local M = {}

local git = require('diffs.git')
local dbg = require('diffs.log').dbg

---@param old_lines string[]
---@param new_lines string[]
---@param old_name string
---@param new_name string
---@return string[]
local function generate_unified_diff(old_lines, new_lines, old_name, new_name)
  local old_content = table.concat(old_lines, '\n')
  local new_content = table.concat(new_lines, '\n')

  local diff_output = vim.diff(old_content, new_content, {
    result_type = 'unified',
    ctxlen = 3,
  })

  if not diff_output or diff_output == '' then
    return {}
  end

  local diff_lines = vim.split(diff_output, '\n', { plain = true })

  local result = {
    'diff --git a/' .. old_name .. ' b/' .. new_name,
    '--- a/' .. old_name,
    '+++ b/' .. new_name,
  }
  for _, line in ipairs(diff_lines) do
    table.insert(result, line)
  end

  return result
end

---@param revision? string
function M.gdiff(revision)
  revision = revision or 'HEAD'

  local bufnr = vim.api.nvim_get_current_buf()
  local filepath = vim.api.nvim_buf_get_name(bufnr)

  if filepath == '' then
    vim.notify('diffs: buffer has no file', vim.log.levels.ERROR)
    return
  end

  local rel_path = git.get_relative_path(filepath)
  if not rel_path then
    vim.notify('diffs: not in a git repository', vim.log.levels.ERROR)
    return
  end

  local old_lines, err = git.get_file_content(revision, filepath)
  if not old_lines then
    vim.notify('diffs: ' .. (err or 'unknown error'), vim.log.levels.ERROR)
    return
  end

  local new_lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)

  local diff_lines = generate_unified_diff(old_lines, new_lines, rel_path, rel_path)

  if #diff_lines == 0 then
    vim.notify('diffs: no changes', vim.log.levels.INFO)
    return
  end

  local diff_buf = vim.api.nvim_create_buf(false, true)
  vim.api.nvim_buf_set_lines(diff_buf, 0, -1, false, diff_lines)
  vim.api.nvim_set_option_value('buftype', 'nofile', { buf = diff_buf })
  vim.api.nvim_set_option_value('bufhidden', 'wipe', { buf = diff_buf })
  vim.api.nvim_set_option_value('modifiable', false, { buf = diff_buf })
  vim.api.nvim_set_option_value('filetype', 'diff', { buf = diff_buf })
  vim.api.nvim_buf_set_name(diff_buf, 'diffs://' .. revision .. ':' .. rel_path)

  vim.cmd('vsplit')
  vim.api.nvim_win_set_buf(0, diff_buf)

  dbg('opened diff buffer %d for %s against %s', diff_buf, rel_path, revision)

  vim.schedule(function()
    require('diffs').attach(diff_buf)
  end)
end

function M.setup()
  vim.api.nvim_create_user_command('Gdiff', function(opts)
    M.gdiff(opts.args ~= '' and opts.args or nil)
  end, {
    nargs = '?',
    desc = 'Show unified diff against git revision (default: HEAD)',
  })
end

return M