Support building gradient index with cat data. (#7371)

src/common/hist_util.h

@@ -16,13 +16,12 @@
 #include <utility>
 #include <map>
 
-#include "row_set.h"
+#include "categorical.h"
 #include "common.h"
+#include "quantile.h"
+#include "row_set.h"
 #include "threading_utils.h"
-#include "../tree/param.h"
-#include "./quantile.h"
-#include "./timer.h"
-#include "../include/rabit/rabit.h"
+#include "timer.h"
 
 namespace xgboost {
 class GHistIndexMatrix;
@@ -105,9 +104,29 @@ class HistogramCuts {
     return idx;
   }
 
+  /**
+   * \brief Search the bin index for numerical feature.
+   */
   BinIdx SearchBin(Entry const& e) const {
     return SearchBin(e.fvalue, e.index);
   }
+
+  /**
+   * \brief Search the bin index for categorical feature.
+   */
+  BinIdx SearchCatBin(Entry const &e) const {
+    auto const &ptrs = this->Ptrs();
+    auto const &vals = this->Values();
+    auto end = ptrs.at(e.index + 1) + vals.cbegin();
+    auto beg = ptrs[e.index] + vals.cbegin();
+    // Truncates the value in case it's not perfectly rounded.
+    auto v  = static_cast<float>(common::AsCat(e.fvalue));
+    auto bin_idx = std::lower_bound(beg, end, v) - vals.cbegin();
+    if (bin_idx == ptrs.at(e.index + 1)) {
+      bin_idx -= 1;
+    }
+    return bin_idx;
+  }
 };
 
 inline HistogramCuts SketchOnDMatrix(DMatrix *m, int32_t max_bins,