defmodule Mix.Tasks.LiveInteractionContracts.Compare do
  @shortdoc "Diff two harness result sets (e.g. before/after a LiveView upgrade)"
  @moduledoc """
  The version-upgrade fuse. Runs the harness against two LiveView versions and
  reports which browser-native machines changed patch-safety behavior — so an
  interaction-heavy app can see, before merging a LiveView bump, exactly which
  delegated machines drifted.

      # produce two result sets:
      mix live_interaction_contracts.test --suite harness --lv 1.2.7   # writes priv/harness/results.json
      cp priv/harness/results.json /tmp/a.json
      mix live_interaction_contracts.test --suite harness --lv "github:phoenixframework/phoenix_live_view#main"
      cp priv/harness/results.json /tmp/b.json

      # diff them:
      mix live_interaction_contracts.compare --a /tmp/a.json --b /tmp/b.json

  Exits non-zero if any machine's survival changed (a drift is a merge-blocking
  signal). "No drift" = the observed behavior the contracts rely on is unchanged.
  """
  use Mix.Task

  @nondestructive ~w(siblingPatch contentPatch attributePatch reorderNearby streamNearby pushPatch reconnect)

  @impl true
  def run(argv) do
    {opts, _, _} = OptionParser.parse(argv, strict: [a: :string, b: :string])
    a = load(opts[:a] || Mix.raise("--a required"))
    b = load(opts[:b] || Mix.raise("--b required"))

    va = get_in(a, ["versions", "live_view"]) || "?"
    vb = get_in(b, ["versions", "live_view"]) || "?"
    engines = Map.keys(a["engines"]) |> Enum.filter(&Map.has_key?(b["engines"], &1))

    diffs =
      for e <- engines,
          machine <- machine_keys(a, e),
          patch <- @nondestructive,
          sa = survived(machine, cell(a, e, machine, patch)),
          sb = survived(machine, cell(b, e, machine, patch)),
          sa != sb do
        "  #{e} · #{machine} · #{patch}: #{va}=#{surv(sa)} → #{vb}=#{surv(sb)}"
      end

    Mix.shell().info("== compare LiveView #{va}  →  #{vb}  (engines: #{Enum.join(engines, ", ")})")

    if diffs == [] do
      Mix.shell().info("✓ no drift — every tested machine has identical patch-safety on both versions")
    else
      Mix.shell().error("DRIFT DETECTED (#{length(diffs)}):\n" <> Enum.join(diffs, "\n"))
      Mix.raise("interaction behavior drifted between #{va} and #{vb} — review before upgrading")
    end
  end

  defp load(path), do: File.read!(path) |> JSON.decode!()
  defp machine_keys(data, e), do: (data["engines"][e]["runs"] |> hd() |> Map.keys()) -- ["ime"]
  defp cell(data, e, machine, patch), do: get_in(data, ["engines", e, "runs", Access.at(0), machine, "patches", patch, "after"])

  defp survived(_m, nil), do: nil
  defp survived("pop", a), do: a["open"] == true
  defp survived("dlg", a), do: a["open"] == true
  defp survived("det", a), do: a["open"] == true
  defp survived("sel", a), do: a["value"] == "b"
  defp survived("inp", a), do: a["value"] == "typed-value" and a["focused"] == true
  defp survived("scr", a), do: a["scrollTop"] == 500
  defp survived(_, _), do: nil

  defp surv(true), do: "green"
  defp surv(false), do: "red"
  defp surv(nil), do: "n/a"
end