ConvertBits() - convert from one power-of-2 number base to another.

Function extracted from upstream:
  PR bitcoin#11167
  Commit c091b99

src/Makefile.test.include

@@ -56,6 +56,7 @@ BITCOIN_TESTS =\
   test/checkblock_tests.cpp \
   test/Checkpoints_tests.cpp \
   test/coins_tests.cpp \
+  test/convertbits_tests.cpp \
   test/compress_tests.cpp \
   test/crypto_tests.cpp \
   test/DoS_tests.cpp \

src/test/convertbits_tests.cpp

@@ -0,0 +1,52 @@
+// Copyright (c) 2018 The Zcash developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <utilstrencodings.h>
+#include <test/test_pivx.h>
+#include "sapling/util.h"
+
+#include <boost/test/unit_test.hpp>
+
+BOOST_FIXTURE_TEST_SUITE(convertbits_tests, BasicTestingSetup)
+
+BOOST_AUTO_TEST_CASE(convertbits_deterministic)
+{
+    for (size_t i = 0; i < 256; i++) {
+        std::vector<unsigned char> input(32, i);
+        std::vector<unsigned char> data;
+        std::vector<unsigned char> output;
+        ConvertBits<8, 5, true>(data, input.begin(), input.end());
+        ConvertBits<5, 8, false>(output, data.begin(), data.end());
+        BOOST_CHECK_EQUAL(data.size(), 52);
+        BOOST_CHECK_EQUAL(output.size(), 32);
+        BOOST_CHECK(input == output);
+    }
+
+    for (size_t i = 0; i < 256; i++) {
+        std::vector<unsigned char> input(43, i);
+        std::vector<unsigned char> data;
+        std::vector<unsigned char> output;
+        ConvertBits<8, 5, true>(data, input.begin(), input.end());
+        ConvertBits<5, 8, false>(output, data.begin(), data.end());
+        BOOST_CHECK_EQUAL(data.size(), 69);
+        BOOST_CHECK_EQUAL(output.size(), 43);
+        BOOST_CHECK(input == output);
+    }
+}
+
+BOOST_AUTO_TEST_CASE(convertbits_random)
+{
+    for (size_t i = 0; i < 1000; i++) {
+        auto input = random_uint256();
+        std::vector<unsigned char> data;
+        std::vector<unsigned char> output;
+        ConvertBits<8, 5, true>(data, input.begin(), input.end());
+        ConvertBits<5, 8, false>(output, data.begin(), data.end());
+        BOOST_CHECK_EQUAL(data.size(), 52);
+        BOOST_CHECK_EQUAL(output.size(), 32);
+        BOOST_CHECK(input == uint256(output));
+    }
+}
+
+BOOST_AUTO_TEST_SUITE_END()

src/utilstrencodings.h

@@ -149,4 +149,29 @@ bool TimingResistantEqual(const T& a, const T& b)
     return accumulator == 0;
 }
 
+/** Convert from one power-of-2 number base to another. */
+template<int frombits, int tobits, bool pad, typename O, typename I>
+bool ConvertBits(O& out, I it, I end) {
+     size_t acc = 0;
+     size_t bits = 0;
+     constexpr size_t maxv = (1 << tobits) - 1;
+     constexpr size_t max_acc = (1 << (frombits + tobits - 1)) - 1;
+     while (it != end) {
+         acc = ((acc << frombits) | *it) & max_acc;
+         bits += frombits;
+         while (bits >= tobits) {
+             bits -= tobits;
+             out.push_back((acc >> bits) & maxv);
+         }
+         ++it;
+     }
+     if (pad) {
+         if (bits) out.push_back((acc << (tobits - bits)) & maxv);
+     } else if (bits >= frombits || ((acc << (tobits - bits)) & maxv)) {
+         return false;
+     }
+     return true;
+ }
+
+
 #endif // BITCOIN_UTILSTRENCODINGS_H