(core) hide long sequences of unchanged rows in diffs

Summary:
It can be hard to find changes, even when highlighted, in a table with many rows.  This diff replaces long sequences of unchanged rows with a row containing "..."s.

With daff, I found that it is important to do this for sequences of unchanged columns also, but not tackling that yet.

Test Plan: added test

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2666

app/common/TableData.ts

@@ -17,6 +17,16 @@ interface ColData {
   values: CellValue[];
 }
 
+/**
+ * An interface for a table with rows that may be skipped.
+ */
+export interface SkippableRows {
+  // If there may be skippable rows, return a function to test rowIds for keeping.
+  getKeepFunc(): undefined | ((rowId: number|"new") => boolean);
+  // Get a special row id which represents a skipped sequence of rows.
+  getSkipRowId(): number;
+}
+
 /**
  * TableData class to maintain a single table's data.
  *
@@ -24,7 +34,7 @@ interface ColData {
  * represent it as column-wise arrays. (An early hope was to allow use of TypedArrays, but since
  * types can be mixed, those are not used.)
  */
-export class TableData extends ActionDispatcher {
+export class TableData extends ActionDispatcher implements SkippableRows {
   private _tableId: string;
   private _isLoaded: boolean = false;
   private _fetchPromise?: Promise<void>;
@@ -156,6 +166,16 @@ export class TableData extends ActionDispatcher {
     return function(rowId: number|"new") { return rowId === "new" ? "new" : values[rowMap.get(rowId)!]; };
   }
 
+  // By default, no rows are skippable, all are kept.
+  public getKeepFunc(): undefined | ((rowId: number|"new") => boolean) {
+    return undefined;
+  }
+
+  // By default, no special row id for skip rows is needed.
+  public getSkipRowId(): number {
+    throw new Error('no skip row id defined');
+  }
+
   /**
    * Returns the list of all rowIds in this table, in unspecified and unstable order. Equivalent
    * to getColValues('id').
@@ -171,6 +191,13 @@ export class TableData extends ActionDispatcher {
     return this._rowIdCol.slice(0).sort((a, b) => a - b);
   }
 
+  /**
+   * Returns true if cells may contain multiple versions (e.g. in diffs).
+   */
+  public mayHaveVersions() {
+    return false;
+  }
+
   /**
    * Returns the list of colIds in this table, including 'id'.
    */