Thermostat

> Track an infinite curve of values given their inflection points. Or just
> program a thermostat.

Thermostat represents an infinite function, ƒ(d) = ?, where d is a Date or
Number, and ƒ(d) returns a Number, interpolating between all of the points
previously added to that function. As I'm not really a math person (honest),
I'll show you what I mean with an example.

Imagine the following curve repeats infinitely (each A is the same as the last):

``
1 C---D
/ \
0 A---B A'--
`

If you query a random point on this curve, it should give you a value between
0 and 1. That said, Thermostat (in an attempt to represent this curve as a
function) only cares about A, B, C, and D (and how they repeat).

API

As usual, Thermostat can be included like so:

`
var thermostat = require('thermostat')
`

$3

The simple lerp implementation core to the inner-workings of Thermostat,
provided for convenience. p1 and p2 are Points (Objects with x and y
Numbers), returning the Number result of linearly interpolating between those
two points, to x.

$3

Returns a new Curve instance with the provided options Object. The available
options are:

- period: _Optional._ The period of the curve. Set to 0 to
disable repetition. Defaults to the number of milliseconds in one week.
- points: _Optional._ An Array of { "x": Number|Date|String, "y": Number }
pairs, each of which represents a single point on the curve at x with the
value y. Defaults to the empty set which, while valid, isn't very
interesting: each request returns 0.

$3

Returns the Number value of the curve at x, represented as a Date, Number, or
String.

$3

All x values can be either Dates, Numbers, or Strings, treated as follows:

- Number: Used as-is.
- Date: Coerced to a Number as defined by Date.valueOf()
- String`: Parsed as a Date, then coerced to a Number likewise.

[lerp]: https://en.wikipedia.org/wiki/Linear_interpolation
[period]: https://simple.wikipedia.org/wiki/Periodic_function
[valueOf]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/valueOf