net.maizegenetics.analysis.gbs.neobio

Class NeedlemanWunsch

PairwiseAlignmentAlgorithm

net.maizegenetics.analysis.gbs.neobio.NeedlemanWunsch

public class NeedlemanWunsch
extends PairwiseAlignmentAlgorithm

This class implements the classic global alignment algorithm (with linear gap penalty function) due to S.B.Needleman and C.D.Wunsch (1970).

It is based on a dynamic programming approach. The idea consists of, given two sequences A and B of sizes n and m, respectively, building an (n+1 x m+1) matrix M that contains the similarity of prefixes of A and B. Every position M[i,j] in the matrix holds the score between the subsequences A[1..i] and B[1..j]. The first row and column represent alignments with spaces.

Starting from row 0, column 0, the algorithm computes each position M[i,j] with the following recurrence:

  M[0,0] = 0
  M[i,j] = max { M[i,j-1]   + scoreInsertion (B[j]),
                 M[i-1,j-1] + scoreSubstitution (A[i], B[j]),
                 M[i-1,j]   + scoreDeletion(A[i])             }
  

In the end, the value at the last position (last row, last column) will contain the similarity between the two sequences. This part of the algorithm is accomplished by the net.maizegenetics.analysis.gbs.neobio.NeedlemanWunsch$computeMatrix() method. It has quadratic space complexity since it needs to keep an (n+1 x m+1) matrix in memory. And since the work of computing each cell is constant, it also has quadratic time complexity.

After the matrix has been computed, the alignment can be retrieved by tracing a path back in the matrix from the last position to the first. This step is performed by the net.maizegenetics.analysis.gbs.neobio.NeedlemanWunsch$buildOptimalAlignment() method, and since the path can be roughly as long as (m + n), this method has O(n) time complexity.

If the similarity value only is needed (and not the alignment itself), it is easy to reduce the space requirement to O(n) by keeping just the last row or column in memory. This is precisely what is done by the net.maizegenetics.analysis.gbs.neobio.NeedlemanWunsch$computeScore() method. Note that it still requires O(n2) time.

For a more efficient approach to the global alignment problem, see the class CrochemoreLandauZivUkelson algorithm. For local alignment, see the class SmithWaterman algorithm.

See Also:

net.maizegenetics.analysis.gbs.neobio.NeedlemanWunsch$computeMatrix(), net.maizegenetics.analysis.gbs.neobio.NeedlemanWunsch$buildOptimalAlignment(), net.maizegenetics.analysis.gbs.neobio.NeedlemanWunsch$computeScore(), class CrochemoreLandauZivUkelson, class SmithWaterman, class SmithWaterman, class CrochemoreLandauZivUkelson, class CrochemoreLandauZivUkelsonLocalAlignment, class CrochemoreLandauZivUkelsonGlobalAlignment

Field Detail

seq1

protected CharSequence seq1

The first sequence of an alignment.

seq2

protected CharSequence seq2

The second sequence of an alignment.

matrix

protected kotlin.Array[] matrix

The dynamic programming matrix. Each position (i, j) represents the best score between the firsts i characters of seq1 and j characters of seq2.

Method Detail

loadSequencesInternal

protected void loadSequencesInternal(java.io.Reader input1,
                                     java.io.Reader input2)

Loads sequences into class CharSequence instances. In case of any error, an exception is raised by the constructor of CharSequence (please check the specification of that class for specific requirements).

Parameters:

input1 - Input for first sequence

input2 - Input for second sequence

Throws:

IOException - If an I/O error occurs when reading the sequences

InvalidSequenceException - If the sequences are not valid

See Also:

class CharSequence, class CharSequence

unloadSequencesInternal

protected void unloadSequencesInternal()

Frees pointers to loaded sequences and the dynamic programming matrix so that their data can be garbage collected.

computePairwiseAlignment

protected PairwiseAlignment computePairwiseAlignment()

Builds an optimal global alignment between the loaded sequences after computing the dynamic programming matrix. It calls the buildOptimalAlignment method after the computeMatrix method computes the dynamic programming matrix.

Returns:

org.jetbrains.dokka.NodeRenderContent@50b0719f: an optimal global alignment between the loaded sequences

Throws:

IncompatibleScoringSchemeException - If the scoring scheme is not compatible with the loaded sequences.

See Also:

#computeMatrix, #buildOptimalAlignment

computeMatrix

protected void computeMatrix()

Computes the dynamic programming matrix.

Throws:

IncompatibleScoringSchemeException - If the scoring scheme is not compatible with the loaded sequences.

buildOptimalAlignment

protected PairwiseAlignment buildOptimalAlignment()

Builds an optimal global alignment between the loaded sequences. Before it is executed, the dynamic programming matrix must already have been computed by the computeMatrix method.

Returns:

org.jetbrains.dokka.NodeRenderContent@5dbdb142: an optimal global alignment between the loaded sequences

Throws:

IncompatibleScoringSchemeException - If the scoring scheme is not compatible with the loaded sequences.

See Also:

#computeMatrix

computeScore

protected int computeScore()

Computes the score of the best global alignment between the two sequences using the scoring scheme previously set. This method calculates the similarity value only (doesn't build the whole matrix so the alignment cannot be recovered, however it has the advantage of requiring O(n) space only).

Returns:

org.jetbrains.dokka.NodeRenderContent@20f4aa35: score of the best global alignment between the loaded sequences

Throws:

IncompatibleScoringSchemeException - If the scoring scheme is not compatible with the loaded sequences.