Proper Money Handling (JSON-RPC): Difference between revisions
No need for JSON library to convert to 8 decimal places, bitcoin does proper rounding with full-precision doubles |
But if a library only shows 4 or 6 decimal places, bitcoind *can't* figure out what the lost precision was. And while bitcoind might correct for buggy input beyond 8 decimal places, bitcoind is only an *implementation*, not a protocol spec... |
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else { int64Value = (long long)(dValue * 10e8 - 0.5); | else { int64Value = (long long)(dValue * 10e8 - 0.5); | ||
To convert to a JSON value divide by 100,000,000.0: | To convert to a JSON value divide by 100,000,000.0, and (somehow) ensure your JSON implementation rounds to 8 decimal places: | ||
double dValue = (double) int64Value / 10e8 | double dValue = (double) int64Value / 10e8 | ||
Revision as of 15:53, 3 March 2011
Overview
The original bitcoin client stores all bitcoin values as 64-bit integers, with 1 BTC stored as 100,000,000 (one-hundred-thousand of the smallest possible bitcoin unit). Values are expressed as double-precision Numbers in the JSON API, with 1 BTC expressed as 1.00000000
If you are writing software that uses the JSON-RPC interface you need to be aware of possible floating-point conversion issues. You, or the JSON library you are using, should convert amounts to either a fixed-point Decimal representation (with 8 digits after the decimal point) or a 64-bit integer representation.
Improper value handling can lead to embarrassing errors; for example, if you truncate instead of doing proper rounding and your software will display the value "0.1 BTC" as "0.09999999 BTC" (or, worse, "0.09 BTC").
The rest of this page gives sample code for various JSON libraries and programming languages.
ECMAScript
The ECMAScript Number type is double-precision floating point; ECMAScript does not have an integral numeric type.
JavaScript experts: what is best practice? Convert to an integral-like-double, or just do proper rounding on display and do all calculations using doubles?
- This is not ECMAScript specific: all program internals should use raw/base bitcoin amounts, never floats.
C/C++
C/C++ JSON libraries return the JavaScript Number type as type 'double'. To convert, without loss of precision, from a double to a 64-bit integer multiply by 100,000,000 and round to the nearest integer:
double dValue = ...from JSON library...;
long long int64Value;
if (dValue > 0) { int64Value = (long long)(dValue * 10e8 + 0.5);
else { int64Value = (long long)(dValue * 10e8 - 0.5);
To convert to a JSON value divide by 100,000,000.0, and (somehow) ensure your JSON implementation rounds to 8 decimal places:
double dValue = (double) int64Value / 10e8
Python
Pass the parse_float arguments to python's JSON parsing routines to parse JSON values into Decimal:
import decimal import json # From http://stackoverflow.com/questions/1960516/python-json-serialize-a-decimal-object class DecimalEncoder(json.JSONEncoder): def _iterencode(self, o, markers=None): if isinstance(o, decimal.Decimal): return (str(o) for o in [o]) return super(DecimalEncoder, self)._iterencode(o, markers) decimal.setcontext(decimal.Context(prec=8)) print json.dumps(decimal.Decimal('10.001'), cls=DecimalEncoder) print json.dumps({ "decimal" : decimal.Decimal('1.1'), "float" : 1.1, "string" : "1.1" }, cls=DecimalEncoder) print json.loads('{"amount": 0.333331}', parse_float=decimal.Decimal)
Output is:
10.001
{"decimal": 1.1, "float": 1.1000000000000001, "string": "1.1"}
{u'blaa': Decimal('0.333331')}