High-Level C API

The C API is broken down into a high-level API, which handles compression of entire arrays, and a low-level-api for processing individual blocks and managing the underlying bit stream.

The high-level API should be the API of choice for applications that compress and decompress entire arrays. A low-level API exists for processing individual, possibly partial blocks as well as reduced-precision integer data less than 32 bits wide.

The following sections are available:

Macros

ZFP_VERSION_MAJOR
ZFP_VERSION_MINOR
ZFP_VERSION_PATCH
ZFP_VERSION
ZFP_VERSION_STRING

Macros identifying the zfp library version. ZFP_VERSION is a single integer constructed from the previous three macros. ZFP_VERSION_STRING is a string literal. See also zfp_library_version and zfp_version_string.

ZFP_CODEC

Macro identifying the version of the compression CODEC. See also zfp_codec_version.

ZFP_MIN_BITS
ZFP_MAX_BITS
ZFP_MAX_PREC
ZFP_MIN_EXP

Default compression parameter settings that impose no constraints. The largest possible compressed block size, corresponding to 3D blocks of doubles, is given by ZFP_MAX_BITS. See also zfp_stream.

ZFP_HEADER_MAGIC
ZFP_HEADER_META
ZFP_HEADER_MODE
ZFP_HEADER_FULL

Bit masks for specifying which portions of a header to output (if any). These constants should be bitwise ORed together. Use ZFP_HEADER_FULL to output all header information available. The compressor and decompressor must agree on which parts of the header to read/write.

ZFP_HEADER_META in essence encodes the information stored in the zfp_field struct, while ZFP_HEADER_MODE encodes the compression parameters stored in the zfp_stream struct. The magic can be used to uniquely identify the stream as a zfp stream, and includes the CODEC version.

See zfp_read_header() and zfp_write_header() for how to read and write header information.

ZFP_MAGIC_BITS
ZFP_META_BITS
ZFP_MODE_SHORT_BITS
ZFP_MODE_LONG_BITS
ZFP_HEADER_MAX_BITS
ZFP_MODE_SHORT_MAX

Number of bits used by each portion of the header. These macros are primarily informational and should not be accessed by the user through the high-level API. For most common compression parameter settings, only ZFP_MODE_SHORT_BITS bits of header information are stored to encode the mode (see zfp_stream_mode()).

Types

zfp_stream

The zfp_stream struct encapsulates all information about the compressed stream for a single block or a collection of blocks that represent an array. See the section on compression modes for a description of the members of this struct.

typedef struct {
  uint minbits;      // minimum number of bits to store per block
  uint maxbits;      // maximum number of bits to store per block
  uint maxprec;      // maximum number of bit planes to store
  int minexp;        // minimum floating point bit plane number to store
  bitstream* stream; // compressed bit stream
} zfp_stream;
zfp_type

Enumerates the scalar types supported by the compressor, and is used to describe the uncompressed array. The compressor and decompressor must use the same zfp_type, e.g. one cannot compress doubles and decompress to floats or integers.

typedef enum {
  zfp_type_none   = 0, // unspecified type
  zfp_type_int32  = 1, // 32-bit signed integer
  zfp_type_int64  = 2, // 64-bit signed integer
  zfp_type_float  = 3, // single precision floating point
  zfp_type_double = 4  // double precision floating point
} zfp_type;
zfp_field

The uncompressed array is described by the zfp_field struct, which encodes the array’s scalar type, dimensions, and memory layout.

typedef struct {
  zfp_type type;   // scalar type (e.g. int32, double)
  uint nx, ny, nz; // sizes (zero for unused dimensions)
  int sx, sy, sz;  // strides (zero for contiguous array a[nz][ny][nx])
  void* data;      // pointer to array data
} zfp_field;

For example, a static multidimensional C array declared as

double array[n1][n2][n3];

would be described by a zfp_field with members

type = zfp_type_double;
nx = n3; ny = n2; nz = n1;
sx = 1; sy = n3; sz = n2 * n3;
data = &array[0][0][0];

Constants

const uint zfp_codec_version

The version of the compression CODEC implemented by this version of the zfp library. The library can decompress files generated by the same CODEC only. To ensure that the zfp.h header matches the binary library linked to, zfp_codec_version should match ZFP_CODEC.

const uint zfp_library_version

The library version. The binary library and headers are compatible if zfp_library_version matches ZFP_VERSION.

const char* const zfp_version_string

A constant string representing the zfp library version and release date. One can search for this string in executables and libraries that use zfp to determine which version of the library the application was compiled against.

Functions

size_t zfp_type_size(zfp_type type)

Return byte size of the given scalar type, e.g. zfp_type_size(zfp_type_float) = 4.

Compressed Stream

zfp_stream* zfp_stream_open(bitstream* stream)

Allocate compressed stream and associate it with bit stream for reading and writing bits to/from memory. stream may be NULL and attached later via zfp_stream_set_bit_stream().

void zfp_stream_close(zfp_stream* stream)

Close and deallocate compressed stream. This does not affect the attached bit stream.

bitstream* zfp_stream_bit_stream(const zfp_stream* stream)

Return bit stream associated with compressed stream.

uint64 zfp_stream_mode(const zfp_stream* zfp)

Return compact encoding of compression parameters. If the return value is no larger than ZFP_MODE_SHORT_MAX, then the least significant ZFP_MODE_SHORT_BITS (12 in the current version) suffice to encode the parameters. Otherwise all 64 bits are needed, and the low ZFP_MODE_SHORT_BITS bits will be all ones. Thus, this variable-length encoding can be used to economically encode and decode the compression parameters, which is especially important if the parameters are to vary spatially over small regions. Such spatially adaptive coding would have to be implemented via the low-level API.

void zfp_stream_params(const zfp_stream* stream, uint* minbits, uint* maxbits, uint* maxprec, int* minexp)

Query compression parameters. For any parameter not needed, pass NULL for the corresponding pointer.

size_t zfp_stream_compressed_size(const zfp_stream* stream)

Number of bytes of compressed storage. This function returns the current byte offset within the bit stream from the beginning of the bit stream memory buffer. To ensure all buffered compressed data has been output call zfp_stream_flush() first.

size_t zfp_stream_maximum_size(const zfp_stream* stream, const zfp_field* field)

Conservative estimate of the compressed byte size for the compression parameters stored in stream and the array whose scalar type and dimensions are given by field. This function may be used to determine how large a memory buffer to allocate to safely hold the entire compressed array.

void zfp_stream_set_bit_stream(zfp_stream* stream, bitstream* bs)

Associate bit stream with compressed stream.

void zfp_stream_rewind(zfp_stream* stream)

Rewind bit stream to beginning for compression or decompression.

Compression Parameters

double zfp_stream_set_rate(zfp_stream* stream, double rate, zfp_type type, uint dims, int wra)

Set rate for fixed-rate mode in compressed bits per value. The target scalar type and array dimensionality are needed to correctly translate the rate to the number of bits per block. The parameter wra should be nonzero if random access writes of blocks into the compressed bit stream is needed, for example for implementing zfp’s compressed arrays. This requires blocks to be aligned on bit stream word boundaries, and therefore constrains the rate. The closest supported rate is returned, which may differ from the desired rate.

uint zfp_stream_set_precision(zfp_stream* stream, uint precision)

Set precision for fixed-precision mode. The precision specifies how many uncompressed bits per value to store, and indirectly governs the relative error. The actual precision is returned, e.g. in case the desired precision is out of range. To preserve a certain floating-point mantissa or integer precision in the decompressed data, see FAQ #21.

double zfp_stream_set_accuracy(zfp_stream* stream, double tolerance)

Set absolute error tolerance for fixed-accuracy mode. The tolerance ensures that values in the decompressed array differ from the input array by no more than this tolerance (in all but exceptional circumstances; see FAQ #17). This compression mode should be used only with floating-point (not integer) data.

int zfp_stream_set_mode(zfp_stream* stream, uint64 mode)

Set all compression parameters from compact integer representation. See zfp_stream_mode() for how to encode the parameters. The return value is nonzero upon success.

int zfp_stream_set_params(zfp_stream* stream, uint minbits, uint maxbits, uint maxprec, int minexp)

Set all compression parameters directly. See the section on expert mode for a discussion of the parameters. The return value is nonzero upon success.

Array Metadata

zfp_field* zfp_field_alloc()

Allocates and returns a default initialized zfp_field struct. The caller must free this struct using zfp_field_free().

zfp_field* zfp_field_1d(void* pointer, zfp_type type, uint nx)

Allocate and return a field struct that describes an existing 1D array, a[nx], of nx uncompressed scalars of given type stored at pointer, which may be NULL and specified later.

zfp_field* zfp_field_2d(void* pointer, zfp_type type, uint nx, uint ny)

Allocate and return a field struct that describes an existing 2D array, a[ny][nx], of nx × ny uncompressed scalars of given type stored at pointer, which may be NULL and specified later.

zfp_field* zfp_field_3d(void* pointer, zfp_type type, uint nx, uint ny, uint nz)

Allocate and return a field struct that describes an existing 3D array, a[nz][ny][nx], of nx × ny × nz uncompressed scalars of given type stored at pointer, which may be NULL and specified later.

void zfp_field_free(zfp_field* field)

Free zfp_field struct previously allocated by one of the functions above.

void* zfp_field_pointer(const zfp_field* field)

Return pointer to the first scalar in the array.

zfp_type zfp_field_type(const zfp_field* field)

Return array scalar type.

uint zfp_field_precision(const zfp_field* field)

Return scalar precision in number of bits, e.g. 32 for zfp_type_float.

uint zfp_field_dimensionality(const zfp_field* field)

Return array dimensionality (1, 2, or 3).

size_t zfp_field_size(const zfp_field* field, uint* size)

Return total number of scalars stored in the array, e.g. nx × ny × nz for a 3D array. If size is not NULL, then store the number of scalars for each dimension, e.g. size[0] = nx; size[1] = ny; size[2] = nz for a 3D array.

int zfp_field_stride(const zfp_field* field, int* stride)

Return zero if array is stored contiguously; nonzero if it is strided. If stride is not NULL, then store the stride for each dimension, e.g. stride[0] = sx; stride[1] = sy; stride[2] = sz for a 3D array. See below for more information on strides.

uint64 zfp_field_metadata(const zfp_field* field)

Return 52-bit compact encoding of the scalar type and array dimensions.

void zfp_field_set_pointer(zfp_field* field, void* pointer)

Set pointer to first scalar in the array.

zfp_type zfp_field_set_type(zfp_field* field, zfp_type type)

Set array scalar type.

void zfp_field_set_size_1d(zfp_field* field, uint nx)

Specify dimensions of 1D array a[nx].

void zfp_field_set_size_2d(zfp_field* field, uint nx, uint ny)

Specify dimensions of 2D array a[ny][nx].

void zfp_field_set_size_3d(zfp_field* field, uint nx, uint ny, uint nz)

Specify dimensions of 3D array a[nz][ny][nx].

void zfp_field_set_stride_1d(zfp_field* field, int sx)

Specify stride for 1D array: sx = &a[1] - &a[0].

void zfp_field_set_stride_2d(zfp_field* field, int sx, int sy)

Specify strides for 2D array: sx = &a[0][1] - &a[0][0]; sy = &a[1][0] - &a[0][0].

void zfp_field_set_stride_3d(zfp_field* field, int sx, int sy, int sz)

Specify strides for 3D array: sx = &a[0][0][1] - &a[0][0][0]; sy = &a[0][1][0] - &a[0][0][0]; sz = &a[1][0][0] - &a[0][0][0].

int zfp_field_set_metadata(zfp_field* field, uint64 meta)

Specify array scalar type and dimensions from compact 52-bit representation. Returns nonzero upon success. See zfp_field_metadata() for how to encode meta.

Compression and Decompression

size_t zfp_compress(zfp_stream* stream, const zfp_field* field)

Compress the whole array described by field using parameters given by stream and then flush the stream. The number of bytes of compressed storage is returned, if the stream were rewound before compression, and otherwise the current byte offset within the bit stream. Zero is returned if compression failed.

int zfp_decompress(zfp_stream* stream, zfp_field* field)

Decompress from stream to array described by field and align the stream on the next word boundary. Nonzero is returned upon success.

size_t zfp_write_header(zfp_stream* stream, const zfp_field* field, uint mask)

Write an optional header to the stream that encodes compression parameters, array metadata, etc. The header information written is determined by the bit mask (see macros). The return value is the number of bits written, or zero upon failure.

size_t zfp_read_header(zfp_stream* stream, zfp_field* field, uint mask)

Read header if one was previously written using zfp_write_header(). The return value is the number of bits read, or zero upon failure. The caller must ensure that the bit mask agrees between header read and write calls.