implement minimal update logic

Author: bolinfest

src/log-update.ts

@@ -13,27 +13,63 @@ const create = (stream: Writable, {showCursor = false} = {}): LogUpdate => {
 	let previousOutput = '';
 	let hasHiddenCursor = false;
 
+	function clear(): void {
+		stream.write(ansiEscapes.eraseLines(previousLineCount));
+		previousOutput = '';
+		previousLineCount = 0;
+	}
+
 	const render = (str: string) => {
 		if (!showCursor && !hasHiddenCursor) {
 			cliCursor.hide();
 			hasHiddenCursor = true;
 		}
 
+		if (str === '') {
+			clear();
+			return;
+		}
+
 		const output = str + '\n';
 		if (output === previousOutput) {
 			return;
 		}
 
+		const prevLines = previousOutput.split('\n');
+		const newLines = output.split('\n');
+		const newLinesCount = newLines.length;
+		let updateSequence = '';
+
+		// If new output is shorter than previous output, erase the extra lines.
+		const numExtraOldLines = previousLineCount - newLinesCount;
+		if (numExtraOldLines > 0) {
+			// Note eraseLines() erases upwards.
+			updateSequence += ansiEscapes.eraseLines(numExtraOldLines);
+		}
+
+		// Move back up to the start of previous output.
+		updateSequence +=
+			ansiEscapes.cursorUp(previousLineCount) + ansiEscapes.cursorLeft;
+
+		newLines.forEach((newLine, index) => {
+			if (newLine !== prevLines[index]) {
+				updateSequence += newLine + ansiEscapes.eraseEndLine;
+			}
+
+			if (index < newLinesCount - 1) {
+				// Note this could be more efficient if we used cursorDown() to move
+				// multiple lines at once.
+				updateSequence += ansiEscapes.cursorNextLine;
+			}
+		});
+
+		stream.write(updateSequence);
+
 		previousOutput = output;
-		stream.write(ansiEscapes.eraseLines(previousLineCount) + output);
-		previousLineCount = output.split('\n').length;
+		previousLineCount = newLinesCount;
 	};
 
-	render.clear = () => {
-		stream.write(ansiEscapes.eraseLines(previousLineCount));
-		previousOutput = '';
-		previousLineCount = 0;
-	};
+	render.clear = clear;
 
 	render.done = () => {
 		previousOutput = '';