CODEWITHSUNDEEP
javaprotocol-buffers
Rich
Rich

Reputation: 15767

What is the best approach for serializing BigDecimal/BigInteger to ProtocolBuffers

I am starting to migrate a custom serialization mechanism to Protocol Buffers. One data type that will be used particularly regularly is BigDecimal.

Does anyone know of a good way of serializing this within Protocol Buffers? Our current serialization routine uses BigDecimal.toPlainString() for serialization, and new BigDecimal(String) for deserialization - I'm assuming there's a better way.

My guess is to define a BigDecimal as:

message BDecimal {
    required int32 scale = 1;
    required BInteger int_val = 2;
}

But I am not too sure how to define BigInteger - perhaps using its toByteArray() method?

Upvotes: 37

Views: 40045

Answers (3)

Diego Hermida
Diego Hermida

Reputation: 133

I have recently had the same need as OP, and used a similar solution to the one proposed by @notnoop. Posting it here after seeing this comment from @stikkos:

How would you convert a BigDecimal to a BigInteger and scale? And back ?

According to the BigDecimal class documentation:

The value of the BigDecimal is (unscaledVal × 10-scale), rounded according to the precision and rounding mode settings.

So, a java.math.BigDecimal can be serialized considering three properties:

Now, the code.

  1. Protobuf v3 message definition:

    syntax = "proto3";

message DecimalValue {
  uint32 scale = 1;
  uint32 precision = 2;
  bytes value = 3;
}

  2. How to serialize a java.math.BigDecimal to a Protobuf message:

    import com.google.protobuf.ByteString;
import DecimalValue;

java.math.BigDecimal bigDecimal = new java.math.BigDecimal("1234.56789");
DecimalValue serialized = DecimalValue.newBuilder()
        .setScale(bigDecimal.scale())
        .setPrecision(bigDecimal.precision())
        .setValue(ByteString.copyFrom(bigDecimal.unscaledValue().toByteArray()))
        .build();

  3. How to deserialize the Protobuf message back to a java.math.BigDecimal:

    java.math.MathContext mc = new java.math.MathContext(serialized.getPrecision());
java.math.BigDecimal deserialized = new java.math.BigDecimal(
        new java.math.BigInteger(serialized.getValue().toByteArray()),
        serialized.getScale(),
        mc);

  4. We can check the obtained java.math.BigDecimal yields the same original value as follows:

    assert bigDecimal.equals(deserialized);

Note: OP assumed there is a better way to serialize a java.math.BigDecimal than using a plain String. In terms of storage, this solution is better.

For example, to serialize the first 100 decimals of π:

3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679

System.out.println("DecimalValue: " + serialized.getSerializedSize());
System.out.println("String: " + StringValue.of(pi).getSerializedSize());

This yields:

DecimalValue: 48
String: 104

Upvotes: 11

squiddle
squiddle

Reputation: 1317

Why do you want to change it? Just because you can or is there a real need (like a profiling session confirming, that serialization/deserialization takes most of the time).

I would use a string, just because it is built in :)

The proposed byte array approach (What is the best approach for serializing BigDecimal/BigInteger to ProtocolBuffers) seems fine to me, if string representation seems to be an issue.

Upvotes: 1

notnoop
notnoop

Reputation: 59307

Yes. You should define BigInteger as BigInteger.toByteArray() .

My guess is that BigDecimal would be:


message BDecimal {
  required int32 scale = 1;
  required BInteger int_val = 2;
}

while BigInteger may be defined as


message BInteger {
  required bytes value = 1;
}

The code to handle BigInteger would be:


  BInteger write(BigInteger val) {
    BInteger.Builder builder = BInteger.newBuilder();
    ByteString bytes = ByteString.copyFrom(val.toByteArray());
    builder.setValue(bytes);
    return builder.build();
  }

  BigInteger read(BInteger message) {
    ByteString bytes = message.getValue();
    return new BigInteger(bytes.toByteArray());
  }

Upvotes: 26

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