import { ThemeObject } from '@o/css'

import { weakKey } from '../helpers/WeakKeys'
import { CompiledTheme } from './createTheme'
import { ThemeSelect } from './Theme'

type PartialTheme = Partial<CompiledTheme>

export function selectThemeSubset(
  subTheme: ThemeSelect,
  theme: CompiledTheme,
): CompiledTheme | null {
  if (!subTheme) {
    return theme
  }
  if (!theme._isCoat && subTheme === theme._subTheme) {
    // hasnt changed
    return null
  }
  // allow only subsetting coats or original themes, not subsets
  while (theme._isSubTheme || theme._isCoat) {
    theme = theme.parent
  }
  if (typeof subTheme === 'function') {
    return createSubSetTheme(`subfn${weakKey(subTheme)}`, theme, subTheme(theme), subTheme)
  }
  // generate new subset theme
  const selectedTheme: PartialTheme = theme[subTheme]
  if (!selectedTheme) {
    return null
  }
  return createSubSetTheme(subTheme, theme, selectedTheme, subTheme)
}

const createSubSetTheme = (
  name: string,
  parent: CompiledTheme,
  child: ThemeObject | CompiledTheme,
  subTheme: ThemeSelect,
) => {
  return {
    ...child,
    coats: undefined,
    parent,
    name,
    _subThemeName: name,
    _subTheme: subTheme,
    _isSubTheme: true,
  }
}