Add FxAlignToBoundary<>() template helper function.

Because I nearly botched this trivial calculation in the previous CL.

Change-Id: I7438f9d3476d93b7899c2d7d761234769f53f9e3
Reviewed-on: https://pdfium-review.googlesource.com/43010
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>

3 files changed, 32 insertions, 1 deletions

diff --git a/core/fxcrt/cfx_bitstream.cpp b/core/fxcrt/cfx_bitstream.cpp
index 3d3040d96a..d16fc2f1a4 100644
--- a/core/fxcrt/cfx_bitstream.cpp
+++ b/core/fxcrt/cfx_bitstream.cpp
@@ -8,6 +8,7 @@
 
 #include <limits>
 
+#include "core/fxcrt/fx_memory.h"
 #include "core/fxcrt/fx_system.h"
 
 CFX_BitStream::CFX_BitStream(pdfium::span<const uint8_t> pData)
@@ -18,7 +19,7 @@ CFX_BitStream::CFX_BitStream(pdfium::span<const uint8_t> pData)
 CFX_BitStream::~CFX_BitStream() {}
 
 void CFX_BitStream::ByteAlign() {
-  m_BitPos = (m_BitPos + 7) & ~7;
+  m_BitPos = FxAlignToBoundary<8>(m_BitPos);
 }
 
 uint32_t CFX_BitStream::GetBits(uint32_t nBits) {
diff --git a/core/fxcrt/fx_memory.h b/core/fxcrt/fx_memory.h
index 707e084211..b39911269b 100644
--- a/core/fxcrt/fx_memory.h
+++ b/core/fxcrt/fx_memory.h
@@ -130,6 +130,13 @@ inline void FX_Free(void* ptr) {
 template <typename T, size_t N>
 char (&ArraySizeHelper(T (&array)[N]))[N];
 
+// Round up to the power-of-two boundary N.
+template <int N, typename T>
+inline T FxAlignToBoundary(T size) {
+  static_assert(N > 0 && (N & (N - 1)) == 0, "Not non-zero power of two");
+  return (size + N - 1) & ~(N - 1);
+}
+
 // Used with std::unique_ptr to FX_Free raw memory.
 struct FxFreeDeleter {
   inline void operator()(void* ptr) const { FX_Free(ptr); }
diff --git a/core/fxcrt/fx_memory_unittest.cpp b/core/fxcrt/fx_memory_unittest.cpp
index 8c577835d6..eab11113cd 100644
--- a/core/fxcrt/fx_memory_unittest.cpp
+++ b/core/fxcrt/fx_memory_unittest.cpp
@@ -83,3 +83,26 @@ TEST(fxcrt, DISABLED_FXMEM_DefaultOOM) {
   EXPECT_FALSE(FXMEM_DefaultRealloc(ptr, kMaxByteAlloc));
   FXMEM_DefaultFree(ptr);
 }
+
+TEST(fxcrt, FXAlign) {
+  static_assert(std::numeric_limits<size_t>::max() % 2 == 1,
+                "numeric limit must be odd for this test");
+
+  size_t s0 = 0;
+  size_t s1 = 1;
+  size_t s2 = 2;
+  size_t sbig = std::numeric_limits<size_t>::max() - 2;
+  EXPECT_EQ(0u, FxAlignToBoundary<2>(s0));
+  EXPECT_EQ(2u, FxAlignToBoundary<2>(s1));
+  EXPECT_EQ(2u, FxAlignToBoundary<2>(s2));
+  EXPECT_EQ(std::numeric_limits<size_t>::max() - 1, FxAlignToBoundary<2>(sbig));
+
+  int i0 = 0;
+  int i511 = 511;
+  int i512 = 512;
+  int ineg = -513;
+  EXPECT_EQ(0, FxAlignToBoundary<512>(i0));
+  EXPECT_EQ(512, FxAlignToBoundary<512>(i511));
+  EXPECT_EQ(512, FxAlignToBoundary<512>(i512));
+  EXPECT_EQ(-512, FxAlignToBoundary<512>(ineg));
+}