Support lts/-n aliases

README.md

@@ -39,7 +39,7 @@ The `node-version` input supports the following syntax:
 
 major versions: `12`, `14`, `16`
 more specific versions: `10.15`, `14.2.0`, `16.3.0`
-nvm lts syntax: `lts/erbium`, `lts/fermium`, `lts/*`
+nvm lts syntax: `lts/erbium`, `lts/fermium`, `lts/*`, `lts/-n`
 
 ## Caching global packages data
 

__tests__/installer.test.ts

@@ -77,9 +77,9 @@ describe('setup-node', () => {
     authSpy.mockImplementation(() => {});
 
     // gets
-    getManifestSpy.mockImplementation(
-      () => <tc.IToolRelease[]>nodeTestManifest
-    );
+    getManifestSpy.mockImplementation(() => [
+      ...(<tc.IToolRelease[]>nodeTestManifest)
+    ]);
     getDistSpy.mockImplementation(() => <im.INodeVersion>nodeTestDist);
 
     // writes
@@ -839,6 +839,76 @@ describe('setup-node', () => {
       );
     });
 
+    it('find latest LTS version and resolve it from local cache (lts/-2)', async () => {
+      // arrange
+      inputs['node-version'] = 'lts/-2';
+
+      const toolPath = path.normalize('/cache/node/12.16.2/x64');
+      findSpy.mockReturnValue(toolPath);
+
+      // act
+      await main.run();
+
+      // assert
+      expect(logSpy).toHaveBeenCalledWith(
+        'Attempt to resolve LTS alias from manifest...'
+      );
+      expect(dbgSpy).toHaveBeenCalledWith(
+        'Getting manifest from actions/node-versions@main'
+      );
+      expect(dbgSpy).not.toHaveBeenCalledWith('No manifest cached');
+      expect(dbgSpy).toHaveBeenCalledWith(
+        `LTS alias '-2' for Node version 'lts/-2'`
+      );
+      expect(dbgSpy).toHaveBeenCalledWith(
+        `Found LTS release '12.16.2' for Node version 'lts/-2'`
+      );
+      expect(logSpy).toHaveBeenCalledWith(`Found in cache @ ${toolPath}`);
+      expect(cnSpy).toHaveBeenCalledWith(
+        `::add-path::${path.join(toolPath, 'bin')}${osm.EOL}`
+      );
+    });
+
+    it('find latest LTS version and install it from manifest (lts/-2)', async () => {
+      // arrange
+      inputs['node-version'] = 'lts/-2';
+
+      const toolPath = path.normalize('/cache/node/12.16.2/x64');
+      findSpy.mockImplementation(() => '');
+      dlSpy.mockImplementation(async () => '/some/temp/path');
+      exSpy.mockImplementation(async () => '/some/other/temp/path');
+      cacheSpy.mockImplementation(async () => toolPath);
+      const expectedUrl =
+        'https://github.com/actions/node-versions/releases/download/12.16.2-20200423.28/node-12.16.2-linux-x64.tar.gz';
+
+      // act
+      await main.run();
+
+      // assert
+      expect(logSpy).toHaveBeenCalledWith(
+        'Attempt to resolve LTS alias from manifest...'
+      );
+      expect(dbgSpy).toHaveBeenCalledWith(
+        'Getting manifest from actions/node-versions@main'
+      );
+      expect(dbgSpy).not.toHaveBeenCalledWith('No manifest cached');
+      expect(dbgSpy).toHaveBeenCalledWith(
+        `LTS alias '-2' for Node version 'lts/-2'`
+      );
+      expect(dbgSpy).toHaveBeenCalledWith(
+        `Found LTS release '12.16.2' for Node version 'lts/-2'`
+      );
+      expect(logSpy).toHaveBeenCalledWith('Attempting to download 12...');
+      expect(logSpy).toHaveBeenCalledWith(
+        `Acquiring 12.16.2 - ${os.arch} from ${expectedUrl}`
+      );
+      expect(logSpy).toHaveBeenCalledWith('Extracting ...');
+      expect(logSpy).toHaveBeenCalledWith('Adding to the cache ...');
+      expect(cnSpy).toHaveBeenCalledWith(
+        `::add-path::${path.join(toolPath, 'bin')}${osm.EOL}`
+      );
+    });
+
     it('fail with unable to parse LTS alias (lts/)', async () => {
       // arrange
       inputs['node-version'] = 'lts/';

src/installer.ts

@@ -45,6 +45,7 @@ export async function getNode(
 
     // No try-catch since it's not possible to resolve LTS alias without manifest
     manifest = await getManifest(auth);
+    manifest.reverse();
 
     versionSpec = resolveLtsAliasFromManifest(versionSpec, stable, manifest);
   }
@@ -216,13 +217,20 @@ function resolveLtsAliasFromManifest(
 
   core.debug(`LTS alias '${alias}' for Node version '${versionSpec}'`);
 
-  // Supported formats are `lts/<alias>` and `lts/*`. Where asterisk means highest possible LTS.
+  // Supported formats are `lts/<alias>`, `lts/*`, and `lts/-n`. Where asterisk means highest possible LTS and -n means the nth-highest.
+  const n = Number(alias);
+  const aliases = Object.fromEntries(
+    manifest
+      .filter(x => x.stable === stable)
+      .map(x => [x.lts?.toLowerCase(), x])
+  );
+  const numbered = Object.values(aliases);
   const release =
     alias === '*'
-      ? manifest.find(x => !!x.lts && x.stable === stable)
-      : manifest.find(
-          x => x.lts?.toLowerCase() === alias && x.stable === stable
-        );
+      ? numbered[numbered.length - 1]
+      : n < 0
+      ? numbered[numbered.length - 1 + n]
+      : aliases[alias];
 
   if (!release) {
     throw new Error(