feat: try Unpaywall for open-access PDFs before LibGen

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main.rs

@@ -89,13 +89,19 @@ fn http_client() -> anyhow::Result<reqwest::blocking::Client> {
 
 /// Download a paper PDF by DOI.
 ///
-/// Tries LibGen first (free, no JS challenge).  If that fails and an Anna's
-/// Archive API key is configured via `ANNAS_ARCHIVE_KEY`, tries the fast
-/// download API as a fallback.
+/// Tries Unpaywall first (free open-access, requires `UNPAYWALL_EMAIL`).
+/// Falls back to LibGen (free, no JS challenge), then Anna's Archive if an
+/// API key is configured via `ANNAS_ARCHIVE_KEY`.
 fn download_pdf(doi: &str) -> anyhow::Result<Vec<u8>> {
     let client = http_client()?;
 
-    // Try LibGen first.
+    // Try Unpaywall first (free open-access).
+    match download_via_unpaywall(&client, doi) {
+        Ok(bytes) => return Ok(bytes),
+        Err(e) => eprintln!("Unpaywall: {e:#}"),
+    }
+
+    // Try LibGen.
     match download_via_libgen(&client, doi) {
         Ok(bytes) => return Ok(bytes),
         Err(e) => eprintln!("LibGen failed: {e:#}"),
@@ -276,6 +282,50 @@ fn download_via_annas_archive(
     Ok(bytes.to_vec())
 }
 
+// -- Unpaywall ---------------------------------------------------------------
+
+/// Try downloading an open-access PDF via the Unpaywall API.
+fn download_via_unpaywall(
+    client: &reqwest::blocking::Client,
+    doi: &str,
+) -> anyhow::Result<Vec<u8>> {
+    let email = std::env::var("UNPAYWALL_EMAIL")
+        .context("UNPAYWALL_EMAIL not set")?;
+
+    eprintln!("Checking Unpaywall for open-access PDF…");
+
+    let api_url = format!(
+        "https://api.unpaywall.org/v2/{doi}?email={email}"
+    );
+
+    let resp: serde_json::Value = client
+        .get(&api_url)
+        .send()
+        .context("failed to call Unpaywall API")?
+        .error_for_status()
+        .context("Unpaywall API returned an error status")?
+        .json()
+        .context("failed to parse Unpaywall API response")?;
+
+    let pdf_url = resp
+        .get("best_oa_location")
+        .and_then(|loc| loc.get("url_for_pdf"))
+        .and_then(|u| u.as_str())
+        .context("no open-access PDF available via Unpaywall")?;
+
+    eprintln!("Downloading open-access PDF from {pdf_url}");
+    let bytes = client
+        .get(pdf_url)
+        .send()
+        .context("failed to download from Unpaywall PDF URL")?
+        .error_for_status()
+        .context("Unpaywall PDF URL returned an error status")?
+        .bytes()?;
+
+    validate_pdf(&bytes)?;
+    Ok(bytes.to_vec())
+}
+
 // -- Helpers ----------------------------------------------------------------
 
 fn validate_pdf(bytes: &[u8]) -> anyhow::Result<()> {