Text lcs substringlength

Lcs.SubstringLength

Returns the length of the longest contiguous run of characters that appears in both inputs.

public static int SubstringLength(ReadOnlySpan<char> a, ReadOnlySpan<char> b, TextElement element = TextElement.Utf16Unit)
public static int SubstringLength<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. The second overload compares any two spans of an IEquatable<T>.

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

Example — the same pair as above, where only itt is unbroken.

using Lodestar.Text.Distances;

int n = Lcs.SubstringLength("kitten", "sitting");   // => 3

Remarks — contiguity is the whole difference from SubsequenceLength, and it is what makes this the right call for detecting quoted or copied text, shared identifiers, or a common stem. It matches the size that Python's difflib.SequenceMatcher.find_longest_match reports. Only the size: this returns no position, and it swaps its two operands internally when b is the longer, so which of several equally long runs was measured is not observable from here. If you need the run's location, difflib's tie-break is the one RatcliffObershelp applies.

The trap is how brittle contiguity is. A single character inserted in the middle of an otherwise identical string halves this number, while SubsequenceLength barely moves. If you are measuring "how much do these two share" rather than "what is the longest unbroken quote", you almost certainly want the subsequence.

Applies to — net10.0, netstandard2.0.

See also — Lcs.SubsequenceLength, RatcliffObershelp.Similarity, the Python equivalence table.