1 files changed, 39 insertions, 0 deletions

diff --git a/packages/turbo-ignore/src/getComparison.ts b/packages/turbo-ignore/src/getComparison.ts
new file mode 100644
index 0000000..a2ad61a
--- /dev/null
+++ b/packages/turbo-ignore/src/getComparison.ts
@@ -0,0 +1,39 @@
+import { info } from "./logger";
+import { TurboIgnoreArgs } from "./types";
+
+export interface GetComparisonArgs extends TurboIgnoreArgs {
+  // the workspace to check for changes
+  workspace: string;
+  // A ref/head to compare against if no previously deployed SHA is available
+  fallback?: string;
+}
+
+export function getComparison(args: GetComparisonArgs): {
+  ref: string;
+  type: "previousDeploy" | "headRelative" | "customFallback";
+} | null {
+  const { fallback, workspace } = args;
+  if (process.env.VERCEL === "1") {
+    if (process.env.VERCEL_GIT_PREVIOUS_SHA) {
+      // use the commit SHA of the last successful deployment for this project / branch
+      info(
+        `Found previous deployment ("${process.env.VERCEL_GIT_PREVIOUS_SHA}") for "${workspace}" on branch "${process.env.VERCEL_GIT_COMMIT_REF}"`
+      );
+      return {
+        ref: process.env.VERCEL_GIT_PREVIOUS_SHA,
+        type: "previousDeploy",
+      };
+    } else {
+      info(
+        `No previous deployments found for "${workspace}" on branch "${process.env.VERCEL_GIT_COMMIT_REF}".`
+      );
+      if (fallback) {
+        info(`Falling back to ref ${fallback}`);
+        return { ref: fallback, type: "customFallback" };
+      }
+
+      return null;
+    }
+  }
+  return { ref: "HEAD^", type: "headRelative" };
+}