Continue work on png writer.

src/compression/CyclicRedundancyChecker.h

@@ -0,0 +1,52 @@
+#pragma once
+
+class CyclicRedundancyChecker
+{
+public:
+
+    void createTable()
+    {
+        unsigned long c{0};
+        for (int n = 0; n < 256; n++)
+        {
+            c = (unsigned long) n;
+            for (int k = 0; k < 8; k++)
+            {
+                if (c & 1)
+                {
+                    c = 0xedb88320L ^ (c >> 1);
+                }
+                else
+                {
+                    c = c >> 1;
+                }
+            }
+            mTable[n] = c;
+        }
+        mTableComputed = true;
+        mTableComputed = 1;
+    }
+
+    unsigned long updateCrc(unsigned long crc, unsigned char *buf, int len)
+    {
+        unsigned long c = crc;
+        if (!mTableComputed)
+        {
+            createTable();
+        }
+        for (int n = 0; n < len; n++)
+        {
+            c = mTable[(c ^ buf[n]) & 0xff] ^ (c >> 8);
+        }
+        return c;
+    }
+
+    unsigned long doCrc(unsigned char *buf, int len)
+    {
+        return updateCrc(0xffffffffL, buf, len) ^ 0xffffffffL;
+    }
+
+private:
+    bool mTableComputed{false};
+    unsigned long mTable[256];
+};