rebuild only files that need re-building

we compare the mtimes of src and dst

blag/blag.py

@@ -293,6 +293,18 @@ def process_markdown(convertibles, input_dir, output_dir,
     for src, dst in convertibles:
         logger.debug(f'Processing {src}')
 
+        # see first if the dst actually needs re-building. for that we compare
+        # the mtimes and assume mtime_dst > mtime_src means that it needs not
+        # to be rebuilt
+        if os.path.exists(f'{output_dir}/{dst}'):
+            mtime_src = os.stat(f'{input_dir}/{src}').st_mtime
+            mtime_dst = os.stat(f'{output_dir}/{dst}').st_mtime
+            if mtime_dst >= mtime_src:
+                logger.debug(
+                    'Skipping, as target exists and is newer than source.'
+                )
+                continue
+
         with open(f'{input_dir}/{src}', 'r') as fh:
             body = fh.read()