Fix diff for deleted lines at end of buffer

No long assumes that the old buffer content was the shorter one when
there were different number of lines - fixes the case where we run past
the `line_count`.

Remove some branches that are not reachable.

autoload/lsc/diff.vim

@@ -6,27 +6,14 @@
 " Finds a single change between the common prefix, and common postfix.
 function! lsc#diff#compute(old, new) abort
   let [start_line, start_char] = s:FirstDifference(a:old, a:new)
-  if len(a:old) <= start_line && strlen(a:old[-1]) <= start_char
-    let start_line = len(a:old)
-    let end_line = 0
-    let end_char = 0
-  else
-    let [end_line, end_char] =
-        \ s:LastDifference(a:old[start_line:], a:new[start_line:], start_char)
-  endif
+  let [end_line, end_char] =
+      \ s:LastDifference(a:old[start_line:], a:new[start_line:], start_char)
 
   let text = s:ExtractText(a:new, start_line, start_char, end_line, end_char)
   let length = s:Length(a:old, start_line, start_char, end_line, end_char)
 
   let adj_end_line = len(a:old) + end_line
-  let adj_end_char = strlen(a:old[end_line]) + end_char
-  " End is exclusive
-  if end_char == 1
-    let adj_end_line += 1
-    let adj_end_char = 0
-  else
-    let adj_end_char += 1
-  endif
+  let adj_end_char = end_line == 0 ? 0 : strlen(a:old[end_line]) + end_char + 1
 
   let result = { 'range': {'start': {'line': start_line, 'character': start_char},
       \  'end': {'line': adj_end_line, 'character': adj_end_char}},
@@ -47,7 +34,7 @@ function! s:FirstDifference(old, new) abort
     let i += 1
   endwhile
   if i >= line_count
-    return [len(a:old) - 1, strlen(a:old[-1])]
+    return [line_count - 1, strlen(a:old[line_count - 1])]
   endif
   let old_line = a:old[i]
   let new_line = a:new[i]
@@ -62,6 +49,7 @@ endfunction
 
 function! s:LastDifference(old, new, start_char) abort
   let line_count = min([len(a:old), len(a:new)])
+  if line_count == 0 | return [0, 0] | endif
   let i = -1
   while i >= -1 * line_count
     if a:old[i] !=# a:new[i] | break | endif
@@ -86,6 +74,7 @@ endfunction
 
 function! s:ExtractText(lines, start_line, start_char, end_line, end_char) abort
   if a:start_line == len(a:lines) + a:end_line
+    if a:end_line == 0 | return '' | endif
     let result = a:lines[a:start_line][a:start_char:a:end_char]
     " json_encode treats empty string computed this was as 'null'
     if strlen(result) == 0 | let result = '' | endif
@@ -96,14 +85,20 @@ function! s:ExtractText(lines, start_line, start_char, end_line, end_char) abort
   for line in a:lines[a:start_line + 1:a:end_line - 1]
     let result .= line."\n"
   endfor
-  let result .= a:lines[a:end_line][:a:end_char]
+  if a:end_line != 0
+    let result .= a:lines[a:end_line][:a:end_char]
+  endif
   return result
 endfunction
 
 function! s:Length(lines, start_line, start_char, end_line, end_char)
     \ abort
   let adj_end_line = len(a:lines) + a:end_line
-  let adj_end_char = strlen(a:lines[adj_end_line]) + a:end_char
+  if adj_end_line >= len(a:lines)
+    let adj_end_char = a:end_char - 1
+  else
+    let adj_end_char = strlen(a:lines[adj_end_line]) + a:end_char
+  endif
   if a:start_line == adj_end_line
     return adj_end_char - a:start_char + 1
   endif

test/diff_test.vim

@@ -55,6 +55,7 @@ function! TestDiff() abort
       \ )
 
   " Line inserted at end
+  " It's important this appears to *prefix* the newline
   call s:TestDiff(
       \ [2,3,2,3], 0, "\nanother",
       \ "foo\nbar\nbaz",
@@ -146,6 +147,12 @@ function! TestDiff() abort
       \ "foo\nbar\nbaz",
       \ "foo\nbar\nba")
 
+  " Delete lines at end
+  call s:TestDiff(
+      \ [0, 3, 2, 3], 8, '',
+      \ "foo\nbar\nbaz",
+      \ "foo")
+
   " Handles multiple blank lines
   call s:TestDiff(
       \ [5,1,5,2], 1, 'x',