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Class IdCheckCharacter

Namespace: AdCodicem.ValueObjects.Identifiers
Assembly: AdCodicem.ValueObjects.Identifiers.dll

The trailing check character of an entity identifier.

public static class IdCheckCharacter

Inheritance​

object ← IdCheckCharacter

Inherited Members​

object.Equals(object?), object.Equals(object?, object?), object.GetHashCode(), object.GetType(), object.MemberwiseClone(), object.ReferenceEquals(object?, object?), object.ToString()

Remarks​

The character is (seed(prefix) + Σᵢ wᵢ · vᵢ) mod 32, where vᵢ is the Crockford value of the i-th data character of the body and wᵢ = 2·(i mod 16) + 1. Its purpose is to turn a mistyped or truncated identifier into a rejection at the boundary — offline, without a database round trip — rather than into a lookup that misses, or worse, one that hits something else.

What it guarantees, stated exactly, because a checksum that promises more than it delivers is worse than none at all:

  • every single-character substitution in the body is detected: the weights are odd, hence invertible modulo 32, and a non-zero difference of Crockford values can never be congruent to zero modulo 32;
  • every adjacent transposition is detected unless the two characters' values differ by exactly 16 — the weight difference between adjacent positions is congruent to 2 modulo 32 everywhere, the wrap included;
  • the prefix enters through the seed, so the same body under two different prefixes yields a different check character for 31 prefixes out of 32. A body copied between two identifier types is therefore caught even by a validator that does not yet know which prefix to expect, which is what AnyEntityId needs before it has resolved anything;
  • random corruption slips through with probability 1/32. That is the information-theoretic limit of one check character over a 32-symbol alphabet, and no scheme does better.

Methods​

Compute(ReadOnlySpan<char>, ReadOnlySpan<char>)​

Computes the check character of a body.

public static char Compute(ReadOnlySpan<char> prefix, ReadOnlySpan<char> bodyData)

Parameters​

prefix ReadOnlySpan<char>

Declared prefix of the identifier type, without its trailing separator.

bodyData ReadOnlySpan<char>

The data characters of the body — the time bucket and the random part — with the check character excluded. Every character must decode; a character that does not contributes nothing, which is harmless because validation refuses it before the check character is ever consulted.

Returns​

char

The check character.