/**
 * toknmtr agent — per-file byte-offset cursor.
 *
 * Persists, per transcript file, the byte offset up to which we've already parsed and
 * pushed events, so re-runs only emit NEW bytes appended since last time. Stored as a
 * single JSON file at ~/.toknmtr/cursors.json (one process at a time is assumed — there's
 * no file locking).
 *
 * Handles truncation/rotation: if a file's current size is smaller than the recorded
 * offset, the file was truncated or replaced (e.g. a session id got reused, or the file
 * was edited out from under us) — getOffset() resets to 0 so the whole file is reparsed.
 */
import { mkdirSync, readFileSync, writeFileSync } from 'node:fs';
import { homedir } from 'node:os';
import { dirname, join } from 'node:path';

export const CURSOR_FILE = join(homedir(), '.toknmtr', 'cursors.json');

interface CursorEntry {
	offset: number;
	size: number;
}

type CursorMap = Record<string, CursorEntry>;

let cache: CursorMap | null = null;

function load(): CursorMap {
	if (cache) return cache;
	try {
		const parsed = JSON.parse(readFileSync(CURSOR_FILE, 'utf8')) as unknown;
		cache = parsed && typeof parsed === 'object' ? (parsed as CursorMap) : {};
	} catch {
		cache = {};
	}
	return cache;
}

/** Persist the in-memory cursor map to disk. Call once after a batch of setOffset() calls. */
export function save(): void {
	const map = load();
	mkdirSync(dirname(CURSOR_FILE), { recursive: true });
	writeFileSync(CURSOR_FILE, JSON.stringify(map, null, 2));
}

/**
 * Byte offset to resume reading `filePath` from, given its current size on disk.
 * Returns 0 (full reparse) if there's no recorded cursor, or if the file shrank since
 * last run (truncated/rotated).
 */
export function getOffset(filePath: string, currentSize: number): number {
	const entry = load()[filePath];
	if (!entry) return 0;
	if (entry.offset > currentSize) return 0;
	return entry.offset;
}

/** Record the new offset/size for `filePath` in memory. Call save() to persist. */
export function setOffset(filePath: string, offset: number, size: number): void {
	load()[filePath] = { offset, size };
}

/** Forget a file's cursor entirely (forces a full reparse on next run). */
export function resetOffset(filePath: string): void {
	delete load()[filePath];
}