Add code to compute the root hash for a hash tree

extensions/browser/content_hash_tree_unittest.cc

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/memory/scoped_ptr.h"
+#include "base/stl_util.h"
+#include "crypto/secure_hash.h"
+#include "crypto/sha2.h"
+#include "extensions/browser/content_hash_tree.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+using crypto::kSHA256Length;
+using crypto::SecureHash;
+
+namespace extensions {
+
+class ContentHashTreeTest : public ::testing::Test {
+ public:
+  ContentHashTreeTest() : block_size_(1024) {}
+
+ protected:
+  int block_size_;

[Marijn Kruisselbrink, 2014/05/12 22:57:51]

You're not actually using block_size_ (and I'm not sure why nodes_ is a member
variable, instead of just local in each test case, but if you prefer it that way
I have no problems with it).

[asargent_no_longer_on_chrome, 2014/05/12 23:52:48]

Good observations! This is why we have code-review. =)

I've removed block_size_. For nodes_, I think I originally envisioned needing a
richer set of helper methods, so I've switched the whole thing over to not have
a ::testing::Test base class and use TEST() instead of TEST_F. 

+  std::vector<std::string> nodes_;
+};
+
+TEST_F(ContentHashTreeTest, HashTreeBasics) {
+  // Empty array.
+  EXPECT_EQ(std::string(), ComputeTreeHashRoot(nodes_, 16));
+
+  // One node.
+  std::string node1;
+  node1.append('1', kSHA256Length);

[Marijn Kruisselbrink, 2014/05/12 22:57:51]

I'm not sure why you're creating an empty string and then using append here,
while further down for expected you just use the std::string constructor that
does the same thing directly. But more importantly I think you swapped the order
of the arguments (actually, I'm not sure which append overload this ends up
resolving to, presumably still the (size_t, char) one, despite that involving
various implicit casts).

[asargent_no_longer_on_chrome, 2014/05/12 23:52:48]

Great catch. I definitely did confuse the two versions:

append(const charT* s, size_type n)
append(size_type n, charT c)


I replaced this pattern with a helper function that returns a real signature
based on a seed, and also added a DCHECK in content_hash_tree.cc to help catch
short signatures in the input in the future.

+  nodes_.push_back(node1);
+  EXPECT_EQ(node1, ComputeTreeHashRoot(nodes_, 16));
+
+  // Two nodes.
+  std::string node2;
+  node2.append('2', kSHA256Length);
+  nodes_.push_back(node2);
+
+  std::string expected(kSHA256Length, 0);
+  scoped_ptr<SecureHash> hash(SecureHash::Create(SecureHash::SHA256));
+  hash->Update(node1.data(), node1.size());
+  hash->Update(node2.data(), node2.size());
+  hash->Finish(string_as_array(&expected), expected.size());
+  EXPECT_EQ(expected, ComputeTreeHashRoot(nodes_, 16));
+}
+
+TEST_F(ContentHashTreeTest, HashTreeMultipleLevels) {
+  for (int i = 0; i < 3; i++) {
+    std::string node;
+    for (size_t j = 0; j < kSHA256Length; j++)
+      node.append(static_cast<char>((j + i) % 256), 1);

[Marijn Kruisselbrink, 2014/05/12 22:57:51]

similar here, it seems like this would append X 1-bytes, not 1 X-byte.

[asargent_no_longer_on_chrome, 2014/05/12 23:52:48]

Done.

+    nodes_.push_back(node);
+  }
+
+  // First try a test where our branch factor is >= 3, so we expect the result
+  // to be the hash of all 3 concatenated together. E.g the expected top hash
+  // should be 4 in the following diagram:
+  //   4
+  // 1 2 3
+  std::string expected =
+      crypto::SHA256HashString(nodes_[0] + nodes_[1] + nodes_[2]);
+  EXPECT_EQ(expected, ComputeTreeHashRoot(nodes_, 4));
+
+  // Now try making the branch factor be 2, so that we
+  // should get the following:
+  //   6
+  //  4 5
+  // 1 2 3
+  // where 4 is the hash of 1 and 2, 5 is the hash of 3, and 6 is the
+  // hash of 4 and 5.
+  std::string hash_of_first_2 = crypto::SHA256HashString(nodes_[0] + nodes_[1]);
+  std::string hash_of_third = crypto::SHA256HashString(nodes_[2]);
+  expected = crypto::SHA256HashString(hash_of_first_2 + hash_of_third);
+  EXPECT_EQ(expected, ComputeTreeHashRoot(nodes_, 2));
+}
+
+}  // namespace extensions