piem: Add helper to construct message ID link

There are two spots that use (piem-inbox-get :url ...) and
piem-escape-mid to construct the public-inbox link, and there is about
to be another.  Extract this shared logic.

Cc: Xinglu Chen <public@yoctocell.xyz>
Message-Id: <20210207075738.8752-2-kyle@kyleam.com>

3 files changed, 32 insertions, 12 deletions

diff --git a/piem-notmuch.el b/piem-notmuch.el
index a222f3f..2a5c525 100644
--- a/piem-notmuch.el
+++ b/piem-notmuch.el
@@ -109,13 +109,9 @@ message itself if it looks like a patch."
 (defun piem-notmuch-show-get-public-inbox-link (mid)
   "Given the message-id MID, return the public-inbox url.
 This will lookup the url in the `piem-inboxes' variable."
-  (let* ((inbox (or (piem-notmuch-get-inbox)
-                    (user-error "No inbox associated with current buffer")))
-         (link (or (piem-inbox-get :url inbox)
-                   (user-error "No url was found for %s" inbox))))
-    (concat
-     (piem--ensure-trailing-slash link)
-     (piem-escape-mid mid))))
+  (piem-mid-url mid
+                (or (piem-notmuch-get-inbox)
+                    (user-error "No inbox associated with current buffer"))))
 
 ;;;###autoload
 (define-minor-mode piem-notmuch-mode
diff --git a/piem.el b/piem.el
index b66894f..24d252e 100644
--- a/piem.el
+++ b/piem.el
@@ -444,7 +444,7 @@ buffer."
         nil))))
 
 
-;;;; Download helpers
+;;;; Link handling
 
 (defconst piem--unreserved-chars
   (append url-unreserved-chars
@@ -456,6 +456,20 @@ buffer."
   "Escape MID for use in path part of a public-inbox URL."
   (url-hexify-string mid piem--unreserved-chars))
 
+(defun piem-mid-url (mid &optional inbox)
+  "Return a public-inbox URL for MID.
+The URL is determined by INBOX's entry in `piem-inboxes'.  If
+INBOX is nil, use the inbox returned by `piem-inbox'."
+  (concat
+   (piem--ensure-trailing-slash
+    (or (piem-inbox-get :url inbox)
+        (user-error "Couldn't find URL for %s"
+                    (or inbox "current buffer"))))
+   (piem-escape-mid mid)))
+
+
+;;;; Download helpers
+
 (defvar piem--has-gunzip)
 (defun piem-check-gunzip ()
   "Return non-nil if gunzip is available."
@@ -550,10 +564,7 @@ This function depends on :url being configured for entries in
      "`piem-maildir-directory' does not look like a Maildir directory"))
    ((not (or message-only (piem-check-gunzip)))
     (user-error "gunzip executable not found")))
-  (when-let ((url (concat (or (piem-inbox-get :url)
-                              (user-error
-                               "Could not find inbox URL for current buffer"))
-                          (piem-escape-mid mid)
+  (when-let ((url (concat (piem-mid-url mid)
                           (if message-only "/raw" "/t.mbox.gz")))
              (buffer (url-retrieve-synchronously url 'silent)))
     (unwind-protect
diff --git a/tests/piem-tests.el b/tests/piem-tests.el
index 795686a..969c9d0 100644
--- a/tests/piem-tests.el
+++ b/tests/piem-tests.el
@@ -49,6 +49,19 @@
   (should (equal (piem-escape-mid "msg@id") "msg@id"))
   (should (equal (piem-escape-mid "m/g@id") "m%2Fg@id")))
 
+(ert-deftest piem-mid-url ()
+  (let ((piem-inboxes '(("inbox-a" :url "https://example.com/a/")
+                        ("inbox-b" :url "https://example.com/b/"))))
+    (should (equal (piem-mid-url "msg@id" "inbox-a")
+                   "https://example.com/a/msg@id"))
+    (should (equal (piem-mid-url "m/sg@id" "inbox-b")
+                   "https://example.com/b/m%2Fsg@id"))
+    (should-error (piem-mid-url "msg@id")
+                  :type 'user-error))
+  (let ((piem-inboxes '(("inbox-a"))))
+    (should-error (piem-mid-url "msg@id" "inbox-a")
+                  :type 'user-error)))
+
 (ert-deftest piem-name-branch-who-what-v ()
   (should (equal (piem-name-branch-who-what-v
                   (list :from "Foo Bar <f@example.com>"