Allow compatibility in serialization

When HMAT_READ_NRCHILD_CHAR environment variable is set, files are read
using old format.
We always write in new format though in order to ensure that we can get
rid of this compatibility layer in a near future.

src/serialization.cpp

@@ -21,11 +21,13 @@
 */
 
 #include "serialization.hpp"
-#include <vector>
 #include "compression.hpp"
 #include "rk_matrix.hpp"
 #include "common/my_assert.h"
 
+#include <cstdlib>
+#include <vector>
+
 namespace hmat {
 
 // TODO decide what to todo with read/write return value
@@ -205,7 +207,14 @@ template<typename TR> TR * MatrixStructUnmarshaller<T>::readTree(TR * tree) {
     tree = readTreeNode(tree);
     if(tree != NULL) {
         tree->depth = depth;
-        int nrChild = readValue<int>();
+        // Number of children was initially written as a char, and now as an int.
+        // When environment variable HMAT_READ_NRCHILD_CHAR is set, read_nrchild_char
+        // is set to 1 and number of children is read as a char.
+        static int read_nrchild_char = -1;
+        if (read_nrchild_char == -1) {
+          read_nrchild_char = getenv("HMAT_READ_NRCHILD_CHAR") != nullptr;
+        }
+        int nrChild = read_nrchild_char == 0 ? readValue<int>() : readValue<char>();
         for(int i = 0; i < nrChild; i++) {
             tree->insertChild(i, readTree(tree));
         }