Templetized SkWriter32 readTAt() & overwriteTAt()

include/core/SkWriter32.h

 
 /*
  * Copyright 2008 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #ifndef SkWriter32_DEFINED
@@ skipping 61 matching lines @@
         if (fInternal.isEmpty() && this->externalCount() + count <= fExternalLimit) {
             p = fExternal + fCount;
         } else {
             p = fInternal.append(count);
         }
 
         fCount += count;
         return p;
     }
 
-    // Read or write 4 bytes at offset, which must be a multiple of 4 <= size().
-    uint32_t read32At(size_t offset) { return this->atOffset(offset); }
-    void write32At(size_t offset, uint32_t val) { this->atOffset(offset) = val; }
+    /**
+     *  Read a T record at offset, which must be a multiple of 4. Only legal if the record
+     *  was writtern atomically using the write methods below.
+     */
+    template<typename T>
+    const T& readTAt(size_t offset) const {
+        return this->atOffset<T>(offset);
+    }
+
+    /**
+     *  Overwrite a T record at offset, which must be a multiple of 4. Only legal if the record
+     *  was writtern atomically using the write methods below.
+     */
+    template<typename T>
+    void overwriteTAt(size_t offset, const T& value) {
+        this->atOffset<T>(offset) = value;
+    }
 
     bool writeBool(bool value) {
         this->write32(value);
         return value;
     }
 
     void writeInt(int32_t value) {
         this->write32(value);
     }
 
@@ skipping 65 matching lines @@
         // TODO: If we're going to spill from fExternal to fInternal, we might want to fill
         // fExternal as much as possible before writing to fInternal.
         memcpy(this->reserve(size), values, size);
     }
 
     /**
      *  Reserve size bytes. Does not need to be 4 byte aligned. The remaining space (if any) will be
      *  filled in with zeroes.
      */
     uint32_t* reservePad(size_t size) {
-        uint32_t* p = this->reserve(SkAlign4(size));
-        uint8_t* tail = (uint8_t*)p + size;
-        switch (SkAlign4(size) - size) {
-            default: SkDEBUGFAIL("SkAlign4(x) - x should always be 0, 1, 2, or 3.");
-            case 3: *tail++ = 0x00;  // fallthrough is intentional
-            case 2: *tail++ = 0x00;  // fallthrough is intentional
-            case 1: *tail++ = 0x00;
-            case 0: ;/*nothing to do*/
+        size_t alignedSize = SkAlign4(size);
+        uint32_t* p = this->reserve(alignedSize);
+        if (alignedSize != size) {
+            SkASSERT(alignedSize >= 4);
+            p[alignedSize / 4 - 1] = 0;

[mtklein, 2014/02/11 13:18:10]

Duh.  I don't know why I wrote the old one to be so complicated.  We could even
satisfy the requirements with just this:

SkASSERT(size > 0);
size_t alignedSize = SkAlign4(size);
uint32_t* p = this->reserve(alignedSize);
p[alignedSize / 4 - 1] = 0;

Anyway, silly me.  LGTM.

[f(malita), 2014/02/11 14:09:36]

Yeah, I kept the conditional mainly for handling reservePad(0) gracefully.

         }
         return p;
     }
 
     /**
      *  Write size bytes from src, and pad to 4 byte alignment with zeroes.
      */
     void writePad(const void* src, size_t size) {
         memcpy(this->reservePad(size), src, size);
     }
@@ skipping 43 matching lines @@
             && stream->write(fInternal.begin(), fInternal.bytes());
     }
 
     // read from the stream, and write up to length bytes. Return the actual
     // number of bytes written.
     size_t readFromStream(SkStream* stream, size_t length) {
         return stream->read(this->reservePad(length), length);
     }
 
 private:
-    uint32_t& atOffset(size_t offset) {
+    template<typename T>
+    T& atOffset(size_t offset) const {
         SkASSERT(SkAlign4(offset) == offset);
-        const int count = SkToInt(offset/4);
-        SkASSERT(count < fCount);
+        const int count = SkToInt(offset / 4);
+        SkASSERT(offset + sizeof(T) <= (unsigned)fCount * 4);
 
         if (count < this->externalCount()) {
-            return fExternal[count];
+            // A record should not span fExternal->fInternal.
+            SkASSERT(offset + sizeof(T) <= (unsigned)this->externalCount() * 4);
+            return *(T*)&fExternal[count];
         }
-        return fInternal[count - this->externalCount()];
+        return *(T*)&fInternal[count - this->externalCount()];
     }
 
-
     // Number of uint32_t written into fExternal.  <= fExternalLimit.
     int externalCount() const { return fCount - fInternal.count(); }
 
     int fCount;                     // Total number of uint32_t written.
     int fExternalLimit;             // Number of uint32_t we can write to fExternal.
     uint32_t* fExternal;            // Unmanaged memory block.
     SkTDArray<uint32_t> fInternal;  // Managed memory block.
 };
 
 /**
  *  Helper class to allocated SIZE bytes as part of the writer, and to provide
  *  that storage to the constructor as its initial storage buffer.
  *
  *  This wrapper ensures proper alignment rules are met for the storage.
  */
 template <size_t SIZE> class SkSWriter32 : public SkWriter32 {
 public:
     SkSWriter32() : SkWriter32(fData.fStorage, SIZE) {}
 
 private:
     union {
         void*   fPtrAlignment;
         double  fDoubleAlignment;
         char    fStorage[SIZE];
     } fData;
 };
 
 #endif