Read zstd dict

read_zstd_dict.go

package main

import (
	"bytes"
	"encoding/binary"
	"fmt"
)

// Little Endianed
var ZstdMagic = []byte("\x28\xB5\x2F\xFD")

// ReadDictID returns a dictionary ID from the first zstd frame, if present
//
// If there are multiple frames in data, we will return only the first's
// dictionary ID. This saves us from scanning the entire payload, with a
// tradeoff of assuming incompatible frames aren't being passed together.
func ReadDictID(data []byte) (int32, error) {
	if !bytes.Equal(data[:4], ZstdMagic) {
		return 0, fmt.Errorf("data malformatted: doesn't start with %x", ZstdMagic)
	}

	// To get the Dictionary ID, we need to know the length of the window
	// desccriptor and the ID itself. We can ignore FCS.
	//
	// Format ref: https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#zstandard-frames
	fhd := data[4]

	wd := 1
	ss := (fhd >> 5) & 1
	if ss == 1 {
		wd = 0
	}

	start := 5 + wd
	end := start
	switch fhd & 3 {
	case 0:
		break
	case 1:
		end += 1
	case 2:
		end += 2
	case 3:
		end += 4
	}

	if start == end {
		return 0, nil
	}

	return int32(binary.LittleEndian.Uint32(data[start:end])), nil
}