Move the logic from ComputeTable into the unit test, which means it's run

automatically together with the other tests, and also removes the stray
function ComputeTable() (which was never referenced by anything else
in the open-source version, causing compiler warnings for some)
out of the core library.

Fixes public issue 96.

A=sesse
R=sanjay

snappy_unittest.cc

@@ -64,6 +64,9 @@ DEFINE_bool(write_compressed, false,
 DEFINE_bool(write_uncompressed, false,
             "Write uncompressed versions of each file to <file>.uncomp");
 
+DEFINE_bool(snappy_dump_decompression_table, false,
+            "If true, we print the decompression table during tests.");
+
 namespace snappy {
 
 
@@ -1160,6 +1163,100 @@ TEST(Snappy, FindMatchLengthRandom) {
   }
 }
 
+static uint16 MakeEntry(unsigned int extra,
+                        unsigned int len,
+                        unsigned int copy_offset) {
+  // Check that all of the fields fit within the allocated space
+  assert(extra       == (extra & 0x7));          // At most 3 bits
+  assert(copy_offset == (copy_offset & 0x7));    // At most 3 bits
+  assert(len         == (len & 0x7f));           // At most 7 bits
+  return len | (copy_offset << 8) | (extra << 11);
+}
+
+// Check that the decompression table is correct, and optionally print out
+// the computed one.
+TEST(Snappy, VerifyCharTable) {
+  using snappy::internal::LITERAL;
+  using snappy::internal::COPY_1_BYTE_OFFSET;
+  using snappy::internal::COPY_2_BYTE_OFFSET;
+  using snappy::internal::COPY_4_BYTE_OFFSET;
+  using snappy::internal::char_table;
+  using snappy::internal::wordmask;
+
+  uint16 dst[256];
+
+  // Place invalid entries in all places to detect missing initialization
+  int assigned = 0;
+  for (int i = 0; i < 256; i++) {
+    dst[i] = 0xffff;
+  }
+
+  // Small LITERAL entries.  We store (len-1) in the top 6 bits.
+  for (unsigned int len = 1; len <= 60; len++) {
+    dst[LITERAL | ((len-1) << 2)] = MakeEntry(0, len, 0);
+    assigned++;
+  }
+
+  // Large LITERAL entries.  We use 60..63 in the high 6 bits to
+  // encode the number of bytes of length info that follow the opcode.
+  for (unsigned int extra_bytes = 1; extra_bytes <= 4; extra_bytes++) {
+    // We set the length field in the lookup table to 1 because extra
+    // bytes encode len-1.
+    dst[LITERAL | ((extra_bytes+59) << 2)] = MakeEntry(extra_bytes, 1, 0);
+    assigned++;
+  }
+
+  // COPY_1_BYTE_OFFSET.
+  //
+  // The tag byte in the compressed data stores len-4 in 3 bits, and
+  // offset/256 in 5 bits.  offset%256 is stored in the next byte.
+  //
+  // This format is used for length in range [4..11] and offset in
+  // range [0..2047]
+  for (unsigned int len = 4; len < 12; len++) {
+    for (unsigned int offset = 0; offset < 2048; offset += 256) {
+      dst[COPY_1_BYTE_OFFSET | ((len-4)<<2) | ((offset>>8)<<5)] =
+        MakeEntry(1, len, offset>>8);
+      assigned++;
+    }
+  }
+
+  // COPY_2_BYTE_OFFSET.
+  // Tag contains len-1 in top 6 bits, and offset in next two bytes.
+  for (unsigned int len = 1; len <= 64; len++) {
+    dst[COPY_2_BYTE_OFFSET | ((len-1)<<2)] = MakeEntry(2, len, 0);
+    assigned++;
+  }
+
+  // COPY_4_BYTE_OFFSET.
+  // Tag contents len-1 in top 6 bits, and offset in next four bytes.
+  for (unsigned int len = 1; len <= 64; len++) {
+    dst[COPY_4_BYTE_OFFSET | ((len-1)<<2)] = MakeEntry(4, len, 0);
+    assigned++;
+  }
+
+  // Check that each entry was initialized exactly once.
+  EXPECT_EQ(256, assigned) << "Assigned only " << assigned << " of 256";
+  for (int i = 0; i < 256; i++) {
+    EXPECT_NE(0xffff, dst[i]) << "Did not assign byte " << i;
+  }
+
+  if (FLAGS_snappy_dump_decompression_table) {
+    printf("static const uint16 char_table[256] = {\n  ");
+    for (int i = 0; i < 256; i++) {
+      printf("0x%04x%s",
+             dst[i],
+             ((i == 255) ? "\n" : (((i%8) == 7) ? ",\n  " : ", ")));
+    }
+    printf("};\n");
+  }
+
+  // Check that computed table matched recorded table.
+  for (int i = 0; i < 256; i++) {
+    EXPECT_EQ(dst[i], char_table[i]) << "Mismatch in byte " << i;
+  }
+}
+
 static void CompressFile(const char* fname) {
   string fullinput;
   CHECK_OK(file::GetContents(fname, &fullinput, file::Defaults()));