From 9b95850e86c03d3eb0e6c59cff2796874a052655 Mon Sep 17 00:00:00 2001
From: "Timothy J. Warren" <tim@timshome.page>
Date: Mon, 20 Nov 2023 11:12:22 -0500
Subject: [PATCH] Basic vertical scrolling

---
 justfile             |  3 +++
 src/common/editor.ts | 21 ++++++++++++++++++++-
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/justfile b/justfile
index 6e93f6c..594782c 100644
--- a/justfile
+++ b/justfile
@@ -2,6 +2,9 @@
 default:
 	@just --list
 
+# Test coverage
+coverage: bun-test deno-coverage
+
 # Typescript checking
 check: deno-check bun-check
 
diff --git a/src/common/editor.ts b/src/common/editor.ts
index 88114e3..0c428be 100644
--- a/src/common/editor.ts
+++ b/src/common/editor.ts
@@ -20,6 +20,10 @@ export class Editor {
 	 * @private
 	 */
 	#cursor: IPoint;
+	/**
+	 * The current scrolling offset
+	 */
+	#offset: IPoint;
 	/**
 	 * The document being edited
 	 * @private
@@ -32,6 +36,10 @@ export class Editor {
 			x: 0,
 			y: 0,
 		};
+		this.#offset = {
+			x: 0,
+			y: 0,
+		};
 
 		this.#document = Document.empty();
 	}
@@ -114,6 +122,15 @@ export class Editor {
 		}
 	}
 
+	private scroll(): void {
+		if (this.#cursor.y < this.#offset.y) {
+			this.#offset.y = this.#cursor.y;
+		}
+		if (this.#cursor.y >= this.#offset.y + this.#screen.rows) {
+			this.#offset.y = this.#cursor.y - this.#screen.rows + 1;
+		}
+	}
+
 	// --------------------------------------------------------------------------
 	// Terminal Output / Drawing
 	// --------------------------------------------------------------------------
@@ -122,6 +139,7 @@ export class Editor {
 	 * Clear the screen and write out the buffer
 	 */
 	public async refreshScreen(): Promise<void> {
+		this.scroll();
 		this.#buffer.append(Ansi.HideCursor);
 		this.#buffer.append(Ansi.ResetCursor);
 		this.drawRows();
@@ -142,7 +160,8 @@ export class Editor {
 
 	private drawRows(): void {
 		for (let y = 0; y < this.#screen.rows; y++) {
-			if (y >= this.#document.numRows) {
+			let filerow = y + this.#offset.y;
+			if (filerow >= this.#document.numRows) {
 				this.drawPlaceholderRow(y);
 			} else {
 				this.drawFileRow(y);