Add basic implementation

src/index.ts

@@ -1,10 +1,91 @@
-import * as core from '@actions/core';
+import * as core from "@actions/core";
+import * as tc from "@actions/tool-cache";
+import * as semver from "semver";
+import { Octokit } from "@octokit/rest";
+
+class Release {
+  version: string;
+  downloadUrl: string;
+
+  constructor(version: string, downloadUrl: string) {
+    this.version = version;
+    this.downloadUrl = downloadUrl;
+  }
+}
+
+async function getRelease(versionSpec?: string): Promise<Release | undefined> {
+  const octokit = new Octokit();
+  return octokit
+    .paginate(
+      octokit.repos.listReleases,
+      { owner: "casey", repo: "just" },
+      (response, done) => {
+        const releases = response.data
+          .map((rel) => {
+            const asset = rel.assets.find((ass) =>
+              ass.name.includes(process.platform)
+            );
+            if (asset) {
+              return new Release(
+                rel.tag_name.replace(/^v/, ""),
+                asset.browser_download_url
+              );
+            }
+          })
+          .filter((rel) =>
+            rel && versionSpec
+              ? semver.satisfies(rel.version, versionSpec)
+              : true
+          );
+        if (releases && done) {
+          done();
+        }
+        return releases;
+      }
+    )
+    .then((releases) => releases.find(Boolean));
+}
+
+async function checkOrInstallTool(
+  toolName: string,
+  versionSpec?: string
+): Promise<string> {
+  // first check if we have previously donwloaded the tool
+  const cache = tc.find(toolName, versionSpec || "*");
+  if (cache) {
+    return cache;
+  }
+
+  // find the latest release by querying GitHub API
+  const release = await getRelease(versionSpec);
+  if (release === undefined) {
+    throw new Error(
+      `no release for ${toolName} matching version spec ${versionSpec}`
+    );
+  }
+
+  // download, extract and cache the tool
+  core.info(`Download from "${release.downloadUrl}"`);
+  const artifact = await tc.downloadTool(release.downloadUrl);
+
+  core.info("Extract downloaded archive");
+  const dir = `./just-${release.version}`;
+
+  let extractDir;
+  if (release.downloadUrl.endsWith(".zip")) {
+    extractDir = await tc.extractZip(artifact, dir);
+  } else {
+    extractDir = await tc.extractTar(artifact, dir);
+  }
+
+  return tc.cacheDir(extractDir, "just", release.version);
+}
 
 async function main() {
   try {
-    const version: string | null = core.getInput('version') || null;
-    // TODO
-    core.info(`[setup-just] installed just v${version}`);
+    const version = core.getInput("version");
+    const cacheDir = await checkOrInstallTool("just", version);
+    core.addPath(cacheDir);
   } catch (err) {
     core.setFailed(err.message);
   }