let s:path = get(g:, 'vim_dir', expand('~/.vim')) . '/theme.json'
let s:defaults = {
      \ 'background': 'dark',
      \ 'dark': {'theme': 'everforest', 'transparent': v:false},
      \ 'light': {'theme': 'PaperColor', 'transparent': v:false}}
let s:config = deepcopy(s:defaults)
let s:applying = v:false
let s:transparent_groups = [
      \ 'Normal', 'NormalNC', 'EndOfBuffer', 'Folded', 'LineNr', 'SignColumn',
      \ 'StatusLine', 'StatusLineNC', 'VertSplit', 'WinSeparator', 'NormalFloat']

function! s:Merge(defaults, override) abort
  let l:result = deepcopy(a:defaults)
  if type(a:override) != v:t_dict
    return l:result
  endif
  for [l:key, l:value] in items(a:override)
    if type(l:value) == v:t_dict && type(get(l:result, l:key)) == v:t_dict
      let l:result[l:key] = s:Merge(l:result[l:key], l:value)
    else
      let l:result[l:key] = l:value
    endif
  endfor
  return l:result
endfunction

function! s:Available(name) abort
  return !empty(globpath(&runtimepath, 'colors/' . a:name . '.vim', 1, 1))
endfunction

function! s:ApplyTransparency() abort
  if !s:config[&background].transparent
    return
  endif
  for l:group in s:transparent_groups
    execute 'highlight ' . l:group . ' guibg=NONE ctermbg=NONE'
  endfor
endfunction

function! s:Load() abort
  if !filereadable(s:path)
    return deepcopy(s:defaults)
  endif
  try
    let l:saved = json_decode(join(readfile(s:path), "\n"))
    return s:Merge(s:defaults, l:saved)
  catch
    return deepcopy(s:defaults)
  endtry
endfunction

function! theme#save() abort
  try
    call mkdir(fnamemodify(s:path, ':h'), 'p')
    call writefile([json_encode(s:config)], s:path)
  catch
    " A read-only home directory should not prevent Vim from exiting cleanly.
  endtry
endfunction

function! theme#apply(name) abort
  let l:background = &background
  let l:name = s:Available(a:name) ? a:name : 'sunbather'
  let s:applying = v:true
  try
    execute 'colorscheme ' . l:name
  catch
    " A scheme that fails to load (for example, one that needs true color in a
    " terminal without it) must not leave Vim without a color scheme. The
    " saved preference is kept so the next capable session restores it.
    colorscheme sunbather
  finally
    let s:applying = v:false
  endtry
  let &background = l:background
  let s:config.background = l:background
  let s:config[l:background].theme = l:name
  call s:ApplyTransparency()
endfunction

function! theme#toggleBg() abort
  let l:background = &background ==# 'dark' ? 'light' : 'dark'
  let l:theme = s:config[l:background].theme
  let s:applying = v:true
  try
    let &background = l:background
  finally
    let s:applying = v:false
  endtry
  let s:config.background = l:background
  call theme#apply(l:theme)
endfunction

function! theme#toggleTransparent() abort
  let s:config[&background].transparent = !s:config[&background].transparent
  if s:config[&background].transparent
    call s:ApplyTransparency()
  else
    call theme#apply(s:config[&background].theme)
  endif
endfunction

function! theme#onColorScheme(name) abort
  if s:applying
    return
  endif
  let s:config.background = &background
  let s:config[&background].theme = a:name
  call s:ApplyTransparency()
endfunction

function! theme#Init() abort
  let s:config = s:Load()
  let &background = get(s:config, 'background', 'dark')
  call theme#apply(s:config[&background].theme)
  augroup DotfilesTheme
    autocmd!
    autocmd ColorScheme * call theme#onColorScheme(expand('<amatch>'))
    autocmd VimLeavePre * call theme#save()
  augroup END
endfunction