Add self-test that reveals SkChecksum weakness
Review URL: https://codereview.appspot.com/6845064
git-svn-id: http://skia.googlecode.com/svn/trunk@6535 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tests/ChecksumTest.cpp b/tests/ChecksumTest.cpp
index 3425bcd..5d5d83b 100644
--- a/tests/ChecksumTest.cpp
+++ b/tests/ChecksumTest.cpp
@@ -68,7 +68,8 @@
// Try checksums of different-sized chunks, but always
// 32-bit aligned and big enough to contain all the
- // nonzero bytes.
+ // nonzero bytes. (Remaining bytes will still be zero
+ // from the initial sk_bzero() call.)
size_t checksum_size = (((i/4)+1)*4);
REPORTER_ASSERT(fReporter, checksum_size <= buf_size);
@@ -80,25 +81,37 @@
}
}
- // Return the checksum of a portion of this static test data
- // (8 bytes repeated twice): "1234567812345678"
- uint32_t GetTestDataChecksum(size_t offset, size_t len) {
- static char testbytes[] = "1234567812345678";
- uint32_t* ptr = reinterpret_cast<uint32_t*>(testbytes);
- return ComputeChecksum(ptr, len);
+ // Return the checksum of a buffer of bytes 'len' long.
+ // The pattern of values within the buffer will be consistent
+ // for every call, based on 'seed'.
+ uint32_t GetTestDataChecksum(size_t len, char seed=0) {
+ SkAutoMalloc storage(len);
+ uint32_t* start = (uint32_t *)storage.get();
+ char* ptr = (char *)start;
+ for (size_t i = 0; i < len; ++i) {
+ *ptr++ = ((seed+i) & 0x7f);
+ }
+ uint32_t result = ComputeChecksum(start, len);
+ return result;
}
void RunTest() {
// Test self-consistency of checksum algorithms.
fWhichAlgorithm = kSkChecksum;
REPORTER_ASSERT(fReporter,
- GetTestDataChecksum(0, 8) ==
- GetTestDataChecksum(8, 8));
+ GetTestDataChecksum(8, 0) ==
+ GetTestDataChecksum(8, 0));
+ REPORTER_ASSERT(fReporter,
+ GetTestDataChecksum(8, 0) !=
+ GetTestDataChecksum(8, 1));
TestChecksumSelfConsistency(128);
fWhichAlgorithm = kSkConsistentChecksum;
REPORTER_ASSERT(fReporter,
- GetTestDataChecksum(0, 8) ==
- GetTestDataChecksum(8, 8));
+ GetTestDataChecksum(8, 0) ==
+ GetTestDataChecksum(8, 0));
+ REPORTER_ASSERT(fReporter,
+ GetTestDataChecksum(8, 0) !=
+ GetTestDataChecksum(8, 1));
TestChecksumSelfConsistency(128);
// Test checksum results that should be consistent across
@@ -107,9 +120,25 @@
REPORTER_ASSERT(fReporter, ComputeChecksum(NULL, 0) == 0);
fWhichAlgorithm = kSkConsistentChecksum;
REPORTER_ASSERT(fReporter, ComputeChecksum(NULL, 0) == 0);
- REPORTER_ASSERT(fReporter, GetTestDataChecksum(0, 8) == 0xa12fac2c);
- REPORTER_ASSERT(fReporter, GetTestDataChecksum(8, 8) == 0xa12fac2c);
- REPORTER_ASSERT(fReporter, GetTestDataChecksum(8, 4) == 0x34333231);
+ REPORTER_ASSERT(fReporter, GetTestDataChecksum(4) == 0x03020100);
+ REPORTER_ASSERT(fReporter, GetTestDataChecksum(8) == 0x07860485);
+
+ // TODO: note the weakness exposed by these collisions...
+ // We need to improve the SkConsistentChecksum algorithm
+ // (and maybe SkChecksum too?)
+ // We would prefer that these asserts FAIL!
+ // Filed as https://code.google.com/p/skia/issues/detail?id=981
+ // ('SkChecksum algorithm allows for way too many collisions')
+ fWhichAlgorithm = kSkChecksum;
+ REPORTER_ASSERT(fReporter,
+ GetTestDataChecksum(128) == GetTestDataChecksum(256));
+ REPORTER_ASSERT(fReporter,
+ GetTestDataChecksum(132) == GetTestDataChecksum(260));
+ fWhichAlgorithm = kSkConsistentChecksum;
+ REPORTER_ASSERT(fReporter, GetTestDataChecksum(128) == 0);
+ REPORTER_ASSERT(fReporter, GetTestDataChecksum(132) == 0x03020100);
+ REPORTER_ASSERT(fReporter, GetTestDataChecksum(256) == 0);
+ REPORTER_ASSERT(fReporter, GetTestDataChecksum(260) == 0x03020100);
}
Reporter* fReporter;