--- /dev/null
+'use strict';
+
+var toPosInt = require('../../number/to-pos-integer')
+ , callable = require('../../object/valid-callable')
+ , value = require('../../object/valid-value')
+
+ , floor = Math.floor;
+
+module.exports = function (compareFn) {
+ var length, low, high, middle;
+
+ value(this);
+ callable(compareFn);
+
+ length = toPosInt(this.length);
+ low = 0;
+ high = length - 1;
+
+ while (low <= high) {
+ middle = floor((low + high) / 2);
+ if (compareFn(this[middle]) < 0) high = middle - 1;
+ else low = middle + 1;
+ }
+
+ if (high < 0) return 0;
+ if (high >= length) return length - 1;
+ return high;
+};