Refine modus-themes-contrast and write a test for it

2 files changed, 21 insertions, 6 deletions

diff --git a/modus-themes.el b/modus-themes.el
index ed902cd..f22aa05 100644
--- a/modus-themes.el
+++ b/modus-themes.el
@@ -3775,12 +3775,11 @@ The value is defined in hexadecimal RGB notation, such #123456."
 (defun modus-themes-contrast (hex-color-1 hex-color-2)
   "Measure WCAG contrast ratio between HEX-COLOR-1 and HEX-COLOR-2.
 HEX-COLOR-1 and HEX-COLOR-2 are color values written in hexadecimal RGB."
-  (let ((ct (/ (+ (modus-themes-wcag-formula hex-color-1) 0.05)
-               (+ (modus-themes-wcag-formula hex-color-2) 0.05))))
-    (max ct (/ ct))))
-
-(defun modus-themes--color-six-digits (hex-color)
-  "Reduce representation of hexadecimal RGB HEX-COLOR to six digits."
+  (if-let* ((hex1-weight (modus-themes-wcag-formula hex-color-1))
+            (hex2-weight (modus-themes-wcag-formula hex-color-2)))
+      (let ((contrast (/ (+ hex1-weight 0.05) (+ hex2-weight 0.05))))
+        (max contrast (/ contrast)))
+    (error "Both `%s' and `%s' must be valid hexadecimal RGB colors" hex-color-1 hex-color-2)))
   (let ((color-no-hash (substring hex-color 1)))
     (if (= (length color-no-hash) 6)
         hex-color
diff --git a/tests/modus-themes-test.el b/tests/modus-themes-test.el
index 388b7a2..a6a585b 100644
--- a/tests/modus-themes-test.el
+++ b/tests/modus-themes-test.el
@@ -69,6 +69,22 @@ Also see `modus-themes-test--modus-themes--hex-to-rgb'."
   (should (= (modus-themes-wcag-formula "#ffffff") 1.0))
   (should (= (modus-themes-wcag-formula "#000000") 0.0))
   (should-not (modus-themes-wcag-formula "#00000")))
+
+(ert-deftest mtt-modus-themes-contrast ()
+  "Test that `modus-themes-contrast' works as intended.
+Also see `modus-themes-test--modus-themes--hex-to-rgb'."
+  (should (= (modus-themes-contrast "#ffffff" "#000000") 21.0))
+  (should (= (modus-themes-contrast "#000000" "#ffffff") 21.0))
+  (let ((float-2-fn (lambda (hex1 hex2)
+                      (string-to-number (format "%.2f" (modus-themes-contrast hex1 hex2))))))
+    (should (= (funcall float-2-fn "#ff0000" "#ffffff")  4.0))
+    (should (= (funcall float-2-fn "#00ff00" "#ffffff") 1.37))
+    (should (= (funcall float-2-fn "#0000ff" "#ffffff") 8.59))
+    (should (= (funcall float-2-fn "#ffff00" "#ffffff") 1.07))
+    (should (= (funcall float-2-fn "#00ffff" "#ffffff") 1.25))
+    (should (= (funcall float-2-fn "#ff00ff" "#ffffff") 3.14)))
+  (should-error (modus-themes-contrast "#ffffff" "#00000"))
+  (should-error (modus-themes-contrast "#fffff" "#00000")))
 (ert-deftest mtt-inheritance ()
   "Ensure all faces inherit from valid faces."
   ;; Third-party packages, loaded if possible to better test face inheritance.