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67 lines
5.5 KiB
Markdown
67 lines
5.5 KiB
Markdown
# Currency Formats
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The XRP Ledger has two kinds of money: [XRP](xrp.html), and [issued currencies](issued-currencies.html). In the XRP Ledger, both types have high precision, although their formats are different.
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## String Formatting
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{% include '_snippets/string-number-formatting.md' %}
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<!--{#_ #}-->
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## XRP Precision
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XRP has the same precision as a 64-bit unsigned integer where each unit is equivalent to 0.000001 XRP. Its properties are:
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* Minimum value: `0`
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* Maximum value: `100000000000` (10<sup>11</sup>) XRP
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- `"100000000000000000"` (10<sup>17</sup>) drops of XRP
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* Precise to the nearest `0.000001` (10<sup>-6</sup>) XRP
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- `"1"` drop of XRP
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## Issued Currency Precision
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Issued currencies in the XRP Ledger are represented with a custom format with the following precision:
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* Minimum nonzero absolute value: `1000000000000000e-96`
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* Maximum value: `9999999999999999e80`
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* Minimum value: `-9999999999999999e80`
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* 15 decimal digits of precision
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## Issued Currency Math
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[[Source]](https://github.com/ripple/rippled/blob/35fa20a110e3d43ffc1e9e664fc9017b6f2747ae/src/ripple/protocol/impl/STAmount.cpp "Source")
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Internally, `rippled` represents numbers for issued currencies in a custom number format. This format can store a wide variety of assets, including those typically measured in very small or very large denominations. This format uses significant digits and a power-of-ten exponent in a similar way to scientific notation. The format supports positive and negative significant digits and exponents within the specified range. Unlike typical floating-point representations of non-whole numbers, this format uses integer math for all calculations, so it always maintains 15 decimal digits of precision. Multiplication and division have adjustments to compensate for over-rounding in the least significant digits.
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Unlike "arbitrary precision" number formats, the custom format can be stored in a fixed size of 64 bits. When serialized this way, the format consists of a "not XRP" bit, a sign bit, significant digits, and an exponent. They are present in order:
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1. The first (most significant) bit for an issued currency amount is `1` to indicate that it is not an XRP amount. (XRP amounts always have the most significant bit set to `0` to distinguish them from this format.)
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2. The sign bit indicates whether the amount is positive or negative. Unlike standard [two's complement](https://en.wikipedia.org/wiki/Two%27s_complement) integers, `1` indicates **positive** in the XRP Ledger format, and `0` indicates negative.
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3. The next 8 bits represent the exponent as an unsigned integer. The exponent indicates the scale (what power of 10 the significant digits should be multiplied by) in the range -96 to +80 (inclusive). However, when serializing, we add 97 to the exponent to make it possible to serialize as an unsigned integer. Thus, a serialized value of `1` indicates an exponent of `-96`, a serialized value of `177` indicates an exponent of 80, and so on.
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4. The remaining 54 bits represent the significant digits as an unsigned integer. When serializing, this value is normalized to the range 10<sup>15</sup> (`1000000000000000`) to 10<sup>16</sup>-1 (`9999999999999999`) inclusive, except for the special case of the value 0. There is a special case for the value 0. In this case, the sign bit, exponent, and mantissa are all zeroes, so the 64-bit value is serialized as `0x8000000000000000000000000000000000000000`.
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## Currency Codes
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All non-XRP currencies in the XRP Ledger have a 160-bit currency code. The [`rippled` APIs](rippled-api.html) map 3-character ASCII strings (case-sensitive) to 160-bit currency codes using a standard mapping. The currency code `XRP` is disallowed for issued currencies. Currencies with the same code can [ripple](rippling.html) across connected trustlines. Currency codes have no other behavior built into the XRP Ledger.
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### Standard Currency Codes
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The standard currency mapping allocates the bits as follows:
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1. The first 8 bits must be `0x00`.
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2. The next 88 bits are reserved, and should be all `0`'s.
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3. The next 24 bits represent 3 characters of ASCII.
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Ripple recommends using [ISO 4217](http://www.xe.com/iso4217.php) codes, or popular pseudo-ISO 4217 codes such as "BTC". However, any combination of the following characters is permitted: all uppercase and lowercase letters, digits, as well as the symbols `?`, `!`, `@`, `#`, `$`, `%`, `^`, `&`, `*`, `<`, `>`, `(`, `)`, `{`, `}`, `[`, `]`, and <code>|</code>. The currency code `XRP` (all-uppercase) is reserved for XRP and cannot be used by issued currencies.
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4. The next 40 bits are reserved and should be all `0`'s.
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Usually, XRP amounts are not specified with currency codes. In the rare case that a field specifies a currency code for XRP, the currency code's binary format is all zeroes.
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### Nonstandard Currency Codes
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You can also issue currency of other types by using a 160-bit (40-character) hexadecimal string such as `015841551A748AD2C1F76FF6ECB0CCCD00000000` as the currency code. To prevent this from being treated as a different currency code type, the first 8 bits MUST NOT be `0x00`.
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**Deprecated:** Some previous versions of [ripple-lib](https://github.com/ripple/ripple-lib) supported an "interest-bearing" or "demurraging" currency code type. These currencies have the first 8 bits `0x01`. Demurraging / interest-bearing currencies are no longer supported, but you may encounter them in ledger data. For more information, see [Demurrage](demurrage.html).
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