Support short version syntax

Author: eregon

.github/workflows/test.yml

@@ -6,7 +6,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ 'ubuntu-16.04', 'ubuntu-18.04', 'macos-latest' ]
-        ruby: [ '2.4.9', '2.5.7', '2.6.5', '2.7.0', 'truffleruby', 'jruby' ]
+        ruby: [ 'ruby-2.4', '2.5', 'ruby-2.6.5', '2.7.0', 'truffleruby', 'jruby' ]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2

README.md

@@ -26,7 +26,7 @@ jobs:
     - uses: actions/checkout@v2
     - uses: eregon/use-ruby-action@master
       with:
-        ruby-version: ruby-2.6.5
+        ruby-version: ruby-2.6
     - run: ruby -v
 ```
 
@@ -41,7 +41,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ 'ubuntu-latest', 'macos-latest' ]
-        ruby: [ 'ruby-2.6.5', 'ruby-2.7.0', 'truffleruby-19.3.0', 'jruby-9.2.9.0' ]
+        ruby: [ 'ruby-2.6', 'ruby-2.7', 'truffleruby-19.3.0', 'jruby-9.2.9.0' ]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2
@@ -51,10 +51,12 @@ jobs:
     - run: ruby -v
 ```
 
-If a specific version is not given, it uses the latest stable release of that implementation.
-For instance `truffleruby` is currently the same as `truffleruby-19.3.0`.
+### Supported Version Syntax
 
-If it's just a version number (e.g., `2.6.5`), then MRI is assumed (same as `ruby-2.6.5`).
+* `engine-version` like `ruby-2.6.5` and `truffleruby-19.3.0`
+* short version like `2.6`, automatically using the latest release matching that version (`2.6.5`)
+* version only like `2.6.5`, assumes MRI for the engine
+* engine only like `truffleruby`, uses the latest stable release of that implementation
 
 ### All Stable Versions
 
@@ -69,7 +71,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ 'ubuntu-latest', 'macos-latest' ]
-        ruby: [ '2.4.9', '2.5.7', '2.6.5', '2.7.0', 'truffleruby', 'jruby' ]
+        ruby: [ '2.4', '2.5', '2.6', '2.7', 'truffleruby', 'jruby' ]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2

dist/index.js

@@ -37,17 +37,35 @@ async function getLatestReleaseTag() {
 }
 
 async function getRubyEngineAndVersion(rubyVersion) {
+  let engine, version
   if (rubyVersion.match(/^\d+/)) { // X.Y.Z => ruby-X.Y.Z
-    return 'ruby-' + rubyVersion
+    engine = 'ruby'
+    version = rubyVersion
   } else if (!rubyVersion.includes('-')) { // myruby -> myruby-stableVersion
-    const engine = rubyVersion
-    const response = await axios.get(`${metadataURL}/versions.json`)
-    const stableVersions = response.data[engine]
-    const latestStableVersion = stableVersions[stableVersions.length-1]
-    return engine + '-' + latestStableVersion
-  } else {
-    return rubyVersion
+    engine = rubyVersion
+    version = '' // Let the logic below find the version
+  } else { // engine-X.Y.Z
+    [engine, version] = rubyVersion.split('-', 2)
+  }
+
+  const response = await axios.get(`${metadataURL}/versions.json`)
+  const stableVersions = response.data
+  const engineVersions = stableVersions[engine]
+  if (!engineVersions) {
+    throw new Error(`Unknown engine ${engine} (${rubyVersion})`)
   }
+
+  if (!engineVersions.includes(version)) {
+    engineVersions.reverse() // inplace!
+    let found = engineVersions.find(v => v.startsWith(version))
+    if (found) {
+      version = found
+    } else {
+      throw new Error(`Unknown version ${version} (${rubyVersion})`)
+    }
+  }
+
+  return engine + '-' + version
 }
 
 function getVirtualEnvironmentName() {