Prototype implementation of filter selection

This rewrites the code used previously in the heuristics to make it easier to
debug and introduces the 'methodical' method, which is intended to be an
expensive but reliable way of reducing image size.

The code in this commit does not work; the 'methodical' test for success does
not take account of data buffered inside zlib and, anyway, it changes the
results of pngtest so that the test fails.  This commit is just a checkpoint of
the current state; another commit will temporarily disable the 'methodical'
code.

Signed-off-by: John Bowler <jbowler@acm.org>

pngstruct.h

@@ -63,13 +63,10 @@
 #endif
 
 #ifdef PNG_WRITE_SUPPORTED
-/* The type of a compression buffer list used by the write code. */
-typedef struct png_compression_buffer *png_compression_bufferp;
-#endif
-
-#ifdef PNG_SELECT_FILTER_METHODICALLY_SUPPORTED
-/* Type of the data cache used when selecting filters methodicially */
-typedef struct png_filter_select *png_filter_selectp;
+/* The write compression control (allocated on demand).
+ * TODO: use this for the read state too.
+ */
+typedef struct png_zlib_state *png_zlib_statep;
 #endif
 
 /* Colorspace support; structures used in png_struct, png_info and in internal
@@ -512,9 +509,6 @@ struct png_struct_def
    unsigned int row_input_pixel_depth  :8;
    unsigned int row_output_pixel_depth :8;
    unsigned int row_max_pixel_depth    :8;
-#ifdef PNG_WRITE_FILTER_SUPPORTED
-   unsigned int filter_mask    :8; /* mask of filters to consider on write */
-#endif /* WRITE_FILTER */
 
 #  define PNG_RF_BITS 9 /* Number of bits required for the row format (below) */
 #ifdef PNG_TRANSFORM_MECH_SUPPORTED
@@ -585,28 +579,6 @@ struct png_struct_def
    uInt IDAT_size; /* limit on IDAT read and write IDAT size */
 #endif /* SEQUENTIAL_READ || WRITE */
 
-#ifdef PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION_SUPPORTED
-   int zlib_text_level;       /* holds zlib compression level */
-   int zlib_text_method;      /* holds zlib compression method */
-   int zlib_text_window_bits; /* holds zlib compression window bits */
-   int zlib_text_mem_level;   /* holds zlib compression memory level */
-   int zlib_text_strategy;    /* holds zlib compression strategy */
-#endif
-
-#ifdef PNG_WRITE_SUPPORTED
-   int zlib_level;            /* holds zlib compression level */
-   int zlib_method;           /* holds zlib compression method */
-   int zlib_window_bits;      /* holds zlib compression window bits */
-   int zlib_mem_level;        /* holds zlib compression memory level */
-   int zlib_strategy;         /* holds zlib compression strategy */
-
-   int zlib_set_level;        /* Actual values set into the zstream on write */
-   int zlib_set_method;
-   int zlib_set_window_bits;
-   int zlib_set_mem_level;
-   int zlib_set_strategy;
-#endif /* WRITE */
-
    /* ERROR HANDLING */
 #ifdef PNG_SETJMP_SUPPORTED
    jmp_buf        *jmp_buf_ptr;   /* passed to longjmp_fn */
@@ -649,8 +621,6 @@ struct png_struct_def
 
 #ifdef PNG_WRITE_FLUSH_SUPPORTED
    png_flush_ptr output_flush_fn; /* Function for flushing output */
-   png_uint_32   flush_dist; /* how many rows apart to flush, 0 - no flush */
-   png_uint_32   flush_rows; /* number of rows written since last flush */
 #endif
 
 #endif /* WRITE */
@@ -749,19 +719,12 @@ struct png_struct_def
     * zlib expects a 'zstream' as the fundamental control structure, it allows
     * all the parameters to be passed as one pointer.
     */
-   z_stream     zstream;       /* decompression structure */
-   png_uint_32  zowner;        /* ID (chunk type) of zstream owner, 0 if none */
+   png_uint_32     zowner;       /* ID (chunk type) of zlib owner, 0 if none */
 #  ifdef PNG_WRITE_SUPPORTED
-   png_compression_bufferp  zbuffer_list;  /* Created on demand during write */
-   png_compression_bufferp *zbuffer_end;   /* 'next' field of current buffer */
-   png_uint_32              zbuffer_len;   /* Length of data in list */
-   unsigned int             zbuffer_start; /* Bytes written from start */
-   unsigned int             zbuffer_filters;/*Filters for this row */
-#     ifdef PNG_SELECT_FILTER_METHODICALLY_SUPPORTED
-         png_filter_selectp zbuffer_select;
-#     endif /* SELECT_FILTER_METHODICALLY */
+   png_zlib_statep zlib_state;   /* State of zlib compression */
 #  endif /* WRITE */
 #  ifdef PNG_READ_SUPPORTED
+   z_stream     zstream;         /* decompression structure */
    unsigned int zstream_ended:1; /* no more zlib output available [read] */
    unsigned int zstream_error:1; /* zlib error message has been output [read] */
 #  endif /* READ */
@@ -779,6 +742,8 @@ struct png_struct_def
    /* SCRATCH buffers, used when control returns to the application or a read
     * loop.
     */
+#  ifdef PNG_READ_SUPPORTED
    png_byte scratch[PNG_ROW_BUFFER_SIZE+16U];
+#  endif /* READ */
 };
 #endif /* PNGSTRUCT_H */