Extract slug from file name

To recap what has happened, by prepending the file name with the problem
id (commit 6fc400e), we have two new file names
together with two old ones:
- (old style) `slug.ext`, e.g. `two_sum.cpp`
- (old style) `slug.submission_id.ext`, e.g. `two_sum.12345.cpp`
- (new style) `id.slug.ext`, e.g. `1.two_sum.cpp`
- (new style) `id.slug.submission_id.ext`, e.g. `1.two_sum.12345.cpp`

We need to handle all of them properly.

Author: ianding1

autoload/leetcode.vim

@@ -435,6 +435,25 @@ function! s:SlugToFileName(slug) abort
     return substitute(a:slug, '-', '_', 'g')
 endfunction
 
+function! s:ProblemSlugFromFileName() abort
+    let parts = split(expand('%:t:r'), '\.')  " Filename without the extension
+    if len(parts) == 1
+        " Old style, e.g. two-sum
+        return s:FileNameToSlug(parts[0])
+    elseif len(parts) == 2 && parts[0] =~ '\v^[0-9]+$'
+        " New style, e.g. 1.two-sum
+        return s:FileNameToSlug(parts[1])
+    elseif len(parts) == 2
+        " Old style with submission id, e.g. two-sum.1234 
+        return s:FileNameToSlug(parts[0])
+    elseif len(parts) == 3
+        " New style with submission id, e.g. 1.two-sum.1234
+        return s:FileNameToSlug(parts[1])
+    else
+        throw 'leetcode: invalid file name: ' . expand('%:t:r')
+    endif
+endfunction
+
 function! s:HandleProblemListCR() abort
     " Parse the problem number from the line
     let line_nr = line('.')
@@ -490,7 +509,8 @@ function! s:HandleProblemListCR() abort
         let problem = s:GetProblem(problem_id)
         let problem_slug = problem['slug']
         let problem_ext = s:SolutionFileExt(g:leetcode_solution_filetype)
-        let problem_file_name = printf('%s.%s.%s', problem_id, s:SlugToFileName(problem_slug),
+        let problem_file_name = printf('%s.%s.%s', problem_id,
+                    \  s:SlugToFileName(problem_slug),
                     \ problem_ext)
 
         if buflisted(problem_file_name)
@@ -632,13 +652,7 @@ function! leetcode#ResetSolution(with_latest_submission) abort
         return
     endif
 
-    let problem_file_name = expand('%:t:r')
-    let problem_file_names = split(problem_file_name, '\.')
-    let problem_name = problem_file_names[0]
-    if len(problem_file_names) > 1
-        let problem_name = problem_file_names[1]
-    endif
-    let problem_slug = s:FileNameToSlug(problem_name)
+    let problem_slug = s:ProblemSlugFromFileName()
     let expr = printf('leetcode.get_problem("%s")', problem_slug)
     let problem = py3eval(expr)
     if type(problem) != v:t_dict
@@ -780,13 +794,7 @@ function! leetcode#TestSolution() abort
         return
     endif
 
-    let file_name = s:FileNameToSlug(expand('%:t:r'))
-    if file_name == ''
-        echo 'no file name'
-        return
-    endif
-
-    let slug = split(file_name, '\.')[0]
+    let slug = s:ProblemSlugFromFileName()
     let filetype = s:GuessFileType()
 
     if exists('b:leetcode_problem')
@@ -889,13 +897,7 @@ function! leetcode#SubmitSolution() abort
         return
     endif
 
-    let file_name = s:FileNameToSlug(expand('%:t:r'))
-    if file_name == ''
-        echo 'no file name'
-        return
-    endif
-
-    let slug = split(file_name, '\.')[0]
+    let slug = s:ProblemSlugFromFileName()
     let filetype = s:GuessFileType()
 
     if has('timers')
@@ -1174,7 +1176,9 @@ function! s:HandleSubmissionsCR() abort
         return
     endif
 
-    let file_name = printf('%s.%s.%s', s:SlugToFileName(submission['slug']),
+    let file_name = printf('%s.%s.%s.%s', 
+                \ submission['problem_id'],
+                \ s:SlugToFileName(submission['slug']),
                 \ submission_id, s:SolutionFileExt(submission['filetype']))
 
     if bufexists(file_name)