trivial refine CheckElementsIntervalClosed & ObtainMinMaxSum (#1049)

* refine common.h

* fix typo

* specify captured variables

include/LightGBM/utils/common.h

@@ -596,11 +596,29 @@ static void ParallelSort(_RanIt _First, _RanIt _Last, _Pr _Pred) {
 // Check that all y[] are in interval [ymin, ymax] (end points included); throws error if not
 template <typename T>
 inline void CheckElementsIntervalClosed(const T *y, T ymin, T ymax, int ny, const char *callername) {
-  for (int i = 0; i < ny; ++i) {
-    if (y[i] < ymin || y[i] > ymax) {
-      std::ostringstream os;
-      os << "[%s]: does not tolerate element [#%i = " << y[i] << "] outside [" << ymin << ", " << ymax << "]";
-      Log::Fatal(os.str().c_str(), callername, i);
+  auto fatal_msg = [&y, &ymin, &ymax, &callername](int i) { 
+    std::ostringstream os;
+    os << "[%s]: does not tolerate element [#%i = " << y[i] << "] outside [" << ymin << ", " << ymax << "]";
+    Log::Fatal(os.str().c_str(), callername, i);
+  };
+  for (int i = 1; i < ny; i += 2) {
+    if (y[i - 1] < y[i]) {
+      if (y[i - 1] < ymin) {
+        fatal_msg(i - 1);
+      } else if (y[i] > ymax) {
+        fatal_msg(i);
+      }
+    } else {
+      if (y[i - 1] > ymax) {
+        fatal_msg(i - 1);
+      } else if (y[i] < ymin) {
+        fatal_msg(i);
+      }
+    }
+  }
+  if (ny & 1) { // odd
+    if (y[ny - 1] < ymin || y[ny - 1] > ymax) {
+      fatal_msg(ny - 1);
     }
   }
 }
@@ -609,17 +627,45 @@ inline void CheckElementsIntervalClosed(const T *y, T ymin, T ymax, int ny, cons
 // this is useful for checking weight requirements.
 template <typename T1, typename T2>
 inline void ObtainMinMaxSum(const T1 *w, int nw, T1 *mi, T1 *ma, T2 *su) {
-  T1 minw = w[0];
-  T1 maxw = w[0];
-  T2 sumw = static_cast<T2>(w[0]);
-  for (int i = 1; i < nw; ++i) {
-    sumw += w[i];
-    if (w[i] < minw) minw = w[i];
-    if (w[i] > maxw) maxw = w[i];
-  }
-  if (mi != nullptr) *mi = minw;
-  if (ma != nullptr) *ma = maxw;
-  if (su != nullptr) *su = sumw;
+  T1 minw;
+  T1 maxw;
+  T1 sumw;
+  int i;
+  if (nw & 1) { // odd
+    minw = w[0];
+    maxw = w[0];
+    sumw = w[0];
+    i = 2;
+  } else { // even
+    if (w[0] < w[1]) {
+      minw = w[0];
+      maxw = w[1];
+    } else {
+      minw = w[1];
+      maxw = w[0];
+    }
+    sumw = w[0] + w[1];
+    i = 3;
+  }
+  for (; i < nw; i += 2) {
+    if (w[i - 1] < w[i]) {
+      minw = std::min(minw, w[i - 1]);
+      maxw = std::max(maxw, w[i]);
+    } else {
+      minw = std::min(minw, w[i]);
+      maxw = std::max(maxw, w[i - 1]);
+    }
+    sumw += w[i - 1] + w[i];
+  }
+  if (mi != nullptr) {
+    *mi = minw;
+  }
+  if (ma != nullptr) {
+    *ma = maxw;
+  }
+  if (su != nullptr) {
+    *su = static_cast<T2>(sumw);
+  }
 }
 
 template<class T>