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Returns the length of the longest sequence of characters that appears in both inputs in the same order, with gaps allowed.

public static int SubsequenceLength(ReadOnlySpan<char> a, ReadOnlySpan<char> b, TextElement element = TextElement.Utf16Unit)
public static int SubsequenceLength<T>(ReadOnlySpan<T> a, ReadOnlySpan<T> b) where T : IEquatable<T>

Parameters — a and b are the two strings to compare. element says what counts as one character: TextElement.Utf16Unit by default, or TextElement.CodePoint to count an emoji once. The second overload compares any two spans of an IEquatable<T>, which is how you get the longest common run of words rather than of characters.

Returns — int, at most the length of the shorter input, and 0 when either is empty.

Example — ittn survives in both, out of six and seven characters.

using Lodestar.Text.Distances;

int n = Lcs.SubsequenceLength("kitten", "sitting");   // => 4

Remarks — this is the classic LCS, and it is the thing to call when you want the shared material itself rather than a score derived from it: how much of a document survived an edit, how much of a template a candidate string fills in. Indel.Distance is precisely len(a) + len(b) - 2 × SubsequenceLength(a, b), so if you need both, call this and do the arithmetic rather than paying for two passes.

The trap is that it is a raw length with no upper bound of its own, and the number alone says nothing: a shared run of 4 is most of a six-letter word and nothing at all in a paragraph. It is also not what SubstringLength returns — this one allows gaps, and the two answers differ on almost every real pair.

Applies to — net10.0, netstandard2.0.

See also — Lcs.SubstringLength, Indel.Distance, RatcliffObershelp.Similarity, the Python equivalence table.