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

Function extracted from upstream:
  PR bitcoin/bitcoin#11167
  Commit c091b99379b97cb314c9fa123beabdbc324cf7a4

src/Makefile.test.include

@@ -61,6 +61,7 @@ BITCOIN_TESTS =\
   test/Checkpoints_tests.cpp \
   test/coins_tests.cpp \
   test/compress_tests.cpp \
+  test/convertbits_tests.cpp \
   test/crypto_tests.cpp \
   test/DoS_tests.cpp \
   test/equihash_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_bitcoin.h>
+#include <zcash/NoteEncryption.hpp>
+
+#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 = libzcash::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

@@ -133,4 +133,28 @@ bool TimingResistantEqual(const T& a, const T& b)
  */
 bool ParseFixedPoint(const std::string &val, int decimals, int64_t *amount_out);
 
+/** 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