diff --git a/src/Makefile.am b/src/Makefile.am
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -102,6 +102,7 @@
 BITCOIN_CORE_H = \
   addrdb.h \
   addrman.h \
+  avalanche_impl.h \
   base58.h \
   bloom.h \
   blockencodings.h \
diff --git a/src/Makefile.test.include b/src/Makefile.test.include
--- a/src/Makefile.test.include
+++ b/src/Makefile.test.include
@@ -25,11 +25,12 @@
 
 # test_bitcoin binary #
 BITCOIN_TESTS =\
-  test/arith_uint256_tests.cpp \
   test/scriptnum10.h \
   test/addrman_tests.cpp \
-  test/amount_tests.cpp \
   test/allocator_tests.cpp \
+  test/amount_tests.cpp \
+  test/arith_uint256_tests.cpp \
+  test/avalanche_tests.cpp \
   test/base32_tests.cpp \
   test/base58_tests.cpp \
   test/base64_tests.cpp \
diff --git a/src/avalanche_impl.h b/src/avalanche_impl.h
new file mode 100644
--- /dev/null
+++ b/src/avalanche_impl.h
@@ -0,0 +1,82 @@
+// Copyright (c) 2018 The Bitcoin developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_AVALANCHE_IMPL_H
+#define BITCOIN_AVALANCHE_IMPL_H
+
+#include <cstdint>
+
+namespace {
+/**
+ * Finalization score.
+ */
+static int AVALANCHE_FINALIZATION_SCORE = 128;
+
+/**
+ * Vote history.
+ */
+struct VoteRecord {
+private:
+    // Historical record of votes.
+    uint16_t votes;
+
+    // confidence's LSB bit is the result. Higher bits are actual confidence
+    // score.
+    uint16_t confidence;
+
+    /**
+     * Return the number of bits set in an integer value.
+     * TODO: There are compiler intrinsics to do that, but we'd need to get them
+     * detected so this will do for now.
+     */
+    static uint32_t countBits(uint32_t value) {
+        uint32_t count = 0;
+        while (value) {
+            // If the value is non zero, then at least one bit is set.
+            count++;
+
+            // Clear the rightmost bit set.
+            value &= (value - 1);
+        }
+
+        return count;
+    }
+
+public:
+    VoteRecord() : votes(0xaaaa), confidence(0) {}
+
+    bool isValid() const { return confidence & 0x01; }
+
+    uint16_t getConfidence() const { return confidence >> 1; }
+    bool hasFinalized() const {
+        return getConfidence() >= AVALANCHE_FINALIZATION_SCORE;
+    }
+
+    bool registerVote(bool vote) {
+        votes = (votes << 1) | vote;
+
+        auto bits = countBits(votes & 0xff);
+        bool yes = bits > 6;
+        bool no = bits < 2;
+        if (!yes && !no) {
+            // The vote is inconclusive.
+            return false;
+        }
+
+        if (isValid() == yes) {
+            // If the vote is in agreement with our internal status, increase
+            // confidence.
+            confidence += 2;
+        } else {
+            // The vote did not agree with our internal state, in that case,
+            // reset confidence.
+            confidence = yes;
+        }
+
+        return true;
+    }
+};
+}
+
+#endif // BITCOIN_AVALANCHE_IMPL_H
diff --git a/src/test/CMakeLists.txt b/src/test/CMakeLists.txt
--- a/src/test/CMakeLists.txt
+++ b/src/test/CMakeLists.txt
@@ -44,10 +44,11 @@
 add_dependencies(check check-bitcoin)
 
 add_test_to_suite(bitcoin test_bitcoin
-	arith_uint256_tests.cpp
 	addrman_tests.cpp
-	amount_tests.cpp
 	allocator_tests.cpp
+	amount_tests.cpp
+	arith_uint256_tests.cpp
+	avalanche_tests.cpp
 	base32_tests.cpp
 	base58_tests.cpp
 	base64_tests.cpp
diff --git a/src/test/avalanche_tests.cpp b/src/test/avalanche_tests.cpp
new file mode 100644
--- /dev/null
+++ b/src/test/avalanche_tests.cpp
@@ -0,0 +1,69 @@
+// Copyright (c) 2010 The Bitcoin developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include "avalanche_impl.h"
+
+#include "test/test_bitcoin.h"
+
+#include <boost/test/unit_test.hpp>
+
+BOOST_FIXTURE_TEST_SUITE(avalanche_tests, BasicTestingSetup)
+
+#define REGISTER_VOTE_AND_CHECK(vr, vote, state, finalized, confidence)        \
+    vr.registerVote(vote);                                                     \
+    BOOST_CHECK_EQUAL(vr.isValid(), state);                                    \
+    BOOST_CHECK_EQUAL(vr.hasFinalized(), finalized);                           \
+    BOOST_CHECK_EQUAL(vr.getConfidence(), confidence);
+
+BOOST_AUTO_TEST_CASE(vote_record) {
+    VoteRecord vr;
+
+    // Check initial state.
+    BOOST_CHECK_EQUAL(vr.isValid(), false);
+    BOOST_CHECK_EQUAL(vr.hasFinalized(), false);
+    BOOST_CHECK_EQUAL(vr.getConfidence(), 0);
+
+    // We register one vote for, which keep things at 4/4.
+    REGISTER_VOTE_AND_CHECK(vr, true, false, false, 0);
+
+    // One more and we are at 5/3.
+    REGISTER_VOTE_AND_CHECK(vr, true, false, false, 0);
+
+    // One more and we are at 5/3.
+    REGISTER_VOTE_AND_CHECK(vr, true, false, false, 0);
+
+    // One more and we are at 6/2.
+    REGISTER_VOTE_AND_CHECK(vr, true, false, false, 0);
+
+    // One more and we are at 6/2.
+    REGISTER_VOTE_AND_CHECK(vr, true, false, false, 0);
+
+    // Next vote will flip state, and confidence will increase as long as we
+    // vote yes.
+    for (int i = 0; i < AVALANCHE_FINALIZATION_SCORE; i++) {
+        REGISTER_VOTE_AND_CHECK(vr, true, true, false, i);
+    }
+
+    // The next vote will finalize the decision.
+    REGISTER_VOTE_AND_CHECK(vr, false, true, true,
+                            AVALANCHE_FINALIZATION_SCORE);
+
+    // Now that we have two no votes, confidence stop increasing.
+    for (int i = 0; i < 5; i++) {
+        REGISTER_VOTE_AND_CHECK(vr, false, true, true,
+                                AVALANCHE_FINALIZATION_SCORE);
+    }
+
+    // Next vote will flip state, and confidence will increase as long as we
+    // vote no.
+    for (int i = 0; i < AVALANCHE_FINALIZATION_SCORE; i++) {
+        REGISTER_VOTE_AND_CHECK(vr, false, false, false, i);
+    }
+
+    // The next vote will finalize the decision.
+    REGISTER_VOTE_AND_CHECK(vr, true, false, true,
+                            AVALANCHE_FINALIZATION_SCORE);
+}
+
+BOOST_AUTO_TEST_SUITE_END()