}

function sharedPrefix(current, old, searchLength) {
  for (let i = 0; i < searchLength; i++) {
    if (!equals(current[i], old[i])) {
      return i;
    }
  }
  return searchLength;
}

function sharedSuffix(current, old, searchLength) {
  let index1 = current.length;
  let index2 = old.length;
  let count = 0;
  while (count < searchLength && equals(current[--index1], old[--index2])) {
    count++;
  }

  return count;
}

function equals(currentValue, previousValue) {
  return currentValue === previousValue;
}

export function calculateSplices(current, previous) {
  return calcSplices(current, 0, current.length, previous, 0, previous.length);
}
