fix typo detection at file start

If a file starts with a typo our pattern [%A]typo[%A] can not find it.
However if the same typo appears a second type in the text our pattern
happily finds the second occurence skipping the first and all typos
in between.

In the text

typo1
Correct text typo2 ...
typo1

our pattern will match the second occurence of 'typo1' skipping
'typo2'.
Since we update our search index 'typo2' will never be found and
vis-spellcheck will show the user an information prompt.

Fix this behaviour by adding a special case for the first typo which may
start the text.

1 files changed, 19 insertions, 18 deletions

diff --git a/spellcheck.lua b/spellcheck.lua
index a8dd37d..1d0d6c7 100644
--- a/spellcheck.lua
+++ b/spellcheck.lua
@@ -125,32 +125,33 @@ local function typo_iter(text, typos, ignored) -- luacheck: ignore ignored
     until (not typo or (typo ~= '' and not ignored[typo]))
 
     if typo then
-      -- to prevent typos from being found in correct words before them
-      -- ("stuff stuf", "broken ok", ...)
-      -- we match typos only when they are enclosed in non-letter characters.
-      local start, finish = text:find('[%A]' .. escape_lua_pattern(typo) ..
-                                          '[%A]', index)
-      -- typo was not found by our pattern this means it must be either
-      -- the first or last word in the text
-      if not start then
-        -- check start of text
+      local start, finish
+      -- special case for typos at the beginning of the text
+      -- Leading typos are not found by our used pattern [%A]typo[%A].
+      -- To prevent typos to be skipped if the leading typo accours an
+      -- additional time in the text we need this special case
+      if index == 1 and text:sub(1, #typo) == typo then
         start = 1
         finish = #typo
-        -- typo is not the beginning must be the end of text
-        if text:sub(start, finish) ~= typo then
+      else
+        -- to prevent typos from being found in correct words before them
+        -- ("stuff stuf", "broken ok", ...)
+        -- we match typos only when they are enclosed in non-letter characters.
+        local pattern = '[%A]' .. escape_lua_pattern(typo) .. '[%A]'
+        start, finish = text:find(pattern, index)
+        if start then -- our pattern [%A]typo[%A] found it
+          start = start + 1 -- ignore leading non letter char
+          finish = finish - 1 -- ignore trailing non letter char
+        else -- typo was not found by our pattern this means it must be the last word
           start = #text - #typo + 1
           finish = start + #typo - 1
-        end
 
-        if text:sub(start, finish) ~= typo then
-          vis:info(string.format(
+          if text:sub(start, finish) ~= typo then
+            vis:info(string.format(
                        'can\'t find typo %s after %d. Please report this bug.',
                        typo, index))
+          end
         end
-        -- our pettern [%A]typo[%A] found it
-      else
-        start = start + 1 -- ignore leading non letter char
-        finish = finish - 1 -- ignore trailing non letter char
       end
       index = finish