[Subversion Wiki] Update of "FS2/Theory" by brane

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Comment:
Begin writing up configuration space theory.

New page:
= Configuration Spaces and Histories =

Before we dive into versioned filesystem design and implementation details, let's first take a look at the concepts we're talking about. We'll begin by discussing what a configuration management system actually operates on.

''N.B.'': Although the following text uses mathematical concepts and notation, it's not intended to be a rigorous dissertation on configuration theory. We'll often make statements and draw conclusions without providing proof. Therefore, dear reader, you can either trust we're right, or do your own homework to prove (or disprove) our assertions. We will, however, try to conscientiously flag⚑ all leaps of faith in the text.

'''Definition:''' A ''data configuration'' is a unique pattern of data that can be represented as exactly one distinct sequence of binary digits.
 * The representation of the ''empty configuration'' {} is the zero-length sequence.
 * The ''size'' , │c│, of a configuration c is the length of its representative sequence of binary digits.

'''Definition:''' A ''data configuration space'' is a [[http://en.wikipedia.org/wiki/Metric_space|metric space]] (𝔹, 𝒹) where:
 * 𝔹 is the set of all data configurations;
 * 𝒹: 𝔹 × 𝔹 ⟶ ℝ is a metric on 𝔹.
 
'''Definition:''' A ''configuration history'' is a sequence of elements of the data configuration space.

== Defining the Metric ==

We'll now show that we can indeed define a metric for 𝔹. One example is the ''[[http://en.wikipedia.org/wiki/Levenshtein_distance|Levenshtein distance]'',

 𝓁: 𝔹 × 𝔹 ⟶ ℕ^0^

applied to the binary-digit sequence representation of data configurations. It is easy to show⚑ that 𝓁 has all the required properties of a metric. In addition, it has the following properties:
 * The minimum distance between any two distinct configurations is⚑ 1.
 * The maximum distance between any two distinct configurations is⚑ the greater of their sizes: ∀a,b ∈ 𝔹: 𝓁(a, b) ≤ max(│a│, │b│) .
 * And it follows⚑ from this that: ∀c ∈ 𝔹: 𝓁({}, c) = │c│ .