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Computes the Sørensen-Dice similarity of two inputs: shared q-grams counted twice, over the two bag sizes added.

public static double Similarity(ReadOnlySpan<char> a, ReadOnlySpan<char> b, int qval = 1, TextElement element = TextElement.Utf16Unit)

Parameters — a and b are the two inputs to compare; a string converts implicitly, and swapping them does not change the answer. qval is how many characters make one gram, 1 by default; it must be at least 1. element says what counts as one character: TextElement.Utf16Unit by default, or TextElement.CodePoint to match Python outside the Basic Multilingual Plane.

Returns — double in [0, 1]. 1 when the two bags hold exactly the same grams with the same multiplicities, 0 when they share none.

Exceptions — ArgumentOutOfRangeException when qval is below 1.

Example — the pair from the index, and the two extremes.

using Lodestar.Text.Similarity;

double related = SorensenDice.Similarity("apple", "pineapple");  // => 0.7142…
double same = SorensenDice.Similarity("abc", "abc");  // => 1
double nothing = SorensenDice.Similarity("abc", "xyz");  // => 0

Remarks — the numerator counts every shared gram twice, which is the whole of the difference from Jaccard.Similarity: the same pair reads 0.7142… here and 0.5555… there. Because Dice = 2·Jaccard / (1 + Jaccard) is increasing, the two never disagree about which of two candidates is the better match — only about by how much.

Two empty inputs give 1; one empty input against a non-empty one gives 0. An input shorter than qval holds no gram and counts as empty, except that two such inputs score 1 only when they are equal — "a" against "b" at qval: 2 gives 0, where textdistance divides by zero.

Applies to — net10.0, netstandard2.0.

See also — Jaccard.Similarity, Tversky.Similarity, the Python equivalence table.