fix: use custom fn in place of strcharpart

This partially reverts 66f055a.

The lack of `skipcc` makes the plugin practically unusable if composing
characters are present in the text, and as this applies to the current
latest stable Neovim release (0.9.1), the regression is unacceptable.

Closes #175

Author: ggandor

fnl/leap/highlight.fnl

@@ -1,7 +1,7 @@
 (local {: inc
         : dec
         : get-cursor-pos
-        : strcharpart}
+        : get-char-from}
        (require "leap.util"))
 
 (local api vim.api)
@@ -61,8 +61,8 @@
 so we set a temporary highlight on it to see where we are."
   (let [[line col &as pos] (or ?pos (get-cursor-pos))
         line-str (vim.fn.getline line)
-        start (vim.fn.charidx line-str (- col 1))
-        ch-at-curpos (case (strcharpart line-str start 1)
+        char-idx (vim.fn.charidx line-str (- col 1))
+        ch-at-curpos (case (get-char-from line-str char-idx)
                        "" " "  ; on an emtpy line
                        ch ch)
         id (api.nvim_buf_set_extmark 0 self.ns (dec line) (dec col)

fnl/leap/search.fnl

@@ -2,7 +2,7 @@
         : dec
         : get-cursor-pos
         : ->representative-char
-        : strcharpart}
+        : get-char-from}
        (require "leap.util"))
 
 (local api vim.api)
@@ -117,11 +117,11 @@ Dynamic attributes
         (when (not= line prev-match.line)
           (set line-str (vim.fn.getline line)))
         (local start (vim.fn.charidx line-str (- col 1)))
-        (case (strcharpart line-str start 1)
+        (case (get-char-from line-str start)
           "" (when (= col 1)  ; on an empty line
                (table.insert targets {: wininfo : pos :chars ["\n"]
                                       :empty-line? true}))
-          ch1 (let [ch2 (case (strcharpart line-str (+ start 1) 1)
+          ch1 (let [ch2 (case (get-char-from line-str (+ start 1))
                           "" "\n"  ; before EOL
                           ch ch)
                     xxx? (and

fnl/leap/util.fnl

@@ -50,10 +50,12 @@
   (if opts.case_sensitive ch* (vim.fn.tolower ch*)))
 
 
-(fn strcharpart [src start len]
-  (if (= (vim.fn.has "nvim-0.10") 1)
-      (vim.fn.strcharpart src start len 1)
-      (vim.fn.strcharpart src start len)))
+; Note: on 0.10+ we could replace this with vim.fn.strcharpart (with
+; `skipcc`).
+(fn get-char-from [str idx]  ; zero-based (<- vim.fn.charidx())
+  (local nr (vim.fn.strgetchar str idx))
+  (if (= nr -1) ""
+      (vim.fn.nr2char nr)))
 
 
 ; Input
@@ -116,6 +118,6 @@
  :get_enterable_windows get-enterable-windows
  : get-eq-class-of
  : ->representative-char
- : strcharpart
+ : get-char-from
  : get-input
  : get-input-by-keymap}