alpacadecimal

Similar and compatible with decimal.Decimal, but optimized for Alpaca's data sets.

Goal

• optimize for Alpaca data sets (99% of decimals are within 10 millions with up to 12 precisions).
• compatible with decimal.Decimal so that it could be a drop-in replacement for current decimal.Decimal usage.

Key Ideas

The original decimal.Decimal package has bottleneck on big.Int operations, e.g. sql serialization / deserialization, addition, multiplication etc. These operations took fair amount cpu and memory during our profiling / monitoring.

The optimization this library is to represent most decimal numbers with int64 instead of big.Int. To keep this library to be compatible with original decimal.Decimal package, we use original as a fallback solution when int64 is not enough (e.g. number is too big / small, too many precisions).

The core data struct is like following:

type Decimal struct {
	// represent decimal with 12 precision, 1.23 will have `fixed = 1_230_000_000_000`
	// max support decimal is 9_223_372.000_000_000_000
	// min support decimal is -9_223_372.000_000_000_000
	fixed int64

	// fallback to original decimal.Decimal if necessary
	fallback *decimal.Decimal
}

We pick 12 precisions because it could cover 99% of Alpaca common cases.

Compatibility

In general, alpacadecimal.Decimal is fully compatible with decimal.Decimal package, as decimal.Decimal is used as a fallback solution for overflow cases.

There are a few special cases / APIs that alpacadecimal.Decimal behaves different from decimal.Decimal (behaviour is still correct / valid, just different). Affected APIs:

• Decimal.Exponent()
• Decimal.Coefficient()
• Decimal.CoefficientInt64()
• Decimal.NumDigits()

For optimized case, alpacadecimal.Decimal always assume that exponent is 12, which results in a valid but different decimal representation. For example,

x := alpacadecimal.NewFromInt(123)
require.Equal(t, int32(-12), x.Exponent())
require.Equal(t, "123000000000000", x.Coefficient().String())
require.Equal(t, int64(123000000000000), x.CoefficientInt64())
require.Equal(t, 15, x.NumDigits())

y := decimal.NewFromInt(123)
require.Equal(t, int32(0), y.Exponent())
require.Equal(t, "123", y.Coefficient().String())
require.Equal(t, int64(123), y.CoefficientInt64())
require.Equal(t, 3, y.NumDigits())

Related Issues

• big.NewInt optimization from here might help to speed up some big.Int related operations.
• big.Int.String slowness is tracked by this issue. The approach we reduce this slowness is to use int64 to represent the number if possible to avoid big.Int operations.

Benchmark

Generally, for general case (99%), the speedup varies from 5x to 100x.

$ make bench
go test -bench=. --cpuprofile profile.out --memprofile memprofile.out
goos: darwin
goarch: amd64
pkg: github.com/alpacahq/alpacadecimal
cpu: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
BenchmarkValue/alpacadecimal.Decimal_Cached_Case-16             314870084                3.498 ns/op
BenchmarkValue/alpacadecimal.Decimal_Optimized_Case-16          15383466                70.27 ns/op
BenchmarkValue/alpacadecimal.Decimal_Fallback_Case-16            5603755               209.2 ns/op
BenchmarkValue/decimal.Decimal-16                                6167956               184.5 ns/op
BenchmarkValue/eric.Decimal-16                                   7021383               162.2 ns/op
BenchmarkAdd/alpacadecimal.Decimal-16                           556380649                2.132 ns/op
BenchmarkAdd/decimal.Decimal-16                                 15557970                68.31 ns/op
BenchmarkAdd/eric.Decimal-16                                    27423730                40.34 ns/op
BenchmarkSub/alpacadecimal.Decimal-16                           268269063                4.410 ns/op
BenchmarkSub/decimal.Decimal-16                                 17239782                59.17 ns/op
BenchmarkSub/eric.Decimal-16                                    24690660                40.81 ns/op
BenchmarkScan/alpacadecimal.Decimal-16                          87226915                13.46 ns/op
BenchmarkScan/decimal.Decimal-16                                 6075110               191.1 ns/op
BenchmarkScan/eric.Decimal-16                                    6422792               174.4 ns/op
BenchmarkMul/alpacadecimal.Decimal-16                           168732728                7.176 ns/op
BenchmarkMul/decimal.Decimal-16                                 16051546                66.57 ns/op
BenchmarkMul/eric.Decimal-16                                    39927952                28.20 ns/op
BenchmarkDiv/alpacadecimal.Decimal-16                           152054401                7.772 ns/op
BenchmarkDiv/decimal.Decimal-16                                  4098888               281.7 ns/op
BenchmarkDiv/eric.Decimal-16                                    34245668                31.42 ns/op
BenchmarkString/alpacadecimal.Decimal-16                        385985688                3.032 ns/op
BenchmarkString/decimal.Decimal-16                               7750777               150.9 ns/op
BenchmarkString/eric.Decimal-16                                  6694531               167.0 ns/op
BenchmarkRound/alpacadecimal.Decimal-16                         88814521                11.92 ns/op
BenchmarkRound/decimal.Decimal-16                                4333029               255.7 ns/op
BenchmarkRound/eric.Decimal-16                                  55717095                21.34 ns/op
PASS
ok      github.com/alpacahq/alpacadecimal       37.671s