json_comparator

defmodule JsonComparator do
  @moduledoc """
  Provides functionality for comparing JSON structures with configurable comparison options.
  """

  @doc """
  Compares two JSON structures for equality with configurable options.

  This function performs a deep comparison of two JSON structures, supporting various data types
  including maps, lists, DateTime objects, and structs. It provides configurable behavior for
  list comparison and DateTime precision.

  ## Parameters

    * `json1` - First JSON structure to compare
    * `json2` - Second JSON structure to compare
    * `opts` - Optional keyword list of comparison options:
      * `:strict_list_order` - When `true`, lists must have identical order to be considered equal.
        Defaults to `false`
      * `:truncate_datetime_microseconds` - When `true`, DateTime comparisons ignore microseconds.
        Defaults to `true`
      * `:error_message` - Custom error message template to use when differences are found.
        The string `%{path}` will be replaced with the path where the difference was found.
        Defaults to "Submitted JSONs do not match: %{path}"

  ## Returns

    * `:ok` - When the structures are equal according to the comparison rules
    * `{:error, message}` - When differences are found, where message is a string indicating
      the path where the first difference was encountered
  """
  def compare(json1, json2, opts \\ []) do
    opts =
      [
        strict_list_order: Keyword.get(opts, :strict_list_order, false),
        truncate_datetime_microseconds: Keyword.get(opts, :truncate_datetime_microseconds, true),
        error_message: Keyword.get(opts, :error_message, "Submitted JSONs do not match: %{path}")
      ]

    case deep_compare(json1, json2, opts) do
      {:ok, true} ->
        :ok

      {:ok, false} ->
        {:error, String.replace(opts[:error_message], "%{path}", "")}

      {:ok, {false, path}} ->
        path_str = to_string(path)
        {:error, String.replace(opts[:error_message], "%{path}", path_str)}
    end
  end

  defp deep_compare(%DateTime{} = dt1, %DateTime{} = dt2, opts) do
    compare_result =
      if Keyword.get(opts, :truncate_datetime_microseconds, true) do
        dt1_truncated = %{dt1 | microsecond: {0, 0}}
        dt2_truncated = %{dt2 | microsecond: {0, 0}}
        DateTime.compare(dt1_truncated, dt2_truncated) == :eq
      else
        DateTime.compare(dt1, dt2) == :eq
      end

    {:ok, compare_result}
  end

  defp deep_compare(%_{} = struct1, %_{} = struct2, _opts) do
    {:ok, struct1.__struct__ == struct2.__struct__ and struct1 == struct2}
  end

  defp deep_compare(map1, map2, opts) when is_map(map1) and is_map(map2) do
    compare_map_keys(Map.keys(map1), Map.keys(map2), map1, map2, opts)
  end

  defp deep_compare(list1, list2, opts) when is_list(list1) and is_list(list2) do
    compare_lists(list1, list2, opts)
  end

  defp deep_compare(val1, val2, _opts) do
    {:ok, val1 === val2}
  end

  defp compare_map_keys(keys1, keys2, map1, map2, opts) do
    case keys1 -- keys2 do
      [] ->
        case keys2 -- keys1 do
          [] -> compare_map_values(keys1, map1, map2, opts)
          [missing_key | _] -> {:ok, {false, missing_key}}
        end

      [missing_key | _] ->
        {:ok, {false, missing_key}}
    end
  end

  defp compare_map_values(keys, map1, map2, opts) do
    Enum.reduce_while(keys, {:ok, true}, fn key, acc ->
      case deep_compare(map1[key], map2[key], opts) do
        {:ok, true} -> {:cont, acc}
        {:ok, {false, path}} -> {:halt, {:ok, {false, "#{key}.#{path}"}}}
        {:ok, false} -> {:halt, {:ok, {false, key}}}
      end
    end)
  end

  defp compare_lists(list1, list2, opts) do
    cond do
      length(list1) != length(list2) ->
        {:ok, {false, "[#{min(length(list1), length(list2))}]"}}

      Keyword.get(opts, :strict_list_order, false) ->
        compare_lists_strict(list1, list2, opts)

      true ->
        compare_lists_unordered(list1, list2, opts)
    end
  end

  defp compare_lists_strict(list1, list2, opts) do
    list1
    |> Enum.zip(list2)
    |> Enum.with_index()
    |> Enum.reduce_while({:ok, true}, fn {{item1, item2}, index}, acc ->
      case deep_compare(item1, item2, opts) do
        {:ok, true} -> {:cont, acc}
        {:ok, {false, path}} -> {:halt, {:ok, {false, "[#{index}]#{path}"}}}
        {:ok, false} -> {:halt, {:ok, {false, "[#{index}]"}}}
      end
    end)
  end

  defp compare_lists_unordered(list1, list2, opts) do
    result =
      Enum.reduce_while(list1, list2, fn item1, acc ->
        case Enum.find_index(acc, &match?({:ok, true}, deep_compare(&1, item1, opts))) do
          nil -> {:halt, nil}
          idx -> {:cont, List.delete_at(acc, idx)}
        end
      end)

    {:ok, result == []}
  end
end