Understanding Pijul file graphs and change files
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Pijul represents files internally as graphs, but how does it actually work?
This document is meant to give a broad, general understanding of Pijul’s representation of files in a repository, and how changes are modeled. The goal is to develop the reader’s intuition about the way that Pijul internally represents files and keep track of their states.
NOTE: This is a work in progress, and will updated from time to time.
Files and directories in a repo as graphs.
A Pijul repository tracks files and folders, and changes to those. The way it does that is by modeling those objects as a graph.
Change files
Change files consist of three parts:
- bits that are hashed
- bits that aren’t hashed
- raw byte contents (also hashed, its hash is included as a field in part 1)
Key insight: a change file is not a patch or diff in the sense that git or other diff tools might produce. A change file contains contains precise changes to an existing collection-of-files-as-a-graph (call it “repo-graph” for short).
For example: a change file never contains deleted text (like deleted lines that might appear in a unified diff). A change file may refer to content introduced by other changes and indicate parts that should be marked deleted in the repo-graph. In other words, a change file never contains deleted text, only references to text to delete.
Conversely, a given change file in isolation may only contain text/content that is being added to the existing repo-graph.
Initial graph
The initial graph is empty so there’s not really much to show, the repository graph can essentially be thought of as containing just a single root node:
This node is not introduced by any changes, it exist only in the
internal Pijul graph structure to anchor new graph nodes. Running
pijul debug on an empty repository produces no graph output.
Adding files
The only thing we can really do with an empty repository is to begin
adding files to it. To do that, we create a file, add some contents to
it, run pijul add <file>, then pijul record -m <message>.
Our first file graph looks like this:
The root change (also known as AAAAAAAAAAAAA) is a synthetic node not introduced by any changes, all the other nodes are introduced by changes. The first little surprise here is that we’ve only recorded change, but two were created. The Pijul log command confirms this:
$ pijul log
Change YS4YZQKSU3ZUXKGRK65W52EKET2UIF4V6SOZMVOEFTMXFOXJEBHQC
Author: ************ <****@*****>
Date: Wed, 18 Oct 2023 04:17:49 +0000
add main.c
Change WLNTADNCUR2X6BU4AFUIVAPTX77CORJ3T4OEXW63XPZ4ACCJ7H2AC
Author:
Date: Wed, 18 Oct 2023 04:17:49 +0000
The author-less change looks like this:
$ pijul change WLNTA
message = ''
timestamp = '2023-10-18T04:17:49.400906844Z'
authors = []
# Hunks
1. Root add
up 1.0, new 0:0
The C1 change is introduced by Pijul to “anchor” other changes. In Pijul it’s possible to join histories from different repositories and, as I understand it, the separate, explicit root addition is what makes that possible. The C2 change is the actual file addition:
1. File addition: "main.c" in "" "UTF-8"
up 2.1, new 0:30
+ #include <stdio.h>
+
+ int
+ main(int argc, char *argv[])
+ {
+ printf("hello world\n");
+ }
Edits & Replacements
By far the most common operations after adding files is editing them. Pijul has two flavours of file edits: edits and replacements. The difference between the two is essentially that an edit only introduces new lines where replacements additionally remove lines.
Simple edits
The edit is rendered like this with pijul change:
1. Edit in "main.c":2 2.31 "UTF-8"
up 2.51, new 0:41, down 2.51
+
+ void saymsg(char *msg) { printf(msg); }
So all we’ve done is add two lines. The internal graph representation in Pijul ends up looking like this:
From the C2 change, Pijul has split the node C2 [32;115[ into two
nodes: C2 [32;51[ and C2 [51;115[, and C3 introduces a new node
whose contents go between them. The range notation on a node label
refers to a slice of bytes from the contents section of a given
change.
The green arrows indicate file content nodes and the order between them, and the red back arrows indicate the immediate predecessor. To render a file from this file graph, Pijul would attempt to do a total topological ordering of the content nodes, traverse them, grab the bytes from respective change files and concatenate them together to produce the final file.
In our example, the content nodes are now C2 [32;51], C2 [51;115[,
and C3 [0;41[, and Pijul orders them by the outgoing green arrows:
If you look closely you’ll note that the there’s one edge labeled C3
instead of [C3]. This means the edge exists only to provide a total
ordering on the nodes. The current contents of our file are literally
the concatenation of the byte sequences indicated by the three nodes.
Replacements
Next we’ll try replacing some of the existing file content with new content. The change itself is quite simple:
1. Replacement in "main.c":8 2.31 "UTF-8"
B:BD 2.87 -> 2.87:113/2
up 2.87, new 0:26, down 2.113
- printf("hello world\n");
+ saymsg("hello world\n");
Now our file graph looks like this:
Like C3, a new change is added with new contents, but there are some new things to observe:
-
As with C3, the nodes of C2 have again been split. The
C2 [51;115[node has been split into three nodes with the ranges [51;87[, [87;113[, and [113;115[. -
The middle node of range [87;113[ is the text that we have removed, and it is indicated in the graph by dashed edges introduced by C4.
-
The node with deleted content retains edges to other content nodes. Pijul considers a node “dead” if all its incoming (ie green) edges are marked deleted. It is still allowed for a dead node to have live edges to other nodes.
-
There are new dotted edges between nodes
C2 [51;87[andC2 [113;115[. These are pseudo-edges added by Pijul and are not explicitly introduced by any change. These edges exist to make it faster for Pijul to compute the live nodes without having to traverse all the dead nodes.
In general, Pijul “deletes” text by splitting a node into three: a node to hold the deleted contents, and two nodes for the remaining (alive) contents before and after. Pseudo-edges are added to make file rendering faster by allowing it to skip dead nodes.
The total number of nodes that make up the contents of our file is now five and are ordered as follows:
Adding another file
Files exist as independent subgraphs that only share a common root in the repository structure, so if we add another file to the repository it will not depend on any of the changes that introduced or modified our first file, it will only depend on the first change that introduced the root.
This it what it looks like:
In the above graph, I have removed the subgraphs that grouped nodes
from a given change and instead shown the subgraphs that represents
each of the two files now tracked by Pijul. This display is much
closer to the result from pijul debug piped through dot(1).
Do note that a single change can introduce more than one file at a time, and also modify many files at the same time. The presentation I have chosen here is in a sense the “cleanest” in that we have considered small incremental changes at a time.
One of the powers of the graph representation begins to come to light: If we decided that we wanted to undo the change called C4 (the last change on main.c) we could undo it without needing to consider the C5 change. Our last change (adding readme.txt) does not in any way depend on C4, so clearly we should be able to modify the part of the file graph that represents main.c without touching the part that represents readme.txt. If, in our last change, we had also edited main.c, then we could have ended up depending on C4 and undoing it would be more cumbersome, but in the current file graph that is not the case.
Conflicts
The file graph can quickly become complicated and challenging to read, so for this section we will reset the file graph to something simpler:
file graph here
How, then, are conflicts modeled? TBD
Idea: reset to a simpler file graph, fork and make conflicting changes.