implement 'page-reverse' option (#1854)

docs/configuration.rst

@@ -2262,6 +2262,15 @@ Description
     * ``false``: Start with the first chapter
 
 
+extractor.[manga-extractor].page-reverse
+----------------------------------------
+Type
+    ``bool``
+Default
+    ``false``
+Description
+    Download manga chapter pages in reverse order.
+
 
 Downloader Options
 ==================

gallery_dl/extractor/common.py

@@ -447,16 +447,23 @@ class GalleryExtractor(Extractor):
         imgs = self.images(page)
 
         if "count" in data:
-            images = zip(
-                range(1, data["count"]+1),
-                imgs,
-            )
+            if self.config("page-reverse"):
+                images = util.enumerate_reversed(imgs, 1, data["count"])
+            else:
+                images = zip(
+                    range(1, data["count"]+1),
+                    imgs,
+                )
         else:
+            enum = enumerate
             try:
                 data["count"] = len(imgs)
             except TypeError:
                 pass
-            images = enumerate(imgs, 1)
+            else:
+                if self.config("page-reverse"):
+                    enum = util.enumerate_reversed
+            images = enum(imgs, 1)
 
         yield Message.Directory, data
         for data[self.enum], (url, imgdata) in images:

gallery_dl/extractor/foolslide.py

@@ -88,7 +88,9 @@ class FoolslideChapterExtractor(FoolslideExtractor):
         data["chapter_id"] = text.parse_int(imgs[0]["chapter_id"])
 
         yield Message.Directory, data
-        for data["page"], image in enumerate(imgs, 1):
+        enum = util.enumerate_reversed if self.config(
+            "page-reverse") else enumerate
+        for data["page"], image in enum(imgs, 1):
             try:
                 url = image["url"]
                 del image["url"]

gallery_dl/extractor/mangadex.py

@@ -122,7 +122,10 @@ class MangadexChapterExtractor(MangadexExtractor):
         data["_http_headers"] = self._headers
         base = "{}/data/{}/".format(
             self.api.athome_server(self.uuid)["baseUrl"], cattributes["hash"])
-        for data["page"], page in enumerate(cattributes["data"], 1):
+
+        enum = util.enumerate_reversed if self.config(
+            "page-reverse") else enumerate
+        for data["page"], page in enum(cattributes["data"], 1):
             text.nameext_from_url(page, data)
             yield Message.Url, base + page, data
 

gallery_dl/util.py

@@ -139,6 +139,17 @@ def delete_items(obj, keys):
             del obj[key]
 
 
+def enumerate_reversed(iterable, start=0, length=None):
+    """Enumerate 'iterable' and return its elements in reverse order"""
+    start -= 1
+    if length is None:
+        length = len(iterable)
+    return zip(
+        range(length - start, start, -1),
+        reversed(iterable),
+    )
+
+
 def number_to_string(value, numbers=(int, float)):
     """Convert numbers (int, float) to string; Return everything else as is."""
     return str(value) if value.__class__ in numbers else value