[core] add option to trim file name length with integer

https://github.com/blackjack4494/youtube-dlc/issues/85

youtube_dlc/YoutubeDL.py

@@ -164,7 +164,8 @@ class YoutubeDL(object):
     simulate:          Do not download the video files.
     format:            Video format code. See options.py for more information.
     outtmpl:           Template for output names.
-    restrictfilenames: Do not allow "&" and spaces in file names
+    restrictfilenames: Do not allow "&" and spaces in file names.
+    trim_file_name:    Limit length of filename (extension excluded).
     ignoreerrors:      Do not stop on download errors.
     force_generic_extractor: Force downloader to use the generic extractor
     nooverwrites:      Prevent overwriting files.
@@ -732,6 +733,16 @@ def prepare_filename(self, info_dict):
             # title "Hello $PATH", we don't want `$PATH` to be expanded.
             filename = expand_path(outtmpl).replace(sep, '') % template_dict
 
+            # https://github.com/blackjack4494/youtube-dlc/issues/85
+            trim_file_name = self.params.get('trim_file_name', False)
+            if trim_file_name:
+                fn_groups = filename.rsplit('.')
+                ext = fn_groups[-1]
+                sub_ext = ''
+                if len(fn_groups) > 2:
+                    sub_ext = fn_groups[-2]
+                filename = '.'.join(filter(None, [fn_groups[0][:trim_file_name], sub_ext, ext]))
+
             # Temporary fix for #4787
             # 'Treat' all problem characters by passing filename through preferredencoding
             # to workaround encoding issues with subprocess on python2 @ Windows

youtube_dlc/__init__.py

@@ -390,6 +390,7 @@ def parse_retries(retries):
         'rejecttitle': decodeOption(opts.rejecttitle),
         'max_downloads': opts.max_downloads,
         'prefer_free_formats': opts.prefer_free_formats,
+        'trim_file_name': opts.trim_file_name,
         'verbose': opts.verbose,
         'dump_intermediate_pages': opts.dump_intermediate_pages,
         'write_pages': opts.write_pages,

youtube_dlc/options.py

@@ -775,6 +775,9 @@ def _comma_separated_values_options_callback(option, opt_str, value, parser):
         '--rm-cache-dir',
         action='store_true', dest='rm_cachedir',
         help='Delete all filesystem cache files')
+    filesystem.add_option(
+        '--trim-file-name', dest='trim_file_name', default=0, type=int,
+        help='Limit the filename length (extension excluded)')
 
     thumbnail = optparse.OptionGroup(parser, 'Thumbnail images')
     thumbnail.add_option(