coin: a radiation-safe Boolean

A Coin is data type for representing Boolean values that is resistant to bit flips. Prefer Coin to bool in safety-critical environments with long-lived variables, such as global variables.

A standard bool is truth-biased, because false matches a single bit pattern (all zeros). A single bit flip invalidates the value. Coin counts the number of bits to determine its truth value. When 4 or more bits are 1, the value is interpreted as true. Coin can tolerate 3 bit flips per byte before an incorrect value is returned.