3.1.1. Switch#

A key feature that caterpillar provides out of the box is support for switch-case structures. This feature allows you to model conditional data processing in a clean, efficient way without the need for a custom implementation.

While there are some limitations to the usage of certain operators, their basic functionality should be more than sufficient for most use cases. To illustrate this, let’s revisit the PNG format implementation and define a common chunk type using a switch-case structure.

Implementation of the general chunk layout#
@struct(order=BigEndian)
class PNGChunk:
    length: uint32          # length of the data field (1)
    type: Bytes(4)
    data: F(this.type) >> { # just use right shift (2)
        b"IHDR": IHDRChunk,
        b"PLTE": PLTEChunk, # the key must be comparable to this.type
        b"pHYs": PHYSChunk,
        b"iTXt": ITXTChunk,
        b"tIME": TIMEChunk,
        b"tEXt": TEXTChunk,
        b"iTXt": ITXTChunk,
        b"fDAT": FDATChunk,
        b"zTXt": ZTXTChunk,
        b"gAMA": GAMAChunk,
        # Special case: IEND chunk, which doesn’t need to parse any data
        b"IEND": Pass # (4)
        # As we don't define all chunks here for now, a default
        # option must be provided
        DEFAULT_OPTION: Memory(this.length) # (3)
    }

  1. Length and Type: The PNGChunk struct has two main fields: length (the length of the data field) and type (the 4-byte identifier of the chunk type). These fields are standard across all PNG chunks.

  2. Dynamic Chunk Handling (Switch-Case): The data field uses a dynamic mapping of chunk types to specific chunk structs. We use the >> operator (right shift) to define this mapping, where each chunk type corresponds to a specific struct (e.g., IHDRChunk, PLTEChunk, etc.).

  3. Default Option: For chunk types that are not explicitly defined in the mapping, we provide a default option. In this case, DEFAULT_OPTION stores the raw chunk data as a Memory object, which is useful for handling unrecognized or unknown chunk types.

  4. Special Handling for `IEND`: The IEND chunk, which marks the end of a PNG file, does not require any data parsing. We simply map it to Pass, a special directive indicating that no further processing is needed for this chunk.

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