# Patch Theory, Take N

This page contains discussion on properties of a Darcs-like patch system where the order of the patches within the repository does not affect its state.

The discussion and the patch system presented here is based on David’s patch theory, GaneshPatchAlgebra and Marnix Klooster’s patch calculus.

## Concepts and Some Notation

First we define central concepts, repository and patch, and some notation for them.

A patch represents modification to the repository state. The patch P below modifies repository from state s to state e:

||||<:>‘’ P ’’||

Basic patches describe the change between the states. With Darcs, such patches include `addfile`, `rmfile`, `hunk` (remove and/or add text lines to a file), `adddir`, `rmdir`, `move` (change name of file or directory), `replace` and `binary` (remove and/or add binary content to a file).

When multiple patches are applied to a repository the end state of previous patch is always the beginning state of next patch:

||||<:>‘’ A B ’’||

The intermediate state can be shown with single dot, the beginning state with <> and end state with <\$>:

||||<:>‘’ <> A . B <\$>’’||

Sequence of patches can be shown also as {‘’ A B’’}:

||||<:>{‘’ A B’’}.||

A repository is a sequence of patches with a well-known start state. The state of repository refers to the end-state of last patch in the sequence, and defined by the effect of all the patches in the sequence.

## Patches and Identity

Definition 1. Each patch has identity. Basic patches with same identity have identical effect when applied to same state.

The identity of the patch P is shown as [P].

It is pretty clear from the definition that two repositories have same state if they both have one basic patch in them and both patches have same identity.

## Inverse

Each patch P has an inverse patch. A patch followed by its inverse results in the original state.

Definition 2. Inverse of patch P is ~P, where

||||<:>‘’ <> P ~P <>’’||

The right inverse of a patch is its left inverse:

||||‘’ ~P <> P ’’||

Inverse of inverse is patch itself.

||||<:>‘’ <> ~~P ’’||

Theorem 1. Inverse of sequence of two patches

||(1)||‘’ ~{ A . B } = { ~B . ~A }’’||

Proof:

|| ||‘’ <> { A . B } ~ { A . B } <> || ||<)> <=> || <> A B ~B ~A <>’’||

The simplest inverse of a basic patch is a basic patch. However, a sequence of patches can be a inverse of a basic patch, e.g., {BCD} is inverse of A below:

|| ||‘’ <> A { B . C . D } <>’’||

## Merge

Identical repositories with one patch in each are not very interesting. Let us now consider two repositories, one patch in each, where the patches are different.

Note that because our definition of repository is recursive, repositories with patches in identical order and identity have identical state. So we could have two almost identical repositories where only the last patch differ from each other.

We can combine the repositories (or branches they represent) in two different ways. We can take patch from repository S first and then patch from repository Q, or first patch from Q and then from S. Regardless of the order, we want to end up in the same state — the combined effect of S followed by P should equal to the combined effect of Q followed by the effect of R. If we want to take first patch from repository S, we have to modify the start state and effect of the patch from repository Q, and vice versa. Despite the modification the identity of the patches should not change, however.

Definition 3. Merge is an operation where two basic patches are combined together in unambiguous way. Merge of the pair of basic patches (S, Q) is the unique pair of patches (R, P) where the sequences ’’{S

• P}’’ and { Q . R } start from a common state, end up in a common state, and the identities of the patches S and R and the patches P and Q are same.

|| ||‘’ (R, P) = merge (S, Q) || ||<)> where || <> S . P <$\phantom{\rule{0.222em}{0ex}}*\mid \mid \mid \mid <\right)\phantom{\rule{0.222em}{0ex}}\mid \mid *<\phantom{\rule{0.222em}{0ex}}Q.R<$>|| || || [S] = [R] || || || [P] = [Q]’’||

Note that the definition is symmetric:

|| If||‘’ (S’, Q’) = merge (S, Q) || ||<)> then || (Q’, S’) = merge (Q, S) ’’||

A note worth of taking: patches may do incompatible changes, or their merge may be ambiguous. There might be multiple reasonable end states that the repository might end up. For instance, consider two patches, one adding “Chapter 1”, another “Section 1” as first line in the file. Should the “Chapter 1” or “Section 1” go first in the merged repository? According to our definition above, the patch system should be able to decide an unambiguous end state, regardless of the ordering of the patches.

### Conflict

Definition 4. If there is no unambiguous merge for the patch pair (S, Q) with a common start state, the patches are in conflict.

## Commute

Merge was defines as an unambiguous way to combine two repositories. What if we wanted to split a repository into two parts — in unambiguous way?

||||‘’ <> S P ’’||

and we aim at two repositories with one patch in each

||||‘’ <> S || || || <> P’ ’’||

where the patch ‘’P’ ’’ has same identity as patch P. As our goal is not permanent divorce but rather separation, we should be able to merge the repositories back together unambiguously:

||||‘’ <> P’ . S’ || || || <> S . P" ’’.||

We can fail finding the patch ‘’P’ ‘’ in two ways. Obviously, there might be no basic patch satisfying the requirement — for example, S adds some text to a file and P removes said text. Alternatively, there might be multiple candidates for’‘P’ ‘’ (or’’ S’ ’’ or *P“*). For example, S adds text”Section 1" as first line and P adds Chapter 1 as second line.

With all those unambiguities under our belt, let us proceed to next definition.

Definition 5. Commutation creates an unique reordering of two patches while keeping the identity of the patches, the start and end state of the sequence same. Pair of basic patches (S, P) commutes with pair of basic patches (Q, R) iff [S] = [R] and [P] = [Q] and the {‘’ Q . R } is the only possible pair of basic patches sharing same end state with patch sequence { S . P’’}.

|| If||‘’ <> S . P <$\phantom{\rule{0.222em}{0ex}}*\mid \mid \mid \mid \mid \mid *<\phantom{\rule{0.222em}{0ex}}Q.R<$> || || || [S] = [R] || || || [P] = [Q] || || || (Q, R) = commute (S, P) || || or || (Q, R) <–> (S, P)’’||

It is clear from the definition that || if||‘’ (Q, R) <–> (S, P) || ||<)> then|| (S, P) <–> (Q, R)’’.||

### Dependence

Definition 6. If the sequential patch pair (S , P) cannot be commuted, they depend on each other.

## Representing Conflicts

When there is is conflicting or ambiguous merge, the basic patches described above are not enough.

In order to handle and mark conflicts we introduce `merger` (or `conflictor` or `duplicate`) patches to our patch system. Below m-notation is used to represent the mergers. The end state of conflict is shown as <*>.

||||merge’’ (S, Q) = ( m(Q , S), m(S , Q) ) || ||<)> or ||<> S m(S, Q) <*> || ||<)> = ||<> Q m(Q, S) <*> ’’||

The identity of a merger patch is same as the identity of the rightmost patch in the pair, [m(X, Q)] is [Q]. The effect of the merger patch differs from the effect the basic patches with same identity, however.

What if the patches depend on each other and commute ends up in some kind of conflict:

||||‘’ ( P, Q ) <~~~> ( Q’, P’) ’’||

Luckily there is no need to introduce more patches, but we can do with merger:

Theorem 2. The sequence { P . Q } equals with sequence { m(~P, Q) . ~m(Q, ~P }.

||||‘’ ( P, Q ) <~~~> ( m(~P, Q), ~m(Q, ~P) )’’||

(A patch sequence equals with another if their start state, end state and the identity of their patches, regardless of their ordering, is are same.)

Proof. Consider two patch sequences

||||‘’ <> P Q || || || <> P ~P <>’’||

Clearly Q and ~P have same start state and they can be merged. After merging we have two sequences with same end state:

||||‘’ <> P Q m(Q, ~P) <*> || || || <> P ~P <> m(~P, Q) <*>’’||

From the definition of inverse we get

||||‘’ <> m(~P, Q) <*> ~m(Q, ~P) ’’||

QED.

The almost-commuted patch m(~P, Q) is in conflict. You might as well ask conflict with what? It is entirely undefined what happens if the repository is split into two branches (<> P and <> m(~P, Q) <*>) after the swap. So the “curly commute” or the swap operation is clearly not the same operation as the commute defined above. Nevertheless it is useful when reordering already conflicting patches, or reordering patches if the intention is not to split the repository.

## Ordering Does Not Matter

The rule #1 follows from Theorem 2: it is possible to freely reorder patches within repository without modifying its state:

||||<> A ||.|| B ||.|| C || ||<)> =||‘’<> B’ ||.|| A’ ||.|| C || ||<)> =||<> B’ ||.|| C’ ||.|| A" || ||<)> =||<> C" ||.|| B" ||.|| A" || ||<)> =||<> C" ||.|| A“’ ||.|| B”’ || ||<)> =||<> A“” ||.|| C“’ ||.|| B”’ || ||<)> =||<> A“” ||.|| B“” ||.|| C“” ’’||

We can also add patch and its inverse to the repository without changing its state:

||||<> A ||.|| B ||.|| C || . || ~C || ||||‘’<> C ||.|| A’ ||.|| B’ || . || ~C || ||||<> C ||.|| B" ||.|| A" || . || ~C ’’||

## Conflict and Dependence

What else we can say about conflicts? Clearly, conflict and dependence (as defined above) seem to have something in common.

Theorem 3. Conflicting patches depend on each other’s inverse.

Proof. Consider patch sequences where A and B are in conflict:

||||‘’ <> A m(A, B)’’ <*> ~m(~B, A) ~B <>|| || ||‘’ <> B m(B, A)’’ <*> ~m(~A, B) ~A <>||

Commute the pairs (m(A, B), ~m(~B, A)) and m(B, A) . ~m(~A, B):

||||‘’ <> A ~A <> B ~B <>’’ || || ||‘’ <> B ~B <> A ~A <>’’.||

According to our definition of dependence, B depends on ~A and A depends on ~B.

QED.

## State After Conflict

The merge of conflicting patches should end in an unambiguous end state. How to determine the end state? It is just a matter of definition, but the definition should be convenient. Darcs 1 uses following definition:

Definition 7(a). The conflict merge of two patches result in start state.

||||‘’ <> A m(A, B)’’ <> || || ||‘’ <> B m(B, A)’’ <> ||

According to the definition above, the effect of the merger patch m(A, B) is same as effect of ~A and its identity is same as the identity of B.

Darcs 2 modified the definition so that two identical patches are handled in different way:

Definition 7(b). The conflict merge of two identical patches result in end state of first patch.

||||‘’ <> A m(A, A) ’’||

## How the Conflicts are Stored in Repository

Darcs 1 stores the conflicting patches pretty much like the m-notation we use above. The patches involved are stored as separate text lines. For historical reasons the merger patches made by Darcs 1 has a version number associated with them.

For instance, merge of two branches where the `file` got renamed to `a` in the first branch and `b` in the second branch is represented as follows:

``````merger 0.0 (
move ./file ./a
move ./file ./b
)``````

### Complex Conflicts

A `merger` has always two patches within it. But conflicts don’t necessarily involve just two patches. If we apply three conflicting patches, A, B and C in the repository, we end up with something like

||||‘’ <> A m(A, B)’’ <*> m(m(A, B), m(A, C)) <*>||

The first patch changes the repository state, the second reverses the change (and records the conflict) but there is no change associated with the third one. It just indicates that this is the patch C and it conflicts with both A and B. The C is formatted as follows:

``````merger 0.0 (
merger 0.0 (
move ./file ./a
move ./file ./b
)
merger 0.0 (
move ./file ./a
move ./file ./c
)
)``````

The limitations of our `merger` notation become clear when we add the fourth conflicting patch, D:

||||‘’ <*> m(m(m(A, B), m(A, C)), m(m(A, B), m(A, D)) <*>’’||

The deep and complex conflicts are commonplace, and they deserve more efficient way of expressing them. The `merger` pairs records the complete history of the patches which got merged — but ordering is not needed beyond first marked conflict. The state does not depend on the order of the merged branches, only the identity of conflicting patches is important.

Darcs 2 uses `conflictor` patches where the set of conflicting branches is simply listed:

||||‘’ <> A c(,A,B)<*>c(A, B, ,C) <*> c(A, B, C, ,D) <*>’’||

The last patch is actually represented like this:

``````conflictor {{
:
move ./file ./a
:
move ./file ./b
:
move ./file ./c
}} [[
]]
:
move ./file ./d``````

(In a rather Haskellish way the colon does not separate branches from each other but it rather indicates that the next patch is the last one in the current branch.)

## Resolving Conflicts

A conflict between two branches can be represented with the following diagram:

||||‘’ || || || || || || || ||A|| ||m(A, B)|| || || ||<>|| || || || <*>|| || || ||B|| ||m(B, A)|| || || || || || || ||’’||

The obvious way to resolve the conflict is to select one of the three states before the conflict as the starting point for the new development: <>, or . In more general case, we can either choose one of the conflicting branches as the basic of new development, or start a completely new branch from the root of the conflict.

How Darcs then resolves conflicts? Darcs 1 has chosen the non-obvious way: the new development is started after the conflict, from the end state of the conflicting patches, shown as <*> above. As you might remember, in order to avoid any ambiguities the start state of the conflict was conveniently selected as its end state, too.

If the new patch resolving the conflict is N and the conflicting patches are A and B, the patch sequence looks pretty simple (again, using our notation):

||||‘’ <> A m(A, B)’’ <> N ||

The patch file created by Darcs 1 looks likewise simple.

However, there is something very important going on, something that does not manifest itself in the patch N or our notation above. Namely, the new patch

• depends on both A and B, or rather the conflicting combination of A and B
• conflicts with either ~A or ~B (or any further patch that depends on A and/or B, for that matter)

The notation above does not reflect the actual behavior of N, so we improve it:

||||‘’ <> A m(A, B)’’ <*> N* ||

Now, it is clear that

1. after the conflict the state of the repository differs in important way from the state before it, and
2. the resolution patch N* differs from plain patch N (which is just an another branch parallel with A and B).

The improved notation also makes it clear that there is no way we can do the trick we did with theorems 2 and 3 above (merge N with A or B).

Darcs 1 enforces the proper behavior of patch N, too. However, from the point of view of Darcs, the end state (<*>) of the conflict is exactly like its start state (<>). Also, there is no new patch type like `resolver` — the patch N* looks exactly like N. Instead Darcs uses contexts — sets of patches that must be present in repository before a certain patch can be applied to it. In other words, Darcs 1 absolutely, definitely requires that both A and B are present before N can be applied to the repository. Darcs 1 also prevents you from moving A or B past the N within the repository. (If N was an ordinary patch, you could do that by unpulling or obliterating A or B).

### Weakness of Darcs 1 Way of Resolving Conflicts

The foremost drawback of Darcs-1-style implicit resolver patches is that they break the rule #1 Darcs started with — ordering of the patches does not matter.

Perhaps more severe problem for usability of Darcs is the destructiveness of the conflicts. There is no way you can continue on any of the conflicting branches (unless you manage to remove all the traces from the other conflicting branches).

Yet another problem is the fact that resolving the conflict in the same way in two different repositories ends up in conflict. (However, while the duplicate is conflict according to the definition above, it is quite harmless and invisible to the Darcs 2 user.)

The resolver patch also make new conflicts more complex. Instead of starting the new branch from the root of conflict, the new branch starts from the neck of conflict. Any new conflicting branch — including the continuation from the one of the original branches — is in recursive conflict, conflict with a conflict. The patches used to mark conflict, mergers or conflictors, need to recognize that. They can not just be a list of a bunch of conflicting branches. They must keep the history of conflict, show the order of conflicting patches and store the new conflict inside the old conflict.

## Resolving Conflicts, Take 2

Above we represented the conflict with a diamond. Let us now extend the diamond with its inverse:

||||‘’ || || || || || || || || || || || ||A|| ||m(A, B)||||~m(~A, ~B)||||~A|| || || ||<>|| || || || <*>|| || || || <>|| || || ||B|| ||m(B, A)|| ||~m(~B, ~A)|| ||~B|| || || || || || || || || || || ||’’||

Also, let us rephrase the paragraph after the diamond diagram:

The obvious way to resolve the conflict is to select one of the three states after the conflict as the starting point for the new development: , or <>. In more general case, we can either choose one of the conflicting branches as the basic of new development, or start a completely new branch from the root of the conflict.

If the development is continued from the branch A, we record inverse of B after the conflict. If it is not clear from the above double diamond, inverse of B, as it would be applied after the conflict, is ~m(~A, ~B).

Likewise, if we want to continue from the branch B, we record inverse of A after the conflict, ~m(~B, ~A).

If we want to start a new branch, we record inverse of both, ~m(~B, ~A) followed by ~B, or ~m(~A, ~B) followed by ~A.

If the conflict is resolved in the same way in another repository, there is no problem — it is just a duplicate. If the conflict is resolved in different way then we have another conflict. However, that is exactly what we expect to happen.

## Resolving Conflicts: Some Practicalities

In retrospective, the definition 7(a) is not so hot. The repository is hosed after a conflict, it has no clear unambiguous state, and if we pretend otherwise, we end up in trouble with the rule #1.

However, Darcs just cannot turn the respective bits in the repository sideways and tell the user that hey, there is a conflict, do something about it.

One way to solve the problem of ambiguous state of the conflict is to prevent live conflicts. Each time Darcs merges two branches and there is a conflict, it will automatically record a inverse patch for all the conflicting branches. Conflicts would be resolved by obliterating one (or none) of those conflict-saving patches.

A bit more advanced approach is to keep the state of pristine just like it is now, but to create special pending patches when conflict occurs. Again, conflicts would be resolved by obliterating one (or none) of the conflict-saving patches in the pending queue, and recording the rest of the patches.

If the user insists in obliterating too many patches and records a Darcs-1-style resolver patch, it is always possible to apply the rule #1:

||||‘’ <> A m(A, B)’’ <> N || becomes || ||<)> ||‘’ <> A m(A, B)’’ <*> m(m(A, B), m(A, N)) <*>||