offsetDecoder

offsetDecoder<TDecoder>(decoder, config): TDecoder

Moves the offset of a given decoder before and/or after decoding.

This function allows a decoder to read its input from a different offset than the one originally provided. It supports both pre-offset adjustments (before decoding) and post-offset adjustments (after decoding).

The pre-offset function determines where decoding should start, while the post-offset function adjusts where the next decoder should continue reading.

For more details, see offsetCodec.

Type Parameters

Type Parameter
`TDecoder` extends `AnyDecoder`

Parameters

Parameter	Type	Description
`decoder`	`TDecoder`	The decoder to adjust.
`config`	`OffsetConfig`	An object specifying how the offset should be modified.

Returns

TDecoder

A new decoder with adjusted offsets.

Examples

Moving the pre-offset forward by 2 bytes.

const decoder = offsetDecoder(getU32Decoder(), {
    preOffset: ({ preOffset }) => preOffset + 2,
});
const bytes = new Uint8Array([0, 0, 42, 0]); // Value starts at offset 2
decoder.read(bytes, 0); // Actually reads from offset 2

Moving the post-offset forward by 2 bytes.

const decoder = offsetDecoder(getU32Decoder(), {
    postOffset: ({ postOffset }) => postOffset + 2,
});
const bytes = new Uint8Array([42, 0, 0, 0]);
const [value, nextOffset] = decoder.read(bytes, 0); // Next decoder starts at offset 6 instead of 4

Using wrapBytes to read from the last 4 bytes of an array.

const decoder = offsetDecoder(getU32Decoder(), {
    preOffset: ({ wrapBytes }) => wrapBytes(-4), // Moves offset to last 4 bytes of the array
});
const bytes = new Uint8Array([0, 0, 0, 0, 0, 0, 0, 42]); // Value stored at the last 4 bytes
decoder.read(bytes, 0); // Reads from bytes.length - 4

Remarks

If you need both encoding and decoding offsets to be adjusted, use offsetCodec.