Implement merging lines of text from either end of a row

src/common/document.ts

@@ -67,12 +67,34 @@ export class Document {
 	}
 
 	public delete(at: Position): void {
-		const row = this.row(at.y);
-		if (row === null) {
+		const len = this.numRows;
+		if (at.y >= len) {
 			return;
 		}
 
-		row.delete(at.x);
+		const row = this.row(at.y)!;
+		const mergeNextRow = at.x === row.size - 1 && at.y < len - 1;
+		const mergeIntoPrevRow = at.x === 0 && at.y > 0;
+
+		// If we are at the end of a line, and press delete,
+		// add the contents of the next row, and delete
+		// the merged row object
+		if (mergeNextRow) {
+			// At the end of a line, pressing delete will merge
+			// the next line into the current on
+			const rowToAppend = this.#rows.at(at.y + 1)!.toString();
+			row.append(rowToAppend);
+			this.deleteRow(at.y + 1);
+		} else if (mergeIntoPrevRow) {
+			// At the beginning of a line, merge the current line
+			// into the previous Row
+			const rowToAppend = row.toString();
+			this.#rows[at.y - 1].append(rowToAppend);
+			this.deleteRow(at.y);
+		} else {
+			row.delete(at.x);
+		}
+
 		row.updateRender();
 
 		this.dirty = true;
@@ -90,6 +112,15 @@ export class Document {
 		this.dirty = true;
 	}
 
+	/**
+	 * Delete the specified row
+	 * @param at - the index of the row to delete
+	 * @private
+	 */
+	private deleteRow(at: number): void {
+		this.#rows.splice(at, 1);
+	}
+
 	private rowsToString(): string {
 		return this.#rows.map((r) => r.toString()).join('\n');
 	}

src/common/row.ts

@@ -33,6 +33,11 @@ export class Row {
 		return this.render.slice(offset).join('');
 	}
 
+	public append(s: string): void {
+		this.chars = this.chars.concat(chars(s));
+		this.updateRender();
+	}
+
 	public insertChar(at: number, c: string): void {
 		const newSlice = Util.chars(c);
 		if (at >= this.size) {