Wrap GMT's data structure GMT_GRID_HEADER for grid/image/cube headers

pygmt/datatypes/grid.py

@@ -1,8 +1,104 @@
 """
-Wrapper for the GMT_GRID data type.
+Wrapper for the GMT_GRID data type and the GMT_GRID_HEADER data structure.
 """
 
 import ctypes as ctp
+from typing import ClassVar
+
+# Constants for lengths of grid header variables.
+#
+# Note: Ideally we should be able to get these constants from the GMT shared library
+# using the ``lib["GMT_GRID_UNIT_LEN80"]`` syntax, but it causes cyclic import error.
+# So we have to hardcode the values here.
+GMT_GRID_UNIT_LEN80 = 80
+GMT_GRID_TITLE_LEN80 = 80
+GMT_GRID_COMMAND_LEN320 = 320
+GMT_GRID_REMARK_LEN160 = 160
+
+# GMT uses single-precision for grids by default, but can be built to use
+# double-precision. Currently, only single-precision is supported.
+gmt_grdfloat = ctp.c_float
+
+
+class _GMT_GRID_HEADER(ctp.Structure):  # noqa: N801
+    """
+    GMT grid header structure for metadata about the grid.
+
+    The class is used in the `GMT_GRID`/`GMT_IMAGE`/`GMT_CUBE` data structure. See the
+    GMT source code gmt_resources.h for the original C structure definitions.
+    """
+
+    _fields_: ClassVar = [
+        # Number of columns
+        ("n_columns", ctp.c_uint32),
+        # Number of rows
+        ("n_rows", ctp.c_uint32),
+        # Grid registration, 0 for gridline and 1 for pixel
+        ("registration", ctp.c_uint32),
+        # Minimum/maximum x and y coordinates
+        ("wesn", ctp.c_double * 4),
+        # Minimum z value
+        ("z_min", ctp.c_double),
+        # Maximum z value
+        ("z_max", ctp.c_double),
+        # x and y increments
+        ("inc", ctp.c_double * 2),
+        # Grid values must be multiplied by this factor
+        ("z_scale_factor", ctp.c_double),
+        # After scaling, add this offset
+        ("z_add_offset", ctp.c_double),
+        # Units in x-directions, in the form "long_name [units]"
+        ("x_units", ctp.c_char * GMT_GRID_UNIT_LEN80),
+        # Units in y-direction, in the form "long_name [units]"
+        ("y_units", ctp.c_char * GMT_GRID_UNIT_LEN80),
+        # Grid value units, in the form "long_name [units]"
+        ("z_units", ctp.c_char * GMT_GRID_UNIT_LEN80),
+        # Name of data set
+        ("title", ctp.c_char * GMT_GRID_TITLE_LEN80),
+        # Name of generating command
+        ("command", ctp.c_char * GMT_GRID_COMMAND_LEN320),
+        # Comments for this data set
+        ("remark", ctp.c_char * GMT_GRID_REMARK_LEN160),
+        # Below are items used internally by GMT
+        # Number of data points (n_columns * n_rows) [paddings are excluded]
+        ("nm", ctp.c_size_t),
+        # Actual number of items (not bytes) required to hold this grid (mx * my),
+        # per band (for images)
+        ("size", ctp.c_size_t),
+        # Bits per data value (e.g., 32 for ints/floats; 8 for bytes).
+        # Only used for ERSI ArcInfo ASCII Exchange grids.
+        ("bits", ctp.c_uint),
+        # For complex grid.
+        # 0 for normal
+        # GMT_GRID_IS_COMPLEX_REAL = real part of complex grid
+        # GMT_GRID_IS_COMPLEX_IMAG = imag part of complex grid
+        ("complex_mode", ctp.c_uint),
+        # Grid format
+        ("type", ctp.c_uint),
+        # Number of bands [1]. Used with GMT_IMAGE containers
+        ("n_bands", ctp.c_uint),
+        # Actual x-dimension in memory. mx = n_columns + pad[0] + pad[1]
+        ("mx", ctp.c_uint),
+        # Actual y-dimension in memory. my = n_rows + pad[2] + pad[3]
+        ("my", ctp.c_uint),
+        # Paddings on west, east, south, north sides [2,2,2,2]
+        ("pad", ctp.c_uint * 4),
+        # Three or four char codes T|B R|C S|R|S (grd) or B|L|P + A|a (img)
+        # describing array layout in mem and interleaving
+        ("mem_layout", ctp.c_char * 4),
+        # Missing value as stored in grid file
+        ("nan_value", gmt_grdfloat),
+        # 0.0 for gridline grids and 0.5 for pixel grids
+        ("xy_off", ctp.c_double),
+        # Referencing system string in PROJ.4 format
+        ("ProjRefPROJ4", ctp.c_char_p),
+        # Referencing system string in WKT format
+        ("ProjRefWKT", ctp.c_char_p),
+        # Referencing system EPSG code
+        ("ProjRefEPSG", ctp.c_int),
+        # Lower-level information for GMT use only
+        ("hidden", ctp.c_void_p),
+    ]
 
 
 class _GMT_GRID(ctp.Structure):  # noqa: N801