1 files changed, 126 insertions, 0 deletions
diff --git a/src/mailurl.rs b/src/mailurl.rs
new file mode 100644
index 0000000..c4285e4
--- /dev/null
+++ b/src/mailurl.rs
@@ -0,0 +1,126 @@
+//! Parsing of `imap[s]://` and `smtp[s]://` endpoint URLs.
+
+use anyhow::{anyhow, bail, Context, Result};
+
+/// A mail server endpoint: who to log in as, where, and how TLS starts.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Endpoint {
+    pub user: String,
+    pub host: String,
+    pub port: u16,
+    /// `true` for TLS-from-the-start (ports 993/465); `false` for STARTTLS.
+    pub implicit_tls: bool,
+}
+
+/// Parses `imap[s]://user@host[:port]` or `smtp[s]://user@host[:port]`.
+///
+/// The user part may itself contain `@` (e.g. `imap://me@example.com@mail.example.com`);
+/// the split happens at the last `@`. Plain schemes upgrade to implicit TLS when the
+/// port is the well-known implicit one (993/465) — STARTTLS makes no sense there.
+pub fn parse_mail_url(url: &str) -> Result<Endpoint> {
+    let (rest, default_port, mut implicit_tls) = if let Some(r) = url.strip_prefix("imaps://") {
+        (r, 993, true)
+    } else if let Some(r) = url.strip_prefix("imap://") {
+        (r, 143, false)
+    } else if let Some(r) = url.strip_prefix("smtps://") {
+        (r, 465, true)
+    } else if let Some(r) = url.strip_prefix("smtp://") {
+        (r, 587, false)
+    } else {
+        bail!("URL must start with imap://, imaps://, smtp:// or smtps://  (got: {url})")
+    };
+
+    let rest = rest.trim_end_matches('/');
+    let (user, hostport) = rest
+        .rsplit_once('@')
+        .ok_or_else(|| anyhow!("URL is missing the user part (user@host): {url}"))?;
+    if user.is_empty() {
+        bail!("URL has an empty user part: {url}");
+    }
+
+    let (host, port) = match hostport.rsplit_once(':') {
+        Some((h, p)) => (
+            h.to_string(),
+            p.parse::<u16>()
+                .with_context(|| format!("invalid port in URL: {url}"))?,
+        ),
+        None => (hostport.to_string(), default_port),
+    };
+    if host.is_empty() {
+        bail!("URL has an empty host: {url}");
+    }
+
+    if port == 993 || port == 465 {
+        implicit_tls = true;
+    }
+
+    Ok(Endpoint {
+        user: user.to_string(),
+        host,
+        port,
+        implicit_tls,
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn imap_starttls_with_default_port() {
+        let ep = parse_mail_url("imap://alice@mail.example.com").unwrap();
+        assert_eq!(ep.user, "alice");
+        assert_eq!(ep.host, "mail.example.com");
+        assert_eq!(ep.port, 143);
+        assert!(!ep.implicit_tls);
+    }
+
+    #[test]
+    fn imaps_defaults_to_993_implicit() {
+        let ep = parse_mail_url("imaps://alice@imap.example.com").unwrap();
+        assert_eq!(ep.port, 993);
+        assert!(ep.implicit_tls);
+    }
+
+    #[test]
+    fn user_may_contain_at_sign() {
+        let ep = parse_mail_url("imap://alice@example.com@mail.example.com:143").unwrap();
+        assert_eq!(ep.user, "alice@example.com");
+        assert_eq!(ep.host, "mail.example.com");
+        assert_eq!(ep.port, 143);
+    }
+
+    #[test]
+    fn smtp_defaults_to_587_starttls() {
+        let ep = parse_mail_url("smtp://bob@smtp.example.com").unwrap();
+        assert_eq!(ep.port, 587);
+        assert!(!ep.implicit_tls);
+    }
+
+    #[test]
+    fn smtps_defaults_to_465_implicit() {
+        let ep = parse_mail_url("smtps://bob@smtp.example.com").unwrap();
+        assert_eq!(ep.port, 465);
+        assert!(ep.implicit_tls);
+    }
+
+    #[test]
+    fn plain_scheme_on_implicit_port_upgrades() {
+        assert!(parse_mail_url("imap://a@h:993").unwrap().implicit_tls);
+        assert!(parse_mail_url("smtp://a@h:465").unwrap().implicit_tls);
+    }
+
+    #[test]
+    fn trailing_slash_is_tolerated() {
+        let ep = parse_mail_url("imap://alice@mail.example.com/").unwrap();
+        assert_eq!(ep.host, "mail.example.com");
+    }
+
+    #[test]
+    fn rejects_unknown_scheme_and_malformed() {
+        assert!(parse_mail_url("http://a@h").is_err());
+        assert!(parse_mail_url("imap://nouser").is_err());
+        assert!(parse_mail_url("imap://@host").is_err());
+        assert!(parse_mail_url("imap://user@host:notaport").is_err());
+    }
+}