import claude/agent/config.{type AgentConfig}
import claude/agent/tool_runner
import claude/messages.{RequestParams}
import claude/types/content.{type ContentBlock, ToolUse}
import claude/types/error.{type ApiError}
import claude/types/message.{
  type Message, type MessageParam, Assistant, BlockContent, MessageParam,
  ToolUseStop,
}
import gleam/list
import gleam/option.{Some}

/// The result of a successful agent loop run.
pub type AgentResult {
  AgentResult(
    messages: List(MessageParam),
    final_message: Message,
    iterations: Int,
    total_input_tokens: Int,
    total_output_tokens: Int,
  )
}

/// Errors that can occur during the agent loop.
pub type AgentError {
  ApiCallFailed(ApiError)
  MaxIterationsReached(messages: List(MessageParam), iterations: Int)
}

/// Events emitted during the agent loop execution.
///
/// These events allow callers to observe the progress of an agent run,
/// whether via a callback function or via BEAM message passing.
pub type AgentEvent {
  /// Agent started processing
  Started(session_id: String)
  /// Assistant sent a response (may include tool calls)
  AssistantResponse(message: Message)
  /// Tool execution started
  ToolExecuting(tool_name: String, tool_id: String)
  /// Tool execution completed
  ToolCompleted(tool_id: String, result: Result(String, String))
  /// Agent finished successfully
  Done(result: AgentResult)
  /// Agent encountered an error
  Failed(error: AgentError)
}

/// Extract tool-use blocks from a list of content blocks.
pub fn extract_tool_calls(content: List(ContentBlock)) -> List(ContentBlock) {
  list.filter(content, fn(block) {
    case block {
      ToolUse(..) -> True
      _ -> False
    }
  })
}

/// Build a user message containing tool results.
///
/// Each result is a tuple of (tool_use_id, Ok(content) | Error(error_message)).
pub fn build_tool_results_message(
  results: List(#(String, Result(String, String))),
) -> MessageParam {
  let blocks =
    list.map(results, fn(result) {
      let #(id, outcome) = result
      case outcome {
        Ok(content) ->
          content.tool_result(tool_use_id: id, content: content, is_error: False)
        Error(err) ->
          content.tool_result(tool_use_id: id, content: err, is_error: True)
      }
    })
  MessageParam(role: message.User, content: BlockContent(blocks))
}

/// Run the agent loop.
///
/// Starts with the given prompt, sends messages to the API, and automatically
/// handles tool-use responses by executing tools and feeding results back.
/// Continues until the model stops with a non-tool-use reason or max_iterations
/// is reached.
pub fn run(
  config: AgentConfig,
  prompt: String,
) -> Result(AgentResult, AgentError) {
  let initial_messages = [message.new_user(prompt)]
  event_loop(
    config,
    list.reverse(initial_messages),
    0,
    0,
    0,
    fn(_) { Nil },
  )
}

/// Run the agent loop with an existing message history.
///
/// Like `run` but allows passing in an existing conversation history.
pub fn run_with_messages(
  config: AgentConfig,
  messages_list: List(MessageParam),
) -> Result(AgentResult, AgentError) {
  event_loop(
    config,
    list.reverse(messages_list),
    0,
    0,
    0,
    fn(_) { Nil },
  )
}

/// Recursive agent loop that emits events at each step.
///
/// This is the single canonical implementation of the agent loop.
/// Both `agent.run` (with a no-op callback) and `actor.run_with_events`
/// delegate to this function.
///
/// The `on_event` callback is called at each notable step. Pass `fn(_) { Nil }`
/// for a silent loop, or a real callback for event-driven usage.
pub fn event_loop(
  config: AgentConfig,
  messages: List(MessageParam),
  iteration: Int,
  total_input: Int,
  total_output: Int,
  on_event: fn(AgentEvent) -> Nil,
) -> Result(AgentResult, AgentError) {
  // Guard: check max iterations
  case iteration >= config.max_iterations {
    True -> {
      let err =
        MaxIterationsReached(
          messages: list.reverse(messages),
          iterations: iteration,
        )
      on_event(Failed(error: err))
      Error(err)
    }
    False -> {
      // Call the API
      case
        messages.create(RequestParams(
          config: config.client,
          model: Some(config.model),
          max_tokens: Some(config.max_tokens),
          messages: list.reverse(messages),
          tools: config.tools,
          system: config.system,
          tool_choice: config.tool_choice,
          thinking: config.thinking,
        ))
      {
        Error(api_error) -> {
          let err = ApiCallFailed(api_error)
          on_event(Failed(error: err))
          Error(err)
        }
        Ok(response) -> {
          // Emit assistant response event
          on_event(AssistantResponse(message: response))

          let new_input = total_input + response.usage.input_tokens
          let new_output = total_output + response.usage.output_tokens

          // Convert assistant response to MessageParam and append
          let assistant_params = message.content_to_params(response.content)
          let assistant_msg =
            MessageParam(role: Assistant, content: BlockContent(assistant_params))
          let updated_messages = [assistant_msg, ..messages]

          // Check stop reason
          case response.stop_reason {
            Some(ToolUseStop) -> {
              // Extract tool calls
              let tool_calls = extract_tool_calls(response.content)

              // Emit ToolExecuting events for each tool call
              list.each(tool_calls, fn(block) {
                case block {
                  ToolUse(id: id, name: name, ..) ->
                    on_event(ToolExecuting(tool_name: name, tool_id: id))
                  _ -> Nil
                }
              })

              // Execute tools
              let results =
                tool_runner.execute_concurrent(
                  tool_calls: tool_calls,
                  tools: config.tools,
                  timeout_ms: config.tool_timeout_ms,
                )

              // Emit ToolCompleted events
              list.each(results, fn(result) {
                let #(id, outcome) = result
                on_event(ToolCompleted(tool_id: id, result: outcome))
              })

              let tool_results_msg = build_tool_results_message(results)
              let next_messages = [tool_results_msg, ..updated_messages]

              // Recurse
              event_loop(
                config,
                next_messages,
                iteration + 1,
                new_input,
                new_output,
                on_event,
              )
            }
            _ -> {
              let result =
                AgentResult(
                  messages: list.reverse(updated_messages),
                  final_message: response,
                  iterations: iteration + 1,
                  total_input_tokens: new_input,
                  total_output_tokens: new_output,
                )
              on_event(Done(result: result))
              Ok(result)
            }
          }
        }
      }
    }
  }
}