multidim-array-index.js

var staticProps = require('static-props')

var pkg = require('./package.json')

/**
 * Prepend package name to error message
 */

function msg (str) {
  return pkg.name + ': ' + str
}

var error = {}

staticProps(error)({
  outOfBoundIndex: msg('Index exceeds its bound')
})

/**
 * Maps multidimensional array indices to monodimensional array index
 *
 * Given
 *
 * dimensions d_1, d_2, d_3 .. d_n
 * and
 * indices i_1, i_2, i_3 .. i_n
 *
 * index is computed by formula
 * index = i_n + i_(n-1) * d_n + i_(n-2) * d_n * d_(n-1) + ... + i_2 * d_n * d_(n-1) * ... * d_3 + i_1 * d_n * ... * d_2
 *
 * @param {Array} dimensions
 * @param {Array} indices
 * @returns {Number} index
 */

function multiDimArrayIndex (dimensions, indices) {
  // Check that indices fit inside dimensions shape.
  for (var i = 0; i < dimensions.length; i++) {
    if (indices[i] > dimensions[i]) {
      throw new TypeError(error.outOfBoundIndex)
    }
  }

  var order = dimensions.length

  // Handle order 1
  if (order === 1) return indices[0]

  //* index = i_n + i_(n-1) * d_n + i_(n-2) * d_n * d_(n-1) + ... + i_2 * d_n * d_(n-1) * ... * d_3 + i_1 * d_n * ... * d_2
  var n = order - 1
  var factor = dimensions[n] // d_n
  var index = indices[n] + factor * indices[n - 1] // i_n + i_(n-1) * d_n

  for (var j = 2; j < order; j++) {
    factor *= dimensions[n - j]

    index += factor * indices[n - j]
  }

  return index
}

staticProps(multiDimArrayIndex)({ error: error })

module.exports = multiDimArrayIndex