lua-scripts-w-api/contrib/darktable-api/preview.lua

-- Serve image previews from darktable's mipmap cache.
--
-- Darktable stores mipmaps as individual JPEG files at:
--   <cache_dir>/mipmaps-<sha1_of_library_realpath>.d/<mip_level>/<imgid>.jpg
--
-- Mip levels (approximate pixel sizes on the long edge):
--   0 → ~67 px   1 → ~135 px   2 → ~270 px   3 → ~540 px
--   4 → ~1080 px (good "medium")   5 → ~2160 px
--
-- If no cached mipmap is found we fall back to exiftool (embedded EXIF
-- thumbnail) and finally to serving the original file path as a redirect.
local ffi    = require "ffi"
local config = require "config"

ffi.cdef[[
  unsigned char *SHA1(const unsigned char *d, size_t n, unsigned char *md);
]]
local crypto = ffi.load("crypto")

local M = {}

-- ── Locate the mipmap cache directory (computed once) ─────────────────────────

local _mipmap_base  -- e.g. "/home/user/.cache/darktable/mipmaps-abc123"

local function mipmap_base()
  if _mipmap_base then return _mipmap_base end

  -- Resolve the real path of library.db (darktable uses realpath before hashing)
  local h = io.popen("realpath " .. config.library_db .. " 2>/dev/null")
  local realpath = h:read("*l"); h:close()
  realpath = realpath or config.library_db

  -- SHA1 hex of the realpath (same algorithm as mipmap_cache.c)
  local md  = ffi.new("unsigned char[20]")
  crypto.SHA1(realpath, #realpath, md)
  local hex = {}
  for i = 0, 19 do hex[i+1] = string.format("%02x", md[i]) end
  local sha1hex = table.concat(hex)

  _mipmap_base = config.cache_dir .. "/mipmaps-" .. sha1hex
  return _mipmap_base
end

-- ── Size → mip level mapping ──────────────────────────────────────────────────

-- size param: "thumb" (~135 px), "small" (~270 px), "medium" (~540 px),
--             "large" (~1080 px), "xlarge" (~2160 px), or an integer level 0-5.
local SIZE_TO_MIP = {
  thumb  = 1,
  small  = 2,
  medium = 3,
  large  = 4,
  xlarge = 5,
}

local function size_to_mip(size)
  if not size or size == "" then return 3 end   -- default: "medium"
  local n = tonumber(size)
  if n then return math.max(0, math.min(5, math.floor(n))) end
  return SIZE_TO_MIP[size:lower()] or 3
end

-- ── Try to read a file, return nil on failure ─────────────────────────────────

local function read_file(path)
  local f = io.open(path, "rb")
  if not f then return nil end
  local data = f:read("*a")
  f:close()
  return data
end

-- ── Public: get preview data for an image id ──────────────────────────────────
--
-- Returns: data (string), content_type (string)
--      or: nil, nil, redirect_url   (caller should issue a 302)
--      or: nil, nil, nil            (404)
function M.get(image_id, image_path, filename, size_param)
  local mip = size_to_mip(size_param)

  -- 1. Walk mip levels: first downward (smaller = smaller file, faster),
  --    then upward (larger mip levels, let the browser scale down).
  --    This ensures we serve something even when only higher-resolution
  --    mipmaps have been cached (e.g. darktable only wrote levels 2-4).
  local base = mipmap_base()
  for level = mip, 0, -1 do
    local path = string.format("%s.d/%d/%d.jpg", base, level, image_id)
    local data = read_file(path)
    if data then return data, "image/jpeg" end
  end
  for level = mip + 1, 5 do
    local path = string.format("%s.d/%d/%d.jpg", base, level, image_id)
    local data = read_file(path)
    if data then return data, "image/jpeg" end
  end

  -- 2. Try exiftool to extract the embedded EXIF thumbnail from the source file.
  if image_path and filename then
    local src = image_path .. "/" .. filename
    -- exiftool writes the thumbnail to stdout with -b
    local cmd = string.format("exiftool -b -ThumbnailImage %q 2>/dev/null", src)
    local fh = io.popen(cmd)
    if fh then
      local data = fh:read("*a"); fh:close()
      if data and #data > 100 then
        return data, "image/jpeg"
      end
    end
  end

  -- 3. For JPEG/PNG source files redirect to the original (client caches it).
  if image_path and filename then
    local src = image_path .. "/" .. filename
    local ext = filename:match("%.([^%.]+)$") or ""
    if ext:lower():match("^jpe?g$") or ext:lower() == "png" then
      return nil, nil, "file://" .. src  -- caller issues redirect or reads directly
    end
  end

  return nil, nil, nil   -- no preview available
end

-- ── Helper: read a local file and serve it inline ─────────────────────────────
-- Used by the API handler when get() returns a file:// redirect.
function M.read_local(path)
  local stripped = path:match("^file://(.+)$") or path
  return read_file(stripped)
end

return M