[R-package] reduce cost of repeated parameter alias checks (#5141)

* [R-package] reduce cost of repeated parameter alias checks

* formatting

R-package/R/aliases.R

@@ -33,11 +33,18 @@
     )])
 }
 
+# [description] Non-exported environment, used for caching details that only need to be
+#               computed once per R session.
+.lgb_session_cache_env <- new.env()
+
 # [description] List of respected parameter aliases. Wrapped in a function to take advantage of
 #               lazy evaluation (so it doesn't matter what order R sources files during installation).
 # [return] A named list, where each key is a main LightGBM parameter and each value is a character
 #          vector of corresponding aliases.
 .PARAMETER_ALIASES <- function() {
+    if (exists("PARAMETER_ALIASES", where = .lgb_session_cache_env)) {
+        return(get("PARAMETER_ALIASES", envir = .lgb_session_cache_env))
+    }
     params_to_aliases <- jsonlite::fromJSON(
         .Call(
             LGBM_DumpParamAliases_R
@@ -47,6 +54,12 @@
         aliases_with_main_name <- c(main_name, unlist(params_to_aliases[[main_name]]))
         params_to_aliases[[main_name]] <- aliases_with_main_name
     }
+    # store in cache so the next call to `.PARAMETER_ALIASES()` doesn't need to recompute this
+    assign(
+        x = "PARAMETER_ALIASES"
+        , value = params_to_aliases
+        , envir = .lgb_session_cache_env
+    )
     return(params_to_aliases)
 }
 

R-package/tests/testthat/test_parameters.R

@@ -58,6 +58,37 @@ test_that(".PARAMETER_ALIASES() returns a named list of character vectors, where
   expect_equal(sort(param_aliases[["task"]]), c("task", "task_type"))
 })
 
+test_that(".PARAMETER_ALIASES() uses the internal session cache", {
+
+  cache_key <- "PARAMETER_ALIASES"
+
+  # clear cache, so this test isn't reliant on the order unit tests are run in
+  if (exists(cache_key, where = .lgb_session_cache_env)) {
+    rm(list = cache_key, envir = .lgb_session_cache_env)
+  }
+  expect_false(exists(cache_key, where = .lgb_session_cache_env))
+
+  # check that result looks correct for at least one parameter
+  iter_aliases <- .PARAMETER_ALIASES()[["num_iterations"]]
+  expect_true(is.character(iter_aliases))
+  expect_true(all(c("num_round", "nrounds") %in% iter_aliases))
+
+  # patch the cache to check that .PARAMETER_ALIASES() checks it
+  assign(
+    x = cache_key
+    , value = list(num_iterations = c("test", "other_test"))
+    , envir = .lgb_session_cache_env
+  )
+  iter_aliases <- .PARAMETER_ALIASES()[["num_iterations"]]
+  expect_equal(iter_aliases, c("test", "other_test"))
+
+  # re-set cache so this doesn't interfere with other unit tests
+  if (exists(cache_key, where = .lgb_session_cache_env)) {
+    rm(list = cache_key, envir = .lgb_session_cache_env)
+  }
+  expect_false(exists(cache_key, where = .lgb_session_cache_env))
+})
+
 test_that("training should warn if you use 'dart' boosting, specified with 'boosting' or aliases", {
   for (boosting_param in .PARAMETER_ALIASES()[["boosting"]]) {
     params <- list(