vim-patch:8.2.1462: Vim9: string slice not supported yet

Problem:    Vim9: string slice not supported yet.
Solution:   Add support for string slicing.

11107bab7e

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/eval.c

@@ -7285,6 +7285,58 @@ char *char_from_string(char *str, varnumber_T index)
   return xstrnsave(str + nbyte, (size_t)utf_ptr2len(str + nbyte));
 }
 
+/// Get the byte index for character index "idx" in string "str" with length
+/// "str_len".
+/// If going over the end return "str_len".
+/// If "idx" is negative count from the end, -1 is the last character.
+/// When going over the start return zero.
+static size_t char_idx2byte(char *str, size_t str_len, varnumber_T idx)
+{
+  varnumber_T nchar = idx;
+  size_t nbyte = 0;
+
+  if (nchar >= 0) {
+    while (nchar > 0 && nbyte < str_len) {
+      nbyte += (size_t)utf_ptr2len(str + nbyte);
+      nchar--;
+    }
+  } else {
+    nbyte = str_len;
+    while (nchar < 0 && nbyte > 0) {
+      nbyte--;
+      nbyte -= (size_t)utf_head_off(str, str + nbyte);
+      nchar++;
+    }
+  }
+  return nbyte;
+}
+
+/// Return the slice "str[first:last]" using character indexes.
+/// Return NULL when the result is empty.
+char *string_slice(char *str, varnumber_T first, varnumber_T last)
+{
+  if (str == NULL) {
+    return NULL;
+  }
+  size_t slen = strlen(str);
+  size_t start_byte = char_idx2byte(str, slen, first);
+  size_t end_byte;
+  if (last == -1) {
+    end_byte = slen;
+  } else {
+    end_byte = char_idx2byte(str, slen, last);
+    if (end_byte < slen) {
+      // end index is inclusive
+      end_byte += (size_t)utf_ptr2len(str + end_byte);
+    }
+  }
+
+  if (start_byte >= slen || end_byte <= start_byte) {
+    return NULL;
+  }
+  return xstrnsave(str + start_byte, end_byte - start_byte);
+}
+
 /// Handle:
 /// - expr[expr], expr[expr:expr] subscript
 /// - ".name" lookup