ans: Cleanup rans writing routines

* Replace struct rans_sym parameter with values. It only had two entries.
* Document functions.
* Use longer variable names.

TODO: Refactor the unit tests.

Change-Id: Idad65117dab565786697015ef07070e1c308d963
diff --git a/test/ans_test.cc b/test/ans_test.cc
index 2e513ea..a553a9e 100644
--- a/test/ans_test.cc
+++ b/test/ans_test.cc
@@ -87,6 +87,11 @@
 const int kRansSymbols =
     static_cast<int>(sizeof(spareto65) / sizeof(spareto65[0]));
 
+struct rans_sym {
+  aom_cdf_prob prob;
+  aom_cdf_prob cum_prob;  // not-inclusive
+};
+
 std::vector<int> ans_encode_build_vals(rans_sym *const tab, int iters) {
   aom_cdf_prob sum = 0;
   for (int i = 0; i < kRansSymbols; ++i) {
@@ -128,7 +133,7 @@
   std::clock_t start = std::clock();
   for (std::vector<int>::const_iterator it = sym_vec.begin();
        it != sym_vec.end(); ++it) {
-    buf_rans_write(&a, &tab[*it]);
+    buf_rans_write(&a, tab[*it].cum_prob, tab[*it].prob);
   }
   aom_buf_ans_flush(&a);
   std::clock_t enc_time = std::clock() - start;