lsp: add incremental text synchronization

* Implementation derived from and validated by vim-lsc authored by Nate
  Bosch

runtime/lua/vim/lsp/util.lua

@@ -226,6 +226,165 @@ end
 -- function M.glob_to_regex(glob)
 -- end
 
+--@private
+--- Finds the first line and column of the difference between old and new lines
+--@param old_lines table list of lines
+--@param new_lines table list of lines
+--@returns (int, int) start_line_idx and start_col_idx of range
+local function first_difference(old_lines, new_lines, start_line_idx)
+  local line_count = math.min(#old_lines, #new_lines)
+  if line_count == 0 then return 1, 1 end
+  if not start_line_idx then
+    for i = 1, line_count do
+      start_line_idx = i
+      if old_lines[start_line_idx] ~= new_lines[start_line_idx] then
+        break
+      end
+    end
+  end
+  local old_line = old_lines[start_line_idx]
+  local new_line = new_lines[start_line_idx]
+  local length = math.min(#old_line, #new_line)
+  local start_col_idx = 1
+  while start_col_idx <= length do
+    if string.sub(old_line, start_col_idx, start_col_idx) ~= string.sub(new_line, start_col_idx, start_col_idx) then
+      break
+    end
+    start_col_idx  = start_col_idx  + 1
+  end
+  return start_line_idx, start_col_idx
+end
+
+
+--@private
+--- Finds the last line and column of the differences between old and new lines
+--@param old_lines table list of lines
+--@param new_lines table list of lines
+--@param start_char integer First different character idx of range
+--@returns (int, int) end_line_idx and end_col_idx of range
+local function last_difference(old_lines, new_lines, start_char, end_line_idx)
+  local line_count = math.min(#old_lines, #new_lines)
+  if line_count == 0 then return 0,0 end
+  if not end_line_idx then
+    end_line_idx = -1
+  end
+  for i = end_line_idx, -line_count, -1  do
+    if old_lines[#old_lines + i + 1] ~= new_lines[#new_lines + i + 1] then
+      end_line_idx = i
+      break
+    end
+  end
+  local old_line
+  local new_line
+  if end_line_idx <= -line_count then
+    end_line_idx = -line_count
+    old_line  = string.sub(old_lines[#old_lines + end_line_idx + 1], start_char)
+    new_line  = string.sub(new_lines[#new_lines + end_line_idx + 1], start_char)
+  else
+    old_line  = old_lines[#old_lines + end_line_idx + 1]
+    new_line  = new_lines[#new_lines + end_line_idx + 1]
+  end
+  local old_line_length = #old_line
+  local new_line_length = #new_line
+  local length = math.min(old_line_length, new_line_length)
+  local end_col_idx = -1
+  while end_col_idx >= -length do
+    local old_char =  string.sub(old_line, old_line_length + end_col_idx + 1, old_line_length + end_col_idx + 1)
+    local new_char =  string.sub(new_line, new_line_length + end_col_idx + 1, new_line_length + end_col_idx + 1)
+    if old_char ~= new_char then
+      break
+    end
+    end_col_idx = end_col_idx - 1
+  end
+  return end_line_idx, end_col_idx
+
+end
+
+--@private
+--- Get the text of the range defined by start and end line/column
+--@param lines table list of lines
+--@param start_char integer First different character idx of range
+--@param end_char integer Last different character idx of range
+--@param start_line integer First different line idx of range
+--@param end_line integer Last different line idx of range
+--@returns string text extracted from defined region
+local function extract_text(lines, start_line, start_char, end_line, end_char)
+  if start_line == #lines + end_line + 1 then
+    if end_line == 0 then return '' end
+    local line = lines[start_line]
+    local length = #line + end_char - start_char
+    return string.sub(line, start_char, start_char + length + 1)
+  end
+  local result = string.sub(lines[start_line], start_char) .. '\n'
+  for line_idx = start_line + 1, #lines + end_line do
+    result = result .. lines[line_idx] .. '\n'
+  end
+  if end_line ~= 0 then
+    local line = lines[#lines + end_line + 1]
+    local length = #line + end_char + 1
+    result = result .. string.sub(line, 1, length)
+  end
+  return result
+end
+
+--@private
+--- Compute the length of the substituted range
+--@param lines table list of lines
+--@param start_char integer First different character idx of range
+--@param end_char integer Last different character idx of range
+--@param start_line integer First different line idx of range
+--@param end_line integer Last different line idx of range
+--@returns (int, int) end_line_idx and end_col_idx of range
+local function compute_length(lines, start_line, start_char, end_line, end_char)
+  local adj_end_line = #lines + end_line + 1
+  local adj_end_char
+  if adj_end_line > #lines then
+    adj_end_char =  end_char - 1
+  else
+    adj_end_char = #lines[adj_end_line] + end_char
+  end
+  if start_line == adj_end_line then
+    return adj_end_char - start_char + 1
+  end
+  local result = #lines[start_line] - start_char + 1
+  for line = start_line + 1, adj_end_line -1 do
+    result = result + #lines[line] + 1
+  end
+  result = result + adj_end_char + 1
+  return result
+end
+
+--- Returns the range table for the difference between old and new lines
+--@param old_lines table list of lines
+--@param new_lines table list of lines
+--@returns table start_line_idx and start_col_idx of range
+function M.compute_diff(old_lines, new_lines, start_line_idx, end_line_idx)
+  local start_line, start_char = first_difference(old_lines, new_lines, start_line_idx)
+  local end_line, end_char = last_difference(vim.list_slice(old_lines, start_line, #old_lines),
+      vim.list_slice(new_lines, start_line, #new_lines), start_char, end_line_idx)
+  local text = extract_text(new_lines, start_line, start_char, end_line, end_char)
+  local length = compute_length(old_lines, start_line, start_char, end_line, end_char)
+
+  local adj_end_line = #old_lines + end_line
+  local adj_end_char
+  if end_line == 0 then
+    adj_end_char = 0
+  else
+    adj_end_char = #old_lines[#old_lines + end_line + 1] + end_char + 1
+  end
+
+  local result = {
+    range = {
+      start = { line = start_line - 1, character = start_char - 1},
+      ["end"] = { line = adj_end_line, character = adj_end_char}
+    },
+    text = text,
+    rangeLength = length + 1,
+  }
+
+  return result
+end
+
 --- Can be used to extract the completion items from a
 --- `textDocument/completion` request, which may return one of
 --- `CompletionItem[]`, `CompletionList` or null.