feat: add 'd' key to make the displayed location the defaultHEADmain

I bound d in the weather buffer to wttrin-make-default, which sets wttrin-favorite-location to the location on screen so it drives the mode-line and future sessions. The footer advertises "[d] to make default".

Persistence rides savehist, not the Emacs custom-variable mechanism: wttrin--savehist-register registers wttrin-favorite-location alongside the search history, at load and on savehist-save-hook. Enable savehist-mode and the favorite survives restarts.

Promoting a location drops it from the search history, the way wttrin-default-locations entries are kept out of history. The favorite shows in the picker instead: wttrin--completion-candidates prepends it when it's a string and not already a default, so it appears exactly once.

The setter only assigns the variable and trims history. It doesn't register with savehist itself, because savehist-additional-variables is unbound until savehist loads, so a direct add-to-list would error for users without savehist. Registration stays on the load and save-hook path.

3 files changed, 141 insertions, 9 deletions

diff --git a/tests/test-wttrin--add-buffer-instructions.el b/tests/test-wttrin--add-buffer-instructions.el
index 0f1c382..425832d 100644
--- a/tests/test-wttrin--add-buffer-instructions.el
+++ b/tests/test-wttrin--add-buffer-instructions.el
@@ -28,7 +28,7 @@
   "Test adding instructions to empty buffer."
   (with-temp-buffer
     (wttrin--add-buffer-instructions)
-    (should (string= "\n\nPress: [a] for another location [g] to refresh [q] to quit"
+    (should (string= "\n\nPress: [a] for another location [g] to refresh [d] to make default [q] to quit"
                      (buffer-string)))))
 
 (ert-deftest test-wttrin--add-buffer-instructions-normal-with-existing-content-appends-instructions ()
@@ -36,7 +36,7 @@
   (with-temp-buffer
     (insert "Weather: Sunny\nTemperature: 20°C")
     (wttrin--add-buffer-instructions)
-    (should (string= "Weather: Sunny\nTemperature: 20°C\n\nPress: [a] for another location [g] to refresh [q] to quit"
+    (should (string= "Weather: Sunny\nTemperature: 20°C\n\nPress: [a] for another location [g] to refresh [d] to make default [q] to quit"
                      (buffer-string)))))
 
 (ert-deftest test-wttrin--add-buffer-instructions-normal-preserves-point-moves-to-end ()
@@ -99,7 +99,7 @@
     (insert "Weather data here")
     (goto-char (point-min))
     (wttrin--add-buffer-instructions)
-    (should (string-suffix-p "Press: [a] for another location [g] to refresh [q] to quit"
+    (should (string-suffix-p "Press: [a] for another location [g] to refresh [d] to make default [q] to quit"
                              (buffer-string)))))
 
 (ert-deftest test-wttrin--add-buffer-instructions-boundary-point-in-middle-appends-at-end ()
@@ -109,7 +109,7 @@
     (goto-char (point-min))
     (forward-line 1)
     (wttrin--add-buffer-instructions)
-    (should (string-suffix-p "Press: [a] for another location [g] to refresh [q] to quit"
+    (should (string-suffix-p "Press: [a] for another location [g] to refresh [d] to make default [q] to quit"
                              (buffer-string)))))
 
 (ert-deftest test-wttrin--add-buffer-instructions-boundary-trailing-newlines-preserves-newlines ()
@@ -117,7 +117,7 @@
   (with-temp-buffer
     (insert "Weather\n\n\n")
     (wttrin--add-buffer-instructions)
-    (should (string= "Weather\n\n\n\n\nPress: [a] for another location [g] to refresh [q] to quit"
+    (should (string= "Weather\n\n\n\n\nPress: [a] for another location [g] to refresh [d] to make default [q] to quit"
                      (buffer-string)))))
 
 (ert-deftest test-wttrin--add-buffer-instructions-boundary-very-large-buffer-appends-at-end ()
@@ -126,7 +126,7 @@
     (insert (make-string 10000 ?x))
     (wttrin--add-buffer-instructions)
     (goto-char (point-max))
-    (should (looking-back "Press: \\[a\\] for another location \\[g\\] to refresh \\[q\\] to quit" nil))))
+    (should (looking-back "Press: \\[a\\] for another location \\[g\\] to refresh \\[d\\] to make default \\[q\\] to quit" nil))))
 
 ;;; Error Cases
 
diff --git a/tests/test-wttrin-location-history.el b/tests/test-wttrin-location-history.el
index d23bdcd..61b495c 100644
--- a/tests/test-wttrin-location-history.el
+++ b/tests/test-wttrin-location-history.el
@@ -211,11 +211,13 @@
     (should (memq 'wttrin--location-history savehist-additional-variables))))
 
 (ert-deftest test-wttrin-location-history-boundary-savehist-register-idempotent ()
-  "Registering when already present does not duplicate the entry."
+  "Registering an already-present variable does not duplicate it."
   (require 'savehist)
-  (let ((savehist-additional-variables '(wttrin--location-history)))
+  (require 'cl-lib)
+  (let ((savehist-additional-variables '(wttrin--location-history wttrin-favorite-location)))
     (wttrin--savehist-register)
-    (should (equal '(wttrin--location-history) savehist-additional-variables))))
+    (should (= 1 (cl-count 'wttrin--location-history savehist-additional-variables)))
+    (should (= 1 (cl-count 'wttrin-favorite-location savehist-additional-variables)))))
 
 (ert-deftest test-wttrin-location-history-integration-savehist-register-on-save-hook ()
   "The registration runs on `savehist-save-hook' so it survives a clobber."
diff --git a/tests/test-wttrin-make-default.el b/tests/test-wttrin-make-default.el
new file mode 100644
index 0000000..c6f845b
--- /dev/null
+++ b/tests/test-wttrin-make-default.el
@@ -0,0 +1,130 @@
+;;; test-wttrin-make-default.el --- Tests for promote-to-default command -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2026 Craig Jennings
+
+;;; Commentary:
+
+;; Unit tests for wttrin--set-favorite-location and wttrin-make-default,
+;; the weather-buffer command (bound to "d") that promotes the displayed
+;; location to the persisted favorite.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+(require 'wttrin)
+
+;;; --------------------------------------------------------------------------
+;;; wttrin--set-favorite-location
+;;; --------------------------------------------------------------------------
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin--set-favorite-location-normal-sets-variable ()
+  "Normal: sets `wttrin-favorite-location' to the given location."
+  (let ((wttrin-favorite-location nil)
+        (savehist-additional-variables nil))
+    (wttrin--set-favorite-location "Paris, FR")
+    (should (equal wttrin-favorite-location "Paris, FR"))))
+
+(ert-deftest test-wttrin--set-favorite-location-error-no-savehist-loaded ()
+  "Error: setting the favorite works even when savehist is not loaded.
+The setter must not touch `savehist-additional-variables' directly (it may be
+unbound); persistence is left to `wttrin--savehist-register'."
+  (let ((wttrin-favorite-location nil))
+    ;; Simulate savehist absent: the variable is unbound.
+    (cl-letf (((symbol-function 'wttrin--savehist-register)
+               (lambda () (error "Should not be called from the setter"))))
+      (wttrin--set-favorite-location "Oslo, NO")
+      (should (equal wttrin-favorite-location "Oslo, NO")))))
+
+(ert-deftest test-wttrin--set-favorite-location-normal-drops-from-history ()
+  "Normal: promoting a location removes it from the search history."
+  (let ((wttrin-favorite-location nil)
+        (wttrin--location-history '("Reykjavik" "Oslo, NO")))
+    (wttrin--set-favorite-location "Reykjavik")
+    (should-not (member "Reykjavik" wttrin--location-history))
+    (should (equal wttrin--location-history '("Oslo, NO")))))
+
+(ert-deftest test-wttrin--set-favorite-location-boundary-not-in-history-is-noop ()
+  "Boundary: promoting a location absent from history leaves history intact."
+  (let ((wttrin-favorite-location nil)
+        (wttrin--location-history '("Oslo, NO")))
+    (wttrin--set-favorite-location "Berkeley, CA")
+    (should (equal wttrin--location-history '("Oslo, NO")))))
+
+(ert-deftest test-wttrin-favorite-savehist-register-includes-favorite ()
+  "Normal: `wttrin--savehist-register' registers the favorite for persistence."
+  (require 'savehist)
+  (let ((savehist-additional-variables '(kill-ring)))
+    (wttrin--savehist-register)
+    (should (memq 'wttrin-favorite-location savehist-additional-variables))))
+
+;;; --------------------------------------------------------------------------
+;;; wttrin-make-default
+;;; --------------------------------------------------------------------------
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin-make-default-normal-sets-favorite-from-current ()
+  "Normal: promotes the buffer's current location to the favorite."
+  (let ((wttrin-favorite-location nil)
+        (savehist-additional-variables nil))
+    (with-temp-buffer
+      (setq-local wttrin--current-location "Tokyo, JP")
+      (wttrin-make-default)
+      (should (equal wttrin-favorite-location "Tokyo, JP")))))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin-make-default-boundary-nil-current-leaves-favorite ()
+  "Boundary: no current location is a no-op that leaves the favorite intact."
+  (let ((wttrin-favorite-location "Berkeley, CA")
+        (savehist-additional-variables nil))
+    (with-temp-buffer
+      (setq-local wttrin--current-location nil)
+      (wttrin-make-default)
+      (should (equal wttrin-favorite-location "Berkeley, CA")))))
+
+;;; --------------------------------------------------------------------------
+;;; favorite in completion candidates
+;;; --------------------------------------------------------------------------
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin-make-default-normal-favorite-prepended-to-candidates ()
+  "Normal: a typed-in favorite is offered in the picker, at the front."
+  (let ((wttrin-default-locations '("Honolulu, HI" "Berkeley, CA"))
+        (wttrin--location-history nil)
+        (wttrin-favorite-location "Reykjavik"))
+    (should (equal (wttrin--completion-candidates)
+                   '("Reykjavik" "Honolulu, HI" "Berkeley, CA")))))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin-make-default-boundary-favorite-default-not-duplicated ()
+  "Boundary: a favorite that is already a default appears exactly once."
+  (require 'cl-lib)
+  (let ((wttrin-default-locations '("Honolulu, HI" "Berkeley, CA"))
+        (wttrin--location-history nil)
+        (wttrin-favorite-location "Berkeley, CA"))
+    (should (= 1 (cl-count "Berkeley, CA" (wttrin--completion-candidates) :test #'equal)))))
+
+(ert-deftest test-wttrin-make-default-boundary-nil-favorite-candidates-unchanged ()
+  "Boundary: nil favorite leaves the candidate list as defaults plus history."
+  (let ((wttrin-default-locations '("Honolulu, HI"))
+        (wttrin--location-history '("Oslo, NO"))
+        (wttrin-favorite-location nil))
+    (should (equal (wttrin--completion-candidates)
+                   '("Honolulu, HI" "Oslo, NO")))))
+
+;;; --------------------------------------------------------------------------
+;;; keymap binding
+;;; --------------------------------------------------------------------------
+
+(ert-deftest test-wttrin-make-default-normal-d-bound-in-mode-map ()
+  "Normal: the weather-buffer keymap binds \"d\" to the command."
+  (should (eq (lookup-key wttrin-mode-map (kbd "d")) 'wttrin-make-default)))
+
+(provide 'test-wttrin-make-default)
+;;; test-wttrin-make-default.el ends here