Pl_Buffer: reduce memory growth (fixes #228)

Rather than keeping a list of buffers for every write, accumulate
bytes in a single buffer, doubling the size of the buffer when needed
to accommodate new data.

This is not the best possible implementation, but the change was
implemented in this way to avoid changing the shape of Pl_Buffer and
thus breaking backward compatibility.

1 files changed, 30 insertions, 13 deletions

diff --git a/libqpdf/Pl_Buffer.cc b/libqpdf/Pl_Buffer.cc
index 45c0c862..1ca9292d 100644
--- a/libqpdf/Pl_Buffer.cc
+++ b/libqpdf/Pl_Buffer.cc
@@ -17,11 +17,32 @@ Pl_Buffer::~Pl_Buffer()
 void
 Pl_Buffer::write(unsigned char* buf, size_t len)
 {
-    Buffer* b = new Buffer(len);
-    memcpy(b->getBuffer(), buf, len);
-    this->data.push_back(b);
+    PointerHolder<Buffer> cur_buf;
+    size_t cur_size = 0;
+    if (! this->data.empty())
+    {
+        cur_buf = this->data.back();
+        cur_size = cur_buf->getSize();
+    }
+    size_t left = cur_size - this->total_size;
+    if (left < len)
+    {
+        size_t new_size = std::max(this->total_size + len, 2 * cur_size);
+        Buffer* b = new Buffer(new_size);
+        if (cur_buf.getPointer())
+        {
+            memcpy(b->getBuffer(), cur_buf->getBuffer(), this->total_size);
+        }
+        this->data.clear();
+        cur_buf = b;
+        this->data.push_back(cur_buf);
+    }
+    if (len)
+    {
+        memcpy(cur_buf->getBuffer() + this->total_size, buf, len);
+        this->total_size += len;
+    }
     this->ready = false;
-    this->total_size += len;
 
     if (getNext(true))
     {
@@ -49,17 +70,13 @@ Pl_Buffer::getBuffer()
 
     Buffer* b = new Buffer(this->total_size);
     unsigned char* p = b->getBuffer();
-    while (! this->data.empty())
+    if (! this->data.empty())
     {
-	PointerHolder<Buffer> bp = this->data.front();
-	this->data.pop_front();
-	size_t bytes = bp->getSize();
-	memcpy(p, bp->getBuffer(), bytes);
-	p += bytes;
-	this->total_size -= bytes;
+        PointerHolder<Buffer> bp = this->data.back();
+        this->data.clear();
+        memcpy(p, bp->getBuffer(), this->total_size);
     }
-
-    assert(this->total_size == 0);
+    this->total_size = 0;
     this->ready = false;
 
     return b;