local M = {}

---
-- Validate that `search_lines` matches exactly once in `buffer_lines`.
-- Returns the start index (1-based) and end index (inclusive) of the match,
-- or errors if there is not exactly one match.
local function find_exact_match(buffer_lines, search_lines)
  local search_text = table.concat(search_lines, "\n")

  local matches = {}
  local search_len = #search_lines

  for i = 1, #buffer_lines - search_len + 1 do
    local candidate = table.concat(vim.list_slice(buffer_lines, i, i + search_len - 1), "\n")
    if candidate == search_text then
      table.insert(matches, { start = i, end_ = i + search_len - 1 })
    end
  end

  if #matches == 0 then
    error("search text not found in buffer")
  elseif #matches > 1 then
    error("search text matches multiple times")
  end

  return matches[1].start, matches[1].end_
end

---
-- Insert `lines` before the lines matched by `search_lines`.
-- Returns a new table of lines with the insertion applied.
-- Errors if `search_lines` does not match exactly once.
function M.insert_before(buffer_lines, search_lines, insert_lines)
  local start, end_ = find_exact_match(buffer_lines, search_lines)

  local result = {}

  for i = 1, start - 1 do
    table.insert(result, buffer_lines[i])
  end

  for _, line in ipairs(insert_lines) do
    table.insert(result, line)
  end

  for i = start, #buffer_lines do
    table.insert(result, buffer_lines[i])
  end

  return result
end

---
-- Append `lines` after the lines matched by `search_lines`.
-- Returns a new table of lines with the append applied.
-- Errors if `search_lines` does not match exactly once.
function M.append_after(buffer_lines, search_lines, append_lines)
  local start, end_ = find_exact_match(buffer_lines, search_lines)

  local result = {}

  for i = 1, end_ do
    table.insert(result, buffer_lines[i])
  end

  for _, line in ipairs(append_lines) do
    table.insert(result, line)
  end

  for i = end_ + 1, #buffer_lines do
    table.insert(result, buffer_lines[i])
  end

  return result
end

---
-- Replace the lines matched by `search_lines` with `replace_lines`.
-- Returns a new table of lines with the replacement applied.
-- Errors if `search_lines` does not match exactly once.
function M.replace(buffer_lines, search_lines, replace_lines)
  local start, end_ = find_exact_match(buffer_lines, search_lines)

  local result = {}

  for i = 1, start - 1 do
    table.insert(result, buffer_lines[i])
  end

  for _, line in ipairs(replace_lines) do
    table.insert(result, line)
  end

  for i = end_ + 1, #buffer_lines do
    table.insert(result, buffer_lines[i])
  end

  return result
end

return M