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64 lines
2.2 KiB
Python
64 lines
2.2 KiB
Python
# Serializes issued currency amounts from string number representations,
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# matching the precision of the XRP Ledger.
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from decimal import getcontext, Decimal
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class IssuedAmount:
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MIN_MANTISSA = 10**15
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MAX_MANTISSA = 10**16 - 1
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MIN_EXP = -96
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MAX_EXP = 80
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def __init__(self, strnum):
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self.context = getcontext()
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self.context.prec = 15
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self.context.Emin = self.MIN_EXP
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self.context.Emax = self.MAX_EXP
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self.dec = Decimal(strnum)
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def to_bytes(self):
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if self.dec.is_zero():
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return self.canonical_zero_serial()
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# Convert components to integers ---------------------------------------
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sign, digits, exp = self.dec.as_tuple()
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mantissa = int("".join([str(d) for d in digits]))
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# Canonicalize to expected range ---------------------------------------
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while mantissa < self.MIN_MANTISSA and exp > self.MIN_EXP:
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mantissa *= 10
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exp -= 1
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while mantissa > self.MAX_MANTISSA:
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if exp >= self.MAX_EXP:
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raise ValueError("amount overflow")
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mantissa //= 10
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exp += 1
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if exp < self.MIN_EXP or mantissa < self.MIN_MANTISSA:
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# Round to zero
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return self.canonical_zero_serial()
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if exp > self.MAX_EXP or mantissa > self.MAX_MANTISSA:
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raise ValueError("amount overflow")
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# Convert to bytes -----------------------------------------------------
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serial = 0x8000000000000000 # "Not XRP" bit set
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if sign == 0:
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serial |= 0x4000000000000000 # "Is positive" bit set
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serial |= ((exp+97) << 54) # next 8 bits are exponent
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serial |= mantissa # last 54 bits are mantissa
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return serial.to_bytes(8, byteorder="big", signed=False)
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def canonical_zero_serial(self):
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"""
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Returns canonical format for zero (a special case):
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- "Not XRP" bit = 1
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- Everything else is zeroes
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- Arguably this means it's canonically written as "negative zero"
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because the encoding usually uses 1 for positive.
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"""
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return (0x8000000000000000).to_bytes(8, byteorder="big", signed=False)
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