micro add --last-changed: use __last_changed__

Make it faster to update moles that have recently changed; re-combine
the capture and compare step as with moles that are not expected to have
changed recently.

By allowing the specification of a time when the mole last changed, we
can compare against that one to see if there have been ongoing changes,
at the same time as capturing the new image.

This seems to speed things up greatly when capturing regular moles.

mel/cmd/microadd.py

@@ -33,6 +33,14 @@ def setup_parser(parser):
         default=0,
         help="The index of the device to take images from.",
     )
+    parser.add_argument(
+        "--last-changed",
+        action="store_true",
+        help=(
+            "Use the image specified in the '__last_changed__' file "
+            "in the mole's directory for comparison, if present."
+        ),
+    )
 
     # From NHS 'Moles' page:
     # http://www.nhs.uk/Conditions/Moles/Pages/Introduction.aspx
@@ -86,8 +94,32 @@ def load_context_images(path):
     return image_list
 
 
-def pick_comparison_path(path_list, min_compare_age_days):
+def pick_comparison_path(
+    path, path_list, min_compare_age_days, use_last_changed
+):
     """Return the most appropriate image path to compare with, or None."""
+
+    # Check for the __last_changed__ file if the --last-changed flag is used
+    if use_last_changed:
+        last_changed_path = os.path.join(path, "__last_changed__")
+        if os.path.exists(last_changed_path):
+            with open(last_changed_path) as file:
+                last_changed_image = file.read().strip()
+                if not last_changed_image:
+                    raise ValueError(
+                        "last changed file must not be empty.",
+                        path,
+                        last_changed_image,
+                    )
+                for p in sorted(path_list):
+                    if p.startswith(last_changed_image):
+                        return p
+                raise ValueError(
+                    "could not find referenced last changed image.",
+                    path,
+                    last_changed_image,
+                )
+
     path_dt_list = [
         (x, mel.lib.datetime.guess_datetime_from_path(x)) for x in path_list
     ]
@@ -109,23 +141,27 @@ def pick_comparison_path(path_list, min_compare_age_days):
     return path_dt_list[-1][0] if path_dt_list else None
 
 
-def get_comparison_image_path(path, min_compare_age_days):
+def get_comparison_image_path(path, min_compare_age_days, use_last_changed):
     micro_path = os.path.join(path, "__micro__")
     if not os.path.exists(micro_path):
         return None
 
     # List all the 'jpg' files in the micro dir
     # TODO: support more than just '.jpg'
     images = [x for x in os.listdir(micro_path) if x.lower().endswith(".jpg")]
-    path = pick_comparison_path(images, min_compare_age_days)
+    path = pick_comparison_path(
+        path, images, min_compare_age_days, use_last_changed
+    )
     if path:
         return os.path.join(micro_path, path)
     else:
         return None
 
 
-def load_comparison_image(path, min_compare_age_days):
-    micro_path = get_comparison_image_path(path, min_compare_age_days)
+def load_comparison_image(path, min_compare_age_days, use_last_changed):
+    micro_path = get_comparison_image_path(
+        path, min_compare_age_days, use_last_changed
+    )
     if micro_path is None:
         return None
     return micro_path, mel.lib.image.load_image(micro_path)
@@ -142,16 +178,24 @@ def process_args(args):
         for mole_path in args.PATH:
             print(mole_path)
             display.reset()
-            process_path(mole_path, args.min_compare_age_days, display, cap)
-
-
-def process_path(mole_path, min_compare_age_days, display, cap):
+            process_path(
+                mole_path,
+                args.min_compare_age_days,
+                display,
+                cap,
+                args.last_changed,
+            )
+
+
+def process_path(
+    mole_path, min_compare_age_days, display, cap, use_last_changed
+):
     # Import pygame as late as possible, to avoid displaying its
     # startup-text where it is not actually used.
     import pygame
 
     comparison_image_data = load_comparison_image(
-        mole_path, min_compare_age_days
+        mole_path, min_compare_age_days, use_last_changed
     )
 
     if comparison_image_data is not None: