Serialization: implement issued currency amounts, pathsets

content/_code-samples/tx-serialization/xrpl_num.py

@@ -0,0 +1,65 @@
+# Serializes issued currency amounts from string number representations,
+# matching the precision of the XRP Ledger.
+
+from decimal import getcontext, Decimal
+
+class IssuedAmount:
+    MIN_MANTISSA = 10**15
+    MAX_MANTISSA = 10**16 - 1
+    MIN_EXP = -96
+    MAX_EXP = 80
+    def __init__(self, strnum):
+        self.context = getcontext()
+        self.context.prec = 15
+        self.context.Emin = self.MIN_EXP
+        self.context.Emax = self.MAX_EXP
+
+        self.dec = Decimal(strnum)
+
+    def to_bytes(self):
+        if self.dec.is_zero():
+            return self.canonical_zero_serial()
+
+        # Convert components to integers ---------------------------------------
+        sign, digits, exp = self.dec.as_tuple()
+        mantissa = int("".join([str(d) for d in digits]))
+
+        # Canonicalize to expected range ---------------------------------------
+        while mantissa < self.MIN_MANTISSA and exp > self.MIN_EXP:
+            mantissa *= 10
+            exp -= 1
+
+        while mantissa > self.MAX_MANTISSA:
+            if exp >= self.MAX_EXP:
+                raise ValueError("amount overflow")
+            mantissa //= 10
+            exp += 1
+
+        if exp < self.MIN_EXP or mantissa < self.MIN_MANTISSA:
+            # Round to zero
+            return self.canonical_zero_serial()
+
+        if exp > self.MAX_EXP or mantissa > self.MAX_MANTISSA:
+            raise ValueError("amount overflow")
+
+        # Convert to bytes -----------------------------------------------------
+        serial = 0x8000000000000000 # "Not XRP" bit set
+        if sign == 0:
+            serial |= 0x4000000000000000 # "Is positive" bit set
+        serial |= ((exp+97) << 54) # next 8 bits are exponent
+        serial |= mantissa # last 54 bits are mantissa
+
+        return serial.to_bytes(8, byteorder="big", signed=False)
+
+
+    def canonical_zero_serial(self):
+        """
+        Returns canonical format for zero:
+        - "Not XRP" bit = 1
+        - "Sign bit" = 1 (for positive !!)
+        - exponent ???
+        """
+        # Mantissa is all zeroes. Must be positive zero.
+        #bitval = 0b1100000001000000000000000000000000000000000000000000000000000000
+        #return bitval.to_bytes(8, byteorder="big", signed=False)
+        return (0x8000000000000000).to_bytes(8, byteorder="big", signed=False)