Simplify BigInt::get_substring a bit

And forbid 0 length substrings, which did not work correctly anyway.

1 files changed, 29 insertions, 0 deletions

diff --git a/src/tests/test_bigint.cpp b/src/tests/test_bigint.cpp
index ad0af03f4..33c617239 100644
--- a/src/tests/test_bigint.cpp
+++ b/src/tests/test_bigint.cpp
@@ -36,6 +36,7 @@ class BigInt_Unit_Tests final : public Test
          results.push_back(test_random_prime());
          results.push_back(test_encode());
          results.push_back(test_bigint_io());
+         results.push_back(test_get_substring());
 
          return results;
          }
@@ -206,6 +207,34 @@ class BigInt_Unit_Tests final : public Test
          return result;
          }
 
+      Test::Result test_get_substring()
+         {
+         const size_t trials = 1000;
+
+         Test::Result result("BigInt get_substring");
+
+         const Botan::BigInt r(Test::rng(), 250);
+
+         for(size_t s = 1; s <= 32; ++s)
+            {
+            for(size_t trial = 0; trial != trials; ++trial)
+               {
+               const size_t offset = Test::rng().next_byte();
+
+               const uint32_t val = r.get_substring(offset, s);
+
+               Botan::BigInt t = r >> offset;
+               t.mask_bits(s);
+
+               const uint32_t cmp = t.to_u32bit();
+
+               result.test_eq("Same value", size_t(val), size_t(cmp));
+               }
+            }
+
+         return result;
+         }
+
       Test::Result test_bigint_io()
          {
          Test::Result result("BigInt IO operators");