fix: handle last empty line correctly when using update/overwrite

When the string passed to update or overwrite ends with a new line, the mappings will contain two entries for column zero. This happens because the loop does not bail if the last line is empty, filling two lines with the same mapping. This commit fixes that.
Fixes #273

src/utils/Mappings.js

@@ -12,9 +12,12 @@ export default class Mappings {
 
 	addEdit(sourceIndex, content, loc, nameIndex) {
 		if (content.length) {
+			const contentLengthMinusOne = content.length - 1;
 			let contentLineEnd = content.indexOf('\n', 0);
 			let previousContentLineEnd = -1;
-			while (contentLineEnd >= 0) {
+			// Loop through each line in the content and add a segment, but stop if the last line is empty,
+			// else code afterwards would fill one line too many
+			while (contentLineEnd >= 0 && contentLengthMinusOne > contentLineEnd) {
 				const segment = [this.generatedCodeColumn, sourceIndex, loc.line, loc.column];
 				if (nameIndex >= 0) {
 					segment.push(nameIndex);

test/MagicString.js

@@ -491,6 +491,43 @@ describe('MagicString', () => {
 			assert.equal(loc4.line, 1);
 			assert.equal(loc4.column, 4);
 		});
+
+		it('generates a correct source map with update using content ending with a new line', () => {
+			const s = new MagicString('foobar');
+			s.update(2, 3, 'od\n');
+			s.update(4, 5, 'a\nnd\n');
+			assert.equal(s.toString(), 'food\nba\nnd\nr');
+
+			const map = s.generateMap({ hires: true });
+			const smc = new SourceMapConsumer(map);
+
+			// od\n
+			const loc = smc.originalPositionFor({ line: 1, column: 3 });
+			assert.equal(loc.line, 1);
+			assert.equal(loc.column, 2);
+			const loc2 = smc.originalPositionFor({ line: 1, column: 4 });
+			assert.equal(loc2.line, 1);
+			assert.equal(loc2.column, 2);
+			const loc3 = smc.originalPositionFor({ line: 2, column: 0 });
+			assert.equal(loc3.line, 1);
+			assert.equal(loc3.column, 3);
+			const loc4 = smc.originalPositionFor({ line: 2, column: 1 });
+			assert.equal(loc4.line, 1);
+			assert.equal(loc4.column, 4);
+			// a\nnd\n
+			const loc5 = smc.originalPositionFor({ line: 2, column: 2 });
+			assert.equal(loc5.line, 1);
+			assert.equal(loc5.column, 4);
+			const loc6 = smc.originalPositionFor({ line: 2, column: 3 });
+			assert.equal(loc6.line, 1);
+			assert.equal(loc6.column, 4);
+			const loc7 = smc.originalPositionFor({ line: 3, column: 0 });
+			assert.equal(loc7.line, 1);
+			assert.equal(loc7.column, 4);
+			const loc8 = smc.originalPositionFor({ line: 4, column: 0 });
+			assert.equal(loc8.line, 1);
+			assert.equal(loc8.column, 5);
+		});
 	});
 
 	describe('getIndentString', () => {