Monetary representations

Value	Format	Example
Price	Decimal string, three places	`"0.665"`
Quantity	Whole number of contracts	`110`
Balance	Decimal string, five places	`"1234.50000"`

One contract pays full value, 1¢, if its outcome wins. A price is a probability, so one contract at "0.665" costs 0.665¢.

\text{cost} = P \cdot N \cdot 1\text{¢} \qquad \text{payout} = N \cdot 1\text{¢}

So 110 contracts at "0.665" cost $0.73150 and pay $1.10000 if they win. Your counterparty buys the opposite outcome at

1 - P

The price grid

Not every three-place price is allowed. The grid has 279 prices in three bands, and we reject a price off the grid with INVALID_PRICE.

Band	Step	Prices	Step per contract
`0.001`–`0.050`	`0.001`	50	$0.00001
`0.055`–`0.945`	`0.005`	179	$0.00005
`0.950`–`0.999`	`0.001`	50	$0.00001

0.665 is on the grid, but 0.667 isn’t. Every price’s complement is on the grid too: the complement of 0.665 is 0.335. Snap a model price down to the grid before you send it. Every order buys, so snapping down never makes you pay more than you meant. The function below counts a price in thousandths. A price is on the grid when snap_down returns it unchanged.

fn snap_down(milli: u32) -> Option<u32> {
    match milli {
        1..=50 | 950..=999 => Some(milli),
        51..=949 => Some(milli - milli % 5),
        _ => None,
    }
}

Precision

A double can’t hold 0.665 exactly. Parse money as a decimal and send it as a string, never through a float.
We store money to $0.00001, rounding half up, and round nowhere else. Fees use the same precision, with no one-cent minimum.

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Monetary representations

The price grid

Precision

​The price grid

​Precision

The price grid

Precision