refactor: restructure plugin layout for better separation

__tests__/test-utils.ts

@@ -1,7 +1,8 @@
 import postcss from 'postcss';
 import nested from 'postcss-nested';
 
-import plugin, { PostcssThemeOptions } from '../src/index';
+import plugin  from '../src/index';
+import { PostcssThemeOptions } from '../src/types';
 
 export function normalizeResult(input: string) {
   return input

src/common/index.ts

@@ -0,0 +1,43 @@
+import path from 'path';
+import { PostcssThemeConfig, PostcssStrictThemeConfig, Theme } from "../types";
+
+/** Get the theme variable name from a string */
+export const parseThemeKey = (value: string) => {
+  const key = value.match(/@theme ([a-zA-Z-_0-9]+)/);
+  return key ? key[1] : '';
+}
+
+/** Replace a theme variable reference with a value */
+export const replaceTheme = (value: string, replace: string) => {
+  return value.replace(/@theme ([a-zA-Z-_0-9]+)/, replace);
+}
+
+/** Get the location of the theme file */
+export function getThemeFilename(cssFile: string) {
+  return path.join(path.dirname(cssFile), 'theme');
+}
+
+/** Remove :theme-root usage from a selector */
+export const replaceThemeRoot = (selector: string) =>
+  selector.replace(/:theme-root\((\S+)\)/g, '$1').replace(/:theme-root/g, '');
+
+/** Make a SimpleTheme into a LightDarkTheme */
+export const normalizeTheme = (config: PostcssThemeConfig | {}): PostcssStrictThemeConfig => {
+  return Object.assign(
+    {},
+    ...Object.entries(config).map(([theme, themeConfig]) => {
+      if ('light' in themeConfig && 'dark' in themeConfig) {
+          return { [theme]: themeConfig };
+      }
+
+      return { [theme]: { light: themeConfig, dark: {} } };
+    })
+  );
+};
+
+/** Determine if a theme has dark mode enabled */
+export const hasDarkMode = (theme: Theme) =>
+  Boolean(
+    Object.keys(theme.dark).length > 0 && Object.keys(theme.light).length > 0
+  );
+