local LrApplication = import 'LrApplication'   -- Import LR namespace which provides access to active catalog
local LrDialogs = import 'LrDialogs'   -- Import LR namespace for user dialog functions
local LrTasks = import 'LrTasks'       -- Import functions for starting async tasks
local LrMobdebug = import 'LrMobdebug' -- Import LR/ZeroBrane debug module

LrMobdebug.start()          

-- Connect with the ZBS debugger server.
LrTasks.startAsyncTask (function()          -- Certain functions in LR which access the catalog need to be wrapped in an asyncTask.
  LrMobdebug.on()                           -- Make this coroutine known to ZBS
  catalog = LrApplication.activeCatalog()   -- Get the active LR catalog. 
  selectedPhoto = catalog:getTargetPhoto()  -- Get the photo currently selected by the user. (type: LrPhoto)
  
  shutterspeed = selectedPhoto:getRawMetadata("shutterSpeed")  -- Receive raw (value-only) metadata from the photo, for shutter speed
  shutterspeedFormatted = selectedPhoto:getFormattedMetadata("shutterSpeed") -- Receive formatted metadata, fit for printing to the user
  aperture = selectedPhoto:getRawMetadata("aperture")          -- Receive aperture metadata from photo
  apertureFormatted = selectedPhoto:getFormattedMetadata("aperture")         -- Formatted version, fit for printing
  iso = selectedPhoto:getRawMetadata("isoSpeedRating")         -- Receive ISO speed rating from photo
  
  temp = 100*aperture^2/(shutterspeed*iso)       -- First part of calculation of exposure value
  exposureValue = math.log10(temp) / math.log10(2) -- take log 10, then change to log base 2 (log base 2 is n/a in Lua math library)   
  exposureValue = round(exposureValue,2)         -- Round to 2 decimal places
  
  LrDialogs.message ("Shutter speed: " .. shutterspeedFormatted .. ", aperture: " .. apertureFormatted .. ", ISO: " ..iso.. "\nThe exposure value of this photo is "..tostring(exposureValue)..".")   -- Output information to user.
end) -- end of startAsyncTask

function round(num, idp)                -- Helper function to round value <num> to <idp> decimal places
  local mult = 10^(idp or 0)
  return math.floor(num * mult + 0.5) / mult
end