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= Case with multiple branch pointers pointing at different parts of a common linear history =

== Incorporating changes from a separate branch ==

'''Before'''. We have a history like this:

<pre>         b1    b2
         |     |
         \/    \/
a--b--c--d--e--f
    \-g--h
         /\
         |
         b3</pre>
Where:

* There's a fork in history after commit <code>b</code>.
* Branch pointer <code>b1</code> points at commit <code>d</code>, <code>b2</code> points at commit <code>f</code>, branch <code>b3</code> points at commit <code>h</code>.

'''After'''. We want the history to look like this:

<pre>                b1      b2
                |       |
                \/      \/
a--b--g--h--c'--d'--e'--f'
         /\
         |
          b3</pre>
That is, we want to cut out commits <code>c--d--e--f</code> and put them on the top of commit <code>h</code>.

'''The backstory'''. Where all these branches came from? That branch <code>b3</code> was under public code review while we were independently privately working on functionalities in <code>b1</code> and after that was done, we were working in the same way on functionalities based on that in branch <code>b2</code>.

'''What we want to do'''. We want to incorporate changes from branch <code>b3</code> to the code in the second arm of the forked history - the one with branch pointers <code>b1</code> and <code>b2</code>. We want to keep the history linear and keep the public history intact (i.e. branch <code>b3</code> points at the tip of the history tree that is public).

'''Technical details'''. In order to do it, execute:

* <code>git checkout b2</code>
* <code>git rebase b3</code>
* resolve merge conflicts in all conflicting commits and make sure that the tests run properly

This results in the following commit tree:

<pre>         b1             b2
         |              |
         \/             \/
a--b--c--d
    \-g--h--c'--d'--e'--f'
         /\
         |
         b3</pre>
Now we need to move branch pointer <code>b1</code> to commit <code>d'</code>. This action will also automatically delete the old branch. This is done by executing:

* <code>git branch -f b1 d'</code>

This results in the desired commit history.

== Changing initial history ==

'''Before'''. We have a history like this:

<pre>         b1    b2    b3
         |     |     |
         \/    \/    \/
a--b--c--d--e--f--g--h</pre>
Where:

* Branch <code>b1</code> points at commit <code>d</code>, <code>b2</code> points at commit <code>f</code>, branch <code>b3</code> points at commit <code>h</code>.

'''After'''. We want the history to look like this:

<pre>   b1     b2      b3
   |      |       |
   \/     \/      \/
a--i--e'--f'--g'--h'</pre>
Where:

* commit <code>i</code> is a commit that was created by squashing commits <code>b</code>, <code>c</code>, <code>d</code>.

'''The backstory'''. Where all these branches came from? The branch <code>b1</code> was under a public code review while branches <code>b2</code> and <code>b3</code> were consecutively created based on previous branch. Now we want to squash commits <code>b</code>, <code>c</code>, <code>d</code> from the code review into a single commit before rebasing branch <code>b1</code> onto <code>master</code> branch.

'''What we want to do'''. We want to change some old history of commits at the beginning of the shown history. To be more precise, we want to squash commits <code>b</code>, <code>c</code>, <code>d</code>.

'''Technical details'''. In order to do it, execute:

* <code>git checkout b3</code>
* <code>git rebase -i a</code>
* during the interactive rebase session squash commits <code>b</code>, <code>c</code>, <code>d</code>

This results in the following commit tree:

<pre>         b1    b2
         |     |
         \/    \/
a--b--c--d--e--f
 \-i--e'--f'--g'--h'
                  /\
                  |
                  b3</pre>
The next step is to move branch pointers <code>b1</code> and <code>b2</code> to the other arm of the forked history:

* <code>git branch -f b2 f'</code>
* <code>git branch -f b1 i'</code>

This results in the desired commit history.

= Moving part of the history to another branch =

We want to cut off top part of one branch and attach it at the top of another branch. Namely:

'''before''' the change we have:

<pre>            b1
            |
            \/
a--b--c--d--e
    \-f--g--h--i--j
            /\    /\
            |     |
            b2    b3</pre>
while '''after''' the change we have:

<pre>            b1     b3
            |      |
            \/     \/
a--b--c--d--e--i'--j'
    \-f--g--h
            /\ 
            | 
            b2</pre>
In order to do it, execute:

* <code>git rebase --onto b1 b2 b3</code>, where:
** <code>b1</code> is the destination branch pointer (or commit hash)
** <code>b2</code> is the cut point - the history will be cut '''after''' this branch pointer (or commit hash)
** <code>b3</code> is the tip of the moved history line.