feat(gl): add immutable postgres ledger storage

domain/ledger/amount.go

@@ -0,0 +1,115 @@
+// Package ledger contains GL's framework-independent financial model.
+package ledger
+
+import (
+	"fmt"
+	"math/big"
+	"strings"
+	"unicode"
+)
+
+const (
+	AmountPrecision = 38
+	AmountScale     = 18
+)
+
+var amountFactor = new(big.Int).Exp(big.NewInt(10), big.NewInt(AmountScale), nil)
+
+// Amount is an exact fixed-point decimal with PostgreSQL numeric(38,18)
+// semantics. Its zero value is a valid zero amount.
+type Amount struct {
+	units big.Int
+}
+
+func ParseAmount(value string) (Amount, error) {
+	if value == "" {
+		return Amount{}, fmt.Errorf("amount is required")
+	}
+
+	negative := value[0] == '-'
+	unsigned := value
+	if negative {
+		unsigned = value[1:]
+	}
+	if unsigned == "" || strings.HasPrefix(unsigned, "+") {
+		return Amount{}, fmt.Errorf("invalid amount %q", value)
+	}
+
+	parts := strings.Split(unsigned, ".")
+	if len(parts) > 2 || parts[0] == "" {
+		return Amount{}, fmt.Errorf("invalid amount %q", value)
+	}
+	for _, part := range parts {
+		if part == "" || strings.IndexFunc(part, func(r rune) bool { return !unicode.IsDigit(r) }) >= 0 {
+			return Amount{}, fmt.Errorf("invalid amount %q", value)
+		}
+	}
+	if len(parts[0]) > 1 && parts[0][0] == '0' {
+		return Amount{}, fmt.Errorf("amount must not contain leading zeroes")
+	}
+	if len(strings.TrimLeft(parts[0], "0")) > AmountPrecision-AmountScale {
+		return Amount{}, fmt.Errorf("amount exceeds integer precision %d", AmountPrecision-AmountScale)
+	}
+
+	fraction := ""
+	if len(parts) == 2 {
+		fraction = parts[1]
+		if len(fraction) > AmountScale {
+			return Amount{}, fmt.Errorf("amount exceeds scale %d", AmountScale)
+		}
+	}
+	digits := strings.TrimLeft(parts[0]+fraction, "0")
+	if len(digits) > AmountPrecision {
+		return Amount{}, fmt.Errorf("amount exceeds precision %d", AmountPrecision)
+	}
+
+	whole := new(big.Int)
+	whole.SetString(parts[0], 10)
+	units := new(big.Int).Mul(whole, amountFactor)
+	if fraction != "" {
+		padded := fraction + strings.Repeat("0", AmountScale-len(fraction))
+		fractional := new(big.Int)
+		fractional.SetString(padded, 10)
+		units.Add(units, fractional)
+	}
+	if negative {
+		units.Neg(units)
+	}
+
+	return Amount{units: *units}, nil
+}
+
+func (a Amount) IsZero() bool {
+	return a.units.Sign() == 0
+}
+
+func (a Amount) Add(other Amount) Amount {
+	var result big.Int
+	result.Add(&a.units, &other.units)
+	return Amount{units: result}
+}
+
+func (a Amount) Negate() Amount {
+	var result big.Int
+	result.Neg(&a.units)
+	return Amount{units: result}
+}
+
+func (a Amount) String() string {
+	if a.units.Sign() == 0 {
+		return "0"
+	}
+
+	abs := new(big.Int).Abs(new(big.Int).Set(&a.units))
+	whole, fraction := new(big.Int), new(big.Int)
+	whole.QuoRem(abs, amountFactor, fraction)
+	value := whole.String()
+	if fraction.Sign() != 0 {
+		fractionText := fmt.Sprintf("%018s", fraction.String())
+		value += "." + strings.TrimRight(fractionText, "0")
+	}
+	if a.units.Sign() < 0 {
+		return "-" + value
+	}
+	return value
+}