mimic/frontend/src/hooks/useTheme.ts

import { useCallback, useEffect, useState } from 'react';

export type Theme = 'light' | 'dark' | 'system';

const STORAGE_KEY = 'mimic-theme';

function resolveTheme(theme: Theme): 'light' | 'dark' {
  if (theme === 'system') {
    return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
  }
  return theme;
}

function applyTheme(theme: Theme) {
  const resolved = resolveTheme(theme);
  document.documentElement.classList.toggle('dark', resolved === 'dark');
}

function readStoredTheme(): Theme {
  try {
    const stored = localStorage.getItem(STORAGE_KEY);
    if (stored === 'light' || stored === 'dark' || stored === 'system') return stored;
  } catch {
    // localStorage unavailable
  }
  return 'system';
}

export function useTheme() {
  const [theme, setThemeState] = useState<Theme>(readStoredTheme);

  useEffect(() => {
    applyTheme(theme);
  }, [theme]);

  // Track system preference changes when theme === 'system'
  useEffect(() => {
    if (theme !== 'system') return;
    const mq = window.matchMedia('(prefers-color-scheme: dark)');
    const handler = () => applyTheme('system');
    mq.addEventListener('change', handler);
    return () => mq.removeEventListener('change', handler);
  }, [theme]);

  const setTheme = useCallback((next: Theme) => {
    try {
      localStorage.setItem(STORAGE_KEY, next);
    } catch {
      // ignore
    }
    setThemeState(next);
  }, []);

  const cycleTheme = useCallback(() => {
    setTheme(theme === 'light' ? 'dark' : theme === 'dark' ? 'system' : 'light');
  }, [theme, setTheme]);

  return { theme, setTheme, cycleTheme };
}