mathjs/lib/function/algebra/solver/lsolve.js

Math.js is an extensive math library for JavaScript and Node.js. It features a flexible expression parser with support for symbolic computation, comes with a large set of built-in functions and constants, and offers an integrated solution to work with dif

'use strict';

function factory(type, config, load, typed) {
  var matrix = load(require('../../../type/matrix/function/matrix'));
  var divideScalar = load(require('../../arithmetic/divideScalar'));
  var multiplyScalar = load(require('../../arithmetic/multiplyScalar'));
  var subtract = load(require('../../arithmetic/subtract'));
  var equalScalar = load(require('../../relational/equalScalar'));
  var solveValidation = load(require('./utils/solveValidation'));
  var DenseMatrix = type.DenseMatrix;
  /**
   * Solves the linear equation system by forwards substitution. Matrix must be a lower triangular matrix.
   *
   * `L * x = b`
   *
   * Syntax:
   *
   *    math.lsolve(L, b)
   *
   * Examples:
   *
   *    const a = [[-2, 3], [2, 1]]
   *    const b = [11, 9]
   *    const x = lsolve(a, b)  // [[-5.5], [20]]
   *
   * See also:
   *
   *    lup, slu, usolve, lusolve
   *
   * @param {Matrix, Array} L       A N x N matrix or array (L)
   * @param {Matrix, Array} b       A column vector with the b values
   *
   * @return {DenseMatrix | Array}  A column vector with the linear system solution (x)
   */

  var lsolve = typed('lsolve', {
    'SparseMatrix, Array | Matrix': function SparseMatrixArrayMatrix(m, b) {
      // process matrix
      return _sparseForwardSubstitution(m, b);
    },
    'DenseMatrix, Array | Matrix': function DenseMatrixArrayMatrix(m, b) {
      // process matrix
      return _denseForwardSubstitution(m, b);
    },
    'Array, Array | Matrix': function ArrayArrayMatrix(a, b) {
      // create dense matrix from array
      var m = matrix(a); // use matrix implementation

      var r = _denseForwardSubstitution(m, b); // result


      return r.valueOf();
    }
  });

  function _denseForwardSubstitution(m, b) {
    // validate matrix and vector, return copy of column vector b
    b = solveValidation(m, b, true); // column vector data

    var bdata = b._data; // rows & columns

    var rows = m._size[0];
    var columns = m._size[1]; // result

    var x = []; // data

    var data = m._data; // forward solve m * x = b, loop columns

    for (var j = 0; j < columns; j++) {
      // b[j]
      var bj = bdata[j][0] || 0; // x[j]

      var xj = void 0; // forward substitution (outer product) avoids inner looping when bj === 0

      if (!equalScalar(bj, 0)) {
        // value @ [j, j]
        var vjj = data[j][j]; // check vjj

        if (equalScalar(vjj, 0)) {
          // system cannot be solved
          throw new Error('Linear system cannot be solved since matrix is singular');
        } // calculate xj


        xj = divideScalar(bj, vjj); // loop rows

        for (var i = j + 1; i < rows; i++) {
          // update copy of b
          bdata[i] = [subtract(bdata[i][0] || 0, multiplyScalar(xj, data[i][j]))];
        }
      } else {
        // zero @ j
        xj = 0;
      } // update x


      x[j] = [xj];
    } // return vector


    return new DenseMatrix({
      data: x,
      size: [rows, 1]
    });
  }

  function _sparseForwardSubstitution(m, b) {
    // validate matrix and vector, return copy of column vector b
    b = solveValidation(m, b, true); // column vector data

    var bdata = b._data; // rows & columns

    var rows = m._size[0];
    var columns = m._size[1]; // matrix arrays

    var values = m._values;
    var index = m._index;
    var ptr = m._ptr; // vars

    var i, k; // result

    var x = []; // forward solve m * x = b, loop columns

    for (var j = 0; j < columns; j++) {
      // b[j]
      var bj = bdata[j][0] || 0; // forward substitution (outer product) avoids inner looping when bj === 0

      if (!equalScalar(bj, 0)) {
        // value @ [j, j]
        var vjj = 0; // lower triangular matrix values & index (column j)

        var jvalues = [];
        var jindex = []; // last index in column

        var l = ptr[j + 1]; // values in column, find value @ [j, j]

        for (k = ptr[j]; k < l; k++) {
          // row
          i = index[k]; // check row (rows are not sorted!)

          if (i === j) {
            // update vjj
            vjj = values[k];
          } else if (i > j) {
            // store lower triangular
            jvalues.push(values[k]);
            jindex.push(i);
          }
        } // at this point we must have a value @ [j, j]


        if (equalScalar(vjj, 0)) {
          // system cannot be solved, there is no value @ [j, j]
          throw new Error('Linear system cannot be solved since matrix is singular');
        } // calculate xj


        var xj = divideScalar(bj, vjj); // loop lower triangular

        for (k = 0, l = jindex.length; k < l; k++) {
          // row
          i = jindex[k]; // update copy of b

          bdata[i] = [subtract(bdata[i][0] || 0, multiplyScalar(xj, jvalues[k]))];
        } // update x


        x[j] = [xj];
      } else {
        // update x
        x[j] = [0];
      }
    } // return vector


    return new DenseMatrix({
      data: x,
      size: [rows, 1]
    });
  }

  return lsolve;
}

exports.name = 'lsolve';
exports.factory = factory;