[R] Add missing DMatrix functions (#9929)

* `XGDMatrixGetQuantileCut`
* `XGDMatrixNumNonMissing`
* `XGDMatrixGetDataAsCSR`

---------

Co-authored-by: Jiaming Yuan <jm.yuan@outlook.com>

R-package/tests/testthat/test_dmatrix.R

@@ -375,3 +375,62 @@ test_that("xgb.DMatrix: can take multi-dimensional 'base_margin'", {
   )
   expect_equal(pred_only_x, pred_w_base - b, tolerance = 1e-5)
 })
+
+test_that("xgb.DMatrix: number of non-missing matches data", {
+  x <- matrix(1:10, nrow = 5)
+  dm1 <- xgb.DMatrix(x)
+  expect_equal(xgb.get.DMatrix.num.non.missing(dm1), 10)
+
+  x[2, 2] <- NA
+  x[4, 1] <- NA
+  dm2 <- xgb.DMatrix(x)
+  expect_equal(xgb.get.DMatrix.num.non.missing(dm2), 8)
+})
+
+test_that("xgb.DMatrix: retrieving data as CSR", {
+  data(mtcars)
+  dm <- xgb.DMatrix(as.matrix(mtcars))
+  csr <- xgb.get.DMatrix.data(dm)
+  expect_equal(dim(csr), dim(mtcars))
+  expect_equal(colnames(csr), colnames(mtcars))
+  expect_equal(unname(as.matrix(csr)), unname(as.matrix(mtcars)), tolerance = 1e-6)
+})
+
+test_that("xgb.DMatrix: quantile cuts look correct", {
+  data(mtcars)
+  y <- mtcars$mpg
+  x <- as.matrix(mtcars[, -1])
+  dm <- xgb.DMatrix(x, label = y)
+  model <- xgb.train(
+    data = dm,
+    params = list(
+      tree_method = "hist",
+      max_bin = 8,
+      nthread = 1
+    ),
+    nrounds = 3
+  )
+  qcut_list <- xgb.get.DMatrix.qcut(dm, "list")
+  qcut_arrays <- xgb.get.DMatrix.qcut(dm, "arrays")
+
+  expect_equal(length(qcut_arrays), 2)
+  expect_equal(names(qcut_arrays), c("indptr", "data"))
+  expect_equal(length(qcut_arrays$indptr), ncol(x) + 1)
+  expect_true(min(diff(qcut_arrays$indptr)) > 0)
+
+  col_min <- apply(x, 2, min)
+  col_max <- apply(x, 2, max)
+
+  expect_equal(length(qcut_list), ncol(x))
+  expect_equal(names(qcut_list), colnames(x))
+  lapply(
+    seq(1, ncol(x)),
+    function(col) {
+      cuts <- qcut_list[[col]]
+      expect_true(min(diff(cuts)) > 0)
+      expect_true(col_min[col] > cuts[1])
+      expect_true(col_max[col] < cuts[length(cuts)])
+      expect_true(length(cuts) <= 9)
+    }
+  )
+})