import { type Plugin, type PluginInput, type PluginOptions, tool } from "@opencode-ai/plugin"
import type { TextPartInput } from "@opencode-ai/sdk"

export const id = "opencode-session-restart"

type PluginClient = PluginInput["client"]

export type RestartPluginOptions = {
  /**
   * Static text always prepended to every restart prompt, for agents that do
   * not define their own `handoff_text`. An agent-defined `handoff_text`
   * overrides this. Default: none.
   */
  static_text?: string
}

const DEFAULT_OPTIONS: RestartPluginOptions = {}

const MAX_TITLE_LENGTH = 80

function buildHandoff(task: string, notes: string | undefined, staticText: string | undefined, parentSessionID: string) {
  const state = notes?.trim()
  return [
    "You are continuing work that was restarted into this fresh session with a clean context window.",
    "",
    `The full earlier conversation remains readable in the parent session ${parentSessionID} (switched to above this one in the session tree). Do not replay it here — only the standing instructions and state notes below, plus the current on-disk state (working tree, git status, staged or uncommitted changes), are available context.`,
    ...(staticText
      ? ["", "STANDING INSTRUCTIONS (always apply):", staticText]
      : []),
    "",
    "STATE NOTES:",
    state && state.length > 0 ? state : "None provided — orient yourself by inspecting git status, the diff, and open files.",
    "",
    "FOCUSED TASK (this session only):",
    task.trim(),
  ].join("\n")
}

async function resolveStaticText(
  client: PluginClient,
  currentAgent: string,
  targetAgent: string | undefined,
  fallback: string | undefined,
) {
  const agentName = targetAgent ?? currentAgent
  try {
    const cfg = await client.config.get()
    const agent = cfg.data?.agent?.[agentName]
    const direct = agent?.handoff_text
    if (typeof direct === "string" && direct.trim()) return direct
    const opts = agent?.options as Record<string, unknown> | undefined
    const folded = opts?.handoff_text
    if (typeof folded === "string" && folded.trim()) return folded
  } catch {
    // config lookup failed — fall through to the plugin-level static_text
  }
  return fallback
}

function normalizeModel(value: string) {
  const slash = value.indexOf("/")
  if (slash <= 0 || slash === value.length - 1) {
    throw new Error(`model must be "providerID/modelID", got "${value}"`)
  }
  return { providerID: value.slice(0, slash), modelID: value.slice(slash + 1) }
}

export const server: Plugin = async ({ client }, rawOptions = {}) => {
  const options: RestartPluginOptions = {
    ...DEFAULT_OPTIONS,
    ...(rawOptions as Partial<RestartPluginOptions>),
  }

  return {
    tool: {
      session_restart: tool({
        description:
          "Start a fresh child session on this project with a clean context window and prompt it to continue a single focused task, " +
          "keeping the current session alive with its full history readable in the session tree. " +
          "Use this when the current session is getting too long or about to compact, so work continues without losing context fidelity. " +
          "The current session is always left running: after calling this tool, stop working here and hand off — the restarted session continues.",
        args: {
          task: tool.schema
            .string()
            .describe("The single focused task for the restarted session, written as a fresh prompt."),
          notes: tool
            .schema.string()
            .optional()
            .describe(
              "Durable context to carry over: decisions, constraints, files touched, unfinished steps, commands run. Omit to rely on the working tree only.",
            ),
          model: tool
            .schema.string()
            .optional()
            .describe('Model for the restarted session as "providerID/modelID". Defaults to this session\'s model.'),
          agent: tool.schema.string().optional().describe("Agent for the restarted session. Defaults to the current agent."),
        },
        async execute(args, ctx) {
          const title = args.task.length > MAX_TITLE_LENGTH ? `${args.task.slice(0, MAX_TITLE_LENGTH - 1)}…` : args.task

          const created = await client.session.create({
            body: { title, parentID: ctx.sessionID },
          })
          const restartedID = created.data?.id
          if (!restartedID) throw new Error("session.create returned no session id")

          const staticText = await resolveStaticText(client, ctx.agent, args.agent, options.static_text)
          const parts: TextPartInput[] = [
            { type: "text", text: buildHandoff(args.task, args.notes, staticText, ctx.sessionID) },
          ]
          await client.session.promptAsync({
            path: { id: restartedID },
            body: {
              parts,
              ...(args.model ? { model: normalizeModel(args.model) } : {}),
              ...(args.agent ? { agent: args.agent } : {}),
            },
          })

          return {
            title: `Restarted in session ${restartedID}`,
            output: `Started a fresh child session ${restartedID} with a clean context window; it is already running the focused task. The current session stays alive with its full history readable. Switch to session ${restartedID} to continue the work.`,
            metadata: {
              restartedSessionID: restartedID,
              parentSessionID: ctx.sessionID,
            },
          }
        },
      }),
    },
  }
}

export default { id, server }