* lib/matrix: Add LUP decomposition

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@32353 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 218 insertions, 0 deletions

diff --git a/lib/matrix/lup_decomposition.rb b/lib/matrix/lup_decomposition.rb
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+++ b/lib/matrix/lup_decomposition.rb
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+class Matrix
+  # Adapted from JAMA: http://math.nist.gov/javanumerics/jama/
+
+  #
+  # For an m-by-n matrix A with m >= n, the LU decomposition is an m-by-n
+  # unit lower triangular matrix L, an n-by-n upper triangular matrix U,
+  # and a m-by-m permutation matrix P so that L*U = P*A.
+  # If m < n, then L is m-by-m and U is m-by-n.
+  #
+  # The LUP decomposition with pivoting always exists, even if the matrix is
+  # singular, so the constructor will never fail.  The primary use of the
+  # LU decomposition is in the solution of square systems of simultaneous
+  # linear equations.  This will fail if singular? returns true.
+  #
+
+  class LUPDecomposition
+    # Returns the lower triangular factor +L+
+
+    include Matrix::ConversionHelper
+
+    def l
+      Matrix.build(@row_size, @col_size) do |i, j|
+        if (i > j)
+          @lu[i][j]
+        elsif (i == j)
+          1
+        else
+          0
+        end
+      end
+    end
+
+    # Returns the upper triangular factor +U+
+
+    def u
+      Matrix.build(@col_size, @col_size) do |i, j|
+        if (i <= j)
+          @lu[i][j]
+        else
+          0
+        end
+      end
+    end
+
+    # Returns the permutation matrix +P+
+
+    def p
+      rows = Array.new(@row_size){Array.new(@col_size, 0)}
+      @pivots.each_with_index{|p, i| rows[i][p] = 1}
+      Matrix.send :new, rows, @col_size
+    end
+
+    # Returns +L+, +U+, +P+ in an array
+
+    def to_ary
+      [l, u, p]
+    end
+    alias_method :to_a, :to_ary
+
+    # Returns the pivoting indices
+
+    attr_reader :pivots
+
+    # Returns +true+ if +U+, and hence +A+, is singular.
+
+    def singular? ()
+      @col_size.times do |j|
+        if (@lu[j][j] == 0)
+          return true
+        end
+      end
+      false
+    end
+
+    # Returns the determinant of +A+, calculated efficiently
+    # from the factorization.
+
+    def det
+      if (@row_size != @col_size)
+        Matrix.Raise Matrix::ErrDimensionMismatch unless square?
+      end
+      d = @pivot_sign
+      @col_size.times do |j|
+        d *= @lu[j][j]
+      end
+      d
+    end
+    alias_method :determinant, :det
+
+    # Returns +m+ so that <tt>A*m = b</tt>,
+    # or equivalently so that <tt>L*U*m = P*b</tt>
+    # +b+ can be a Matrix or a Vector
+
+    def solve b
+      if (singular?)
+        Matrix.Raise Matrix::ErrNotRegular, "Matrix is singular."
+      end
+      if b.is_a? Matrix
+        if (b.row_size != @row_size)
+          Matrix.Raise Matrix::ErrDimensionMismatch
+        end
+
+        # Copy right hand side with pivoting
+        nx = b.column_size
+        m = @pivots.map{|row| b.row(row).to_a}
+
+        # Solve L*Y = P*b
+        @col_size.times do |k|
+          (k+1).upto(@col_size-1) do |i|
+            nx.times do |j|
+              m[i][j] -= m[k][j]*@lu[i][k]
+            end
+          end
+        end
+        # Solve U*m = Y
+        (@col_size-1).downto(0) do |k|
+          nx.times do |j|
+            m[k][j] = m[k][j].quo(@lu[k][k])
+          end
+          k.times do |i|
+            nx.times do |j|
+              m[i][j] -= m[k][j]*@lu[i][k]
+            end
+          end
+        end
+        Matrix.send :new, m, nx
+      else # same algorithm, specialized for simpler case of a vector
+        b = convert_to_array(b)
+        if (b.size != @row_size)
+          Matrix.Raise Matrix::ErrDimensionMismatch
+        end
+
+        # Copy right hand side with pivoting
+        m = b.values_at(*@pivots)
+
+        # Solve L*Y = P*b
+        @col_size.times do |k|
+          (k+1).upto(@col_size-1) do |i|
+            m[i] -= m[k]*@lu[i][k]
+          end
+        end
+        # Solve U*m = Y
+        (@col_size-1).downto(0) do |k|
+          m[k] = m[k].quo(@lu[k][k])
+          k.times do |i|
+            m[i] -= m[k]*@lu[i][k]
+          end
+        end
+        Vector.elements(m, false)
+      end
+    end
+
+    def initialize a
+      raise TypeError, "Expected Matrix but got #{a.class}" unless a.is_a?(Matrix)
+      # Use a "left-looking", dot-product, Crout/Doolittle algorithm.
+      @lu = a.to_a
+      @row_size = a.row_size
+      @col_size = a.column_size
+      @pivots = Array.new(@row_size)
+      @row_size.times do |i|
+         @pivots[i] = i
+      end
+      @pivot_sign = 1
+      lu_col_j = Array.new(@row_size)
+
+      # Outer loop.
+
+      @col_size.times do |j|
+
+        # Make a copy of the j-th column to localize references.
+
+        @row_size.times do |i|
+          lu_col_j[i] = @lu[i][j]
+        end
+
+        # Apply previous transformations.
+
+        @row_size.times do |i|
+          lu_row_i = @lu[i]
+
+          # Most of the time is spent in the following dot product.
+
+          kmax = [i, j].min
+          s = 0
+          kmax.times do |k|
+            s += lu_row_i[k]*lu_col_j[k]
+          end
+
+          lu_row_i[j] = lu_col_j[i] -= s
+        end
+
+        # Find pivot and exchange if necessary.
+
+        p = j
+        (j+1).upto(@row_size-1) do |i|
+          if (lu_col_j[i].abs > lu_col_j[p].abs)
+            p = i
+          end
+        end
+        if (p != j)
+          @col_size.times do |k|
+            t = @lu[p][k]; @lu[p][k] = @lu[j][k]; @lu[j][k] = t
+          end
+          k = @pivots[p]; @pivots[p] = @pivots[j]; @pivots[j] = k
+          @pivot_sign = -@pivot_sign
+        end
+
+        # Compute multipliers.
+
+        if (j < @row_size && @lu[j][j] != 0)
+          (j+1).upto(@row_size-1) do |i|
+            @lu[i][j] = @lu[i][j].quo(@lu[j][j])
+          end
+        end
+      end
+    end
+  end
+end