dec1.mjs

dec1.mjs

import { Uint8ArrayList } from 'uint8arraylist';

var ChunkType_IDENTIFIER = 0xff;
var ChunkType_COMPRESSED = 0x00;
var ChunkType_UNCOMPRESSED = 0x01;
var ChunkType_PADDING = 0xfe;
var IDENTIFIER = Buffer.from([0x73, 0x4e, 0x61, 0x50, 0x70, 0x59]);
var IDENTIFIER_FRAME = Buffer.from([0xff, 0x06, 0x00, 0x00, 0x73, 0x4e, 0x61, 0x50, 0x70, 0x59]);

function uncompress(chunk) {
	var foundIdentifier = false;
    var buffer = new Uint8ArrayList();
	buffer.append(chunk);
    var result = new Uint8ArrayList();
    while (buffer.length > 0) {
      if (buffer.length < 4) break;
		
	  console.log("checking chunk type=" + buffer.get(0));

      var type = getChunkType(buffer.get(0));
      var frameSize = getFrameSize(buffer, 1);

      if (buffer.length - 4 < frameSize) {
        break;
      }

      var data = buffer.subarray(4, 4 + frameSize);
      buffer.consume(4 + frameSize);
	

      if (!foundIdentifier && type !== ChunkType_IDENTIFIER) {
        throw "malformed input: must begin with an identifier";
      }

      if (type === ChunkType_IDENTIFIER) {
        if (!Buffer.prototype.equals.call(data, IDENTIFIER)) {
          throw "malformed input: bad identifier";
        }
        foundIdentifier = true;
        continue;
      }

      if (type === ChunkType_COMPRESSED) {
     //   result.append(uncompress(data.subarray(4)));
      }
      if (type === ChunkType_UNCOMPRESSED) {
		console.log("got uncompressed chunk..");
        result.append(data.subarray(4));
      }
    }
    if (result.length === 0) {
      return null;
    }
    return result;
}



function getFrameSize(buffer, offset) {
  return buffer.get(offset) + (buffer.get(offset + 1) << 8) + (buffer.get(offset + 2) << 16);
}

function getChunkType(value) {
    switch (value) {
        case ChunkType_IDENTIFIER:
            return ChunkType_IDENTIFIER;
        case ChunkType_COMPRESSED:
            return ChunkType_COMPRESSED;
        case ChunkType_UNCOMPRESSED:
            return ChunkType_UNCOMPRESSED;
        case ChunkType_PADDING:
            return ChunkType_PADDING;
        default:
            throw new Error("Unsupported snappy chunk type");
    }
}



var chunk = new Uint8ArrayList();
chunk.append(Uint8Array.from([0xff,0x06,0x00,0x00,0x73,0x4e,0x61,0x50,0x70,0x59]));

var data = [0x01, 0x80, 0x00, 0x00];
for (var i = 0; i < 0x80; i+=1) {
	data.push(0x41);
}
chunk.append(Uint8Array.from(data));
var result = uncompress(chunk);
console.log("Decompressed ok "+result.length + " bytes");