/**
 * System Select — Switch the system prompt via /system
 *
 * Scans .pi/agents/, .claude/agents/, .gemini/agents/, .codex/agents/
 * (project-local and global) for agent definition .md files.
 *
 * /system opens a select dialog to pick a system prompt. The selected
 * agent's body is prepended to Pi's default instructions so tool usage
 * still works. Tools are restricted to the agent's declared tool set
 * if specified.
 *
 * Usage: pi -e extensions/system-select.ts -e extensions/minimal.ts
 */

import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
import { readdirSync, readFileSync, existsSync } from "node:fs";
import { join, basename } from "node:path";
import { homedir } from "node:os";
import { applyExtensionDefaults } from "./themeMap.ts";

interface AgentDef {
	name: string;
	description: string;
	tools: string[];
	body: string;
	source: string;
}

function parseFrontmatter(raw: string): { fields: Record<string, string>; body: string } {
	const match = raw.match(/^---\s*\n([\s\S]*?)\n---\s*\n([\s\S]*)$/);
	if (!match) return { fields: {}, body: raw };
	const fields: Record<string, string> = {};
	for (const line of match[1].split("\n")) {
		const idx = line.indexOf(":");
		if (idx > 0) fields[line.slice(0, idx).trim()] = line.slice(idx + 1).trim();
	}
	return { fields, body: match[2] };
}

function scanAgents(dir: string, source: string): AgentDef[] {
	if (!existsSync(dir)) return [];
	const agents: AgentDef[] = [];
	try {
		for (const file of readdirSync(dir)) {
			if (!file.endsWith(".md")) continue;
			const raw = readFileSync(join(dir, file), "utf-8");
			const { fields, body } = parseFrontmatter(raw);
			agents.push({
				name: fields.name || basename(file, ".md"),
				description: fields.description || "",
				tools: fields.tools ? fields.tools.split(",").map((t) => t.trim()) : [],
				body: body.trim(),
				source,
			});
		}
	} catch {}
	return agents;
}

function displayName(name: string): string {
	return name.split("-").map(w => w.charAt(0).toUpperCase() + w.slice(1)).join(" ");
}

export default function (pi: ExtensionAPI) {
	let activeAgent: AgentDef | null = null;
	let allAgents: AgentDef[] = [];
	let defaultTools: string[] = [];

	pi.on("session_start", async (_event, ctx) => {
		applyExtensionDefaults(import.meta.url, ctx);
		activeAgent = null;
		allAgents = [];

		const home = homedir();
		const cwd = ctx.cwd;

		const dirs: [string, string][] = [
			[join(cwd, ".pi", "agents"), ".pi"],
			[join(cwd, ".claude", "agents"), ".claude"],
			[join(cwd, ".gemini", "agents"), ".gemini"],
			[join(cwd, ".codex", "agents"), ".codex"],
			[join(home, ".pi", "agent", "agents"), "~/.pi"],
			[join(home, ".claude", "agents"), "~/.claude"],
			[join(home, ".gemini", "agents"), "~/.gemini"],
			[join(home, ".codex", "agents"), "~/.codex"],
		];

		const seen = new Set<string>();
		const sourceCounts: Record<string, number> = {};

		for (const [dir, source] of dirs) {
			const agents = scanAgents(dir, source);
			for (const agent of agents) {
				const key = agent.name.toLowerCase();
				if (seen.has(key)) continue;
				seen.add(key);
				allAgents.push(agent);
				sourceCounts[source] = (sourceCounts[source] || 0) + 1;
			}
		}

		defaultTools = pi.getActiveTools();
		ctx.ui.setStatus("system-prompt", "System Prompt: Default");

		const defaultPrompt = ctx.getSystemPrompt();
		const lines = defaultPrompt.split("\n").length;
		const chars = defaultPrompt.length;
		
		const loadedSources = Object.entries(sourceCounts)
			.map(([src, count]) => `${count} from ${src}`)
			.join(", ");
		
		const notifyLines = [];
		if (allAgents.length > 0) {
			notifyLines.push(`Loaded ${allAgents.length} agents (${loadedSources})`);
		}
		notifyLines.push(`System Prompt: Default (${lines} lines, ${chars} chars)`);
		
		ctx.ui.notify(notifyLines.join("\n"), "info");
	});

	pi.registerCommand("system", {
		description: "Select a system prompt from discovered agents",
		handler: async (_args, ctx) => {
			if (allAgents.length === 0) {
				ctx.ui.notify("No agents found in .*/agents/*.md", "warning");
				return;
			}

			const options = [
				"Reset to Default",
				...allAgents.map((a) => `${a.name} — ${a.description} [${a.source}]`),
			];

			const choice = await ctx.ui.select("Select System Prompt", options);
			if (choice === undefined) return;

			if (choice === options[0]) {
				activeAgent = null;
				pi.setActiveTools(defaultTools);
				ctx.ui.setStatus("system-prompt", "System Prompt: Default");
				ctx.ui.notify("System Prompt reset to Default", "success");
				return;
			}

			const idx = options.indexOf(choice) - 1;
			const agent = allAgents[idx];
			activeAgent = agent;

			if (agent.tools.length > 0) {
				pi.setActiveTools(agent.tools);
			} else {
				pi.setActiveTools(defaultTools);
			}

			ctx.ui.setStatus("system-prompt", `System Prompt: ${displayName(agent.name)}`);
			ctx.ui.notify(`System Prompt switched to: ${displayName(agent.name)}`, "success");
		},
	});

	pi.on("before_agent_start", async (event, _ctx) => {
		if (!activeAgent) return;
		return {
			systemPrompt: activeAgent.body + "\n\n" + event.systemPrompt,
		};
	});
}