Buffer Layout and Position Formula#

Every chunk is backed by a flat float32 array in memory. When fields are returned by FDB they are decoded and written into this array one at a time. This page explains how the correct write position is calculated.

Chunk Buffer Shape#

The chunk buffer is a C-order (row-major) multi-dimensional array with one entry per combination of axis positions inside the chunk, times the number of grid-point values num_values:

total floats = chunkSize_0 × chunkSize_1 × … × chunkSize_{N-1} × num_values

chunkSize_i depends on the chunking mode chosen for axis i:

Chunking mode	`chunkSize_i`
`SINGLE_VALUE`	1
`WHOLE_AXIS`	Full axis length
`FixedSizeChunk(k)`	`k`

The grid-point dimension (num_values) is always the trailing dimension and is never chunked.

Buffer Position Formula#

For every GRIB field returned by FDB during a chunk access, the buffer write position along each axis i is:

local_i   = axis.index(key_i) − partAxisOffset[i]
bufPos_i  = local_i + bufferOffset[i]

Where:

axis.index(key_i)

Zero-based position of the MARS key value returned by FDB within the Part’s local axis (range: 0 to axisSize_i − 1).

partAxisOffset[i]

Start of the intersection within the Part’s own local axis:

partAxisOffset[i] = intersection.lower[i] − partBoundingBox.lower[i]

bufferOffset[i]

Start of the intersection within the chunk buffer:

bufferOffset[i] = intersection.lower[i] − chunkBoundingBox.lower[i]

local_i is the zero-based position of the field within the intersection along axis i. Adding bufferOffset[i] shifts it to the correct slot in the chunk buffer.

Flat Buffer Index#

The per-axis positions are combined into a single flat index using C-order (row-major) arithmetic — the rightmost axis varies fastest:

stride_{N-1}  = num_values
stride_{N-2}  = chunkSize_{N-1} × num_values
stride_{N-3}  = chunkSize_{N-2} × stride_{N-2}
…

flatIndex = sum(bufPos_i × stride_i  for i in 0..N-1)

Each position in the flat index corresponds to num_values consecutive float32 values — the decoded grid-point values for that field.

Note

The formula applies independently to every field FDB returns. Fields are written into the buffer in whatever order FDB returns them; the position formula makes the result independent of retrieval order.