Provide values for ranges

rangemap.js

var Rangemap = function(points, values) {
	this.points = points || [];
	this.values = values || [];
};

Rangemap.prototype = {
	"equalInRange": true,
	"compare": function(a, b) {
		if (a === b) {
			return this.equalInRange;
		}
		if (a.constructor === b.constructor === String) {
			// for text ranges
			var arr = [a, b];
			if (this.comparator instanceof Function) {
				// allow a custom comparator
				arr.sort(this.comparator);
			} else {
				arr.sort();
			}
			return arr[0] === a;
		} else {
			// for numbered ranges
			if (this.comparator instanceof Function) {
				return this.comparator(a, b);
			} else {
				return a < b;
			}
		}
	},
	"valueForIndex": function(index) {
		var indexOffset = 0,
			valueOffset = 0;
		// if points.length > values.length, assume a minimum value was specified
		if (this.points.length > this.values.length) {
			if (index < this.points[0]) {
				throw 'the specified index was below the minimum range';
				// return undefined;
			}
			indexOffset++;
		}
		while (indexOffset < this.points.length && valueOffset < this.values.length) {
			if (this.compare(index, this.points[indexOffset])) {
				return this.values[valueOffset];
			}
			indexOffset++;
			valueOffset++;
		}
		// if values.length > points.length, assume a global upper value
		if (this.values.length > valueOffset) {
			return this.values[valueOffset];
		}
		return undefined;
	}
};

rangemap.spec.js

describe('de.Rangemap', function() {

	it('should return the correct value for an index in a range', function() {
		var range = new Rangemap([4, 8, 10], ['a', 'b', 'c']);
		expect(range.valueForIndex(3)).toBe('a');
		expect(range.valueForIndex(4)).toBe('a');
		expect(range.valueForIndex(6)).toBe('b');
		expect(range.valueForIndex(8)).toBe('b');
		expect(range.valueForIndex(9)).toBe('c');
	});

	it('should assume a minimum value when more values provided', function() {
		var range = new Rangemap([0, 4, 8, 10], ['a', 'b', 'c']);
		expect(function(){range.valueForIndex(-1);}).toThrow();
		expect(range.valueForIndex(4)).toBe('a');
		expect(range.valueForIndex(6)).toBe('b');
		expect(range.valueForIndex(8)).toBe('b');
		expect(range.valueForIndex(9)).toBe('c');
	});

	it('should assume a global upper value when fewer values provided', function() {
		var range = new Rangemap([4, 8, 10], ['a', 'b', 'c', 'd']);
		expect(range.valueForIndex(3)).toBe('a');
		expect(range.valueForIndex(6)).toBe('b');
		expect(range.valueForIndex(9)).toBe('c');
		expect(range.valueForIndex(11)).toBe('d');
	});

	it('should work with strings', function() {
		var range = new Rangemap(['clown', 'pony', 'snake'], [1, 2, 3, 4]);
		expect(range.valueForIndex('animal')).toBe(1);
		expect(range.valueForIndex('cat')).toBe(1);
		expect(range.valueForIndex('dog')).toBe(2);
		expect(range.valueForIndex('lion')).toBe(2);
		expect(range.valueForIndex('rabbit')).toBe(3);
		expect(range.valueForIndex('zebra')).toBe(4);
	});
});

Example usage:

var testScores = new Rangemap([60, 70, 80, 90], ['F', 'D', 'C', 'B', 'A']);
// johnny scored 85 on his test. what grade does he get?
testScores.valueForIndex(85); // returns "B"