defmodule Excessibility.TelemetryCapture.Timeline do
  @moduledoc """
  Generates timeline data from telemetry snapshots.

  Extracts key state, computes diffs, and formats timeline entries
  for human and AI consumption.
  """

  alias Excessibility.TelemetryCapture.Diff
  alias Excessibility.TelemetryCapture.Filter

  @default_highlight_fields [:current_user, :live_action, :errors, :form]
  @small_value_threshold 100

  @doc """
  Extracts key state from assigns for timeline display.

  Includes:
  - Highlighted fields (from config)
  - Small primitive values (< #{@small_value_threshold} chars)
  - List counts (products: [3 items] -> products_count: 3)
  - Auto-detected important fields (status, action, etc.)
  """
  def extract_key_state(assigns, highlight_fields \\ @default_highlight_fields) do
    using_defaults? = highlight_fields == @default_highlight_fields

    Enum.reduce(assigns, %{}, fn {key, value}, acc ->
      cond do
        key in highlight_fields -> Map.put(acc, key, value)
        is_list(value) -> Map.put(acc, :"#{key}_count", length(value))
        using_defaults? and small_value?(value) -> Map.put(acc, key, value)
        true -> acc
      end
    end)

    # Always include highlighted fields as-is

    # Convert non-highlighted lists to counts

    # Include small primitives only when using default fields

    # Skip everything else
  end

  defp small_value?(value) when is_integer(value), do: true
  defp small_value?(value) when is_atom(value), do: true
  defp small_value?(value) when is_boolean(value), do: true

  defp small_value?(value) when is_binary(value) do
    byte_size(value) <= @small_value_threshold
  end

  defp small_value?(_), do: false

  @doc """
  Builds a complete timeline from snapshots.

  Returns a map with:
  - :test - test name
  - :duration_ms - total test duration
  - :timeline - list of timeline entries
  """
  def build_timeline([], test_name) do
    %{
      test: test_name,
      timeline: [],
      duration_ms: 0
    }
  end

  def build_timeline(snapshots, test_name, opts \\ []) do
    first_timestamp = List.first(snapshots).timestamp
    last_timestamp = List.last(snapshots).timestamp
    duration_ms = DateTime.diff(last_timestamp, first_timestamp, :millisecond)

    timeline =
      snapshots
      |> Enum.with_index(1)
      |> Enum.map(fn {snapshot, index} ->
        previous = if index > 1, do: Enum.at(snapshots, index - 2)
        build_timeline_entry(snapshot, previous, index, opts)
      end)

    %{
      test: test_name,
      duration_ms: duration_ms,
      timeline: timeline
    }
  end

  @doc """
  Builds a single timeline entry from a snapshot and its predecessor.
  """
  def build_timeline_entry(snapshot, previous, sequence, opts \\ []) do
    filtered_assigns = Filter.filter_assigns(snapshot.assigns, opts)

    key_state =
      extract_key_state(filtered_assigns, opts[:highlight_fields] || @default_highlight_fields)

    previous_assigns =
      if previous do
        Filter.filter_assigns(previous.assigns, opts)
      end

    diff = Diff.compute_diff(filtered_assigns, previous_assigns)
    changes = Diff.extract_changes(diff)

    duration_since_previous =
      if previous do
        DateTime.diff(snapshot.timestamp, previous.timestamp, :millisecond)
      end

    %{
      sequence: sequence,
      event: snapshot.event_type,
      timestamp: snapshot.timestamp,
      view_module: snapshot.view_module,
      key_state: key_state,
      changes: changes,
      duration_since_previous_ms: duration_since_previous
    }
  end
end