skratchdot/array-any-index

'use strict';

var getType = require('get-object-type');
var value = require('d3-interpolate').value;

/**
 * [exports description]
 * @param  {Array} arr        The source array
 * @param  {Number} index    The index to get an interpolated value from. Negative
 *                        and floats are accepted.
 * @return {any}                    If the index is an integer, we return the item in the
 *                          array. If the index is a negative integer, we travel
 *                          back through the array to get a value. If the index
 *                          is a float, we use linear interpolation to find an
 *                          acceptable value.
 */
var arrayAnyIndex = function (arr, index) {
    var abs;
    var pos;
    var lo;
    var hi;
    var fractional;
    if (getType(arr).toLowerCase().indexOf('array') === -1) {
        throw new Error('A valid array must be used.');
    }
    if (arr.length === 0) {
        throw new Error('The array length must be positive.');
    }
    if (!Number.isFinite(index)) {
        throw new Error('The index must be a finite number.');
    }
    abs = Math.abs(index);
    pos = abs % arr.length;
    fractional = abs % 1;
    if (fractional === 0) {
        if (index < 0 && pos > 0) {
            pos = arr.length - pos;
        }
        return arr[pos];
    } else {
        lo = arrayAnyIndex(arr, Math.floor(index));
        hi = arrayAnyIndex(arr, Math.ceil(index));
        return value(lo, hi)(index < 0 ? 1 - fractional : fractional);
    }
};

module.exports = exports = arrayAnyIndex;