• A branch has a purpose. The most obvious is a topic branch you use for developing a new feature. A topic branch 'add-frotz' would be about adding a new 'frotz' feature and shouldn't do anything else. In a well organized project, 'master' branch (or 'trunk' if you are coming from subversion) has the sole purpose of containing proven-to-be-good changes to produce the next release. Your per-customer branch is to contain the changes suitable for the next customer code dump and nothing else. It typically consists of bugfixes and selected features the particular customer asked (and paid for).
• The act of making a commit with one or more parents and advancing the tip of a branch with that commit is to make this statement: I have considered what all of these parent commits represent, and in my belief the state I am committing is more suitable for the purpose of this branch than any of them.

• The commit at the tip of 'add-frotz' (one of the parents of the resulting merge) didn't contain any of the other good changes happened on the 'master' branch', so compared to that commit, the resulting merge commit is more suitable for the purpose of the 'master' branch.
• The commit at the tip of 'master' (the other parent of the resulting merge) didn't contain the new 'frotz' feature, and your wanting to have that feature in the next release makes the resulting merge more suitable for the purpose of the 'master' branch.

$ git checkout add-frotz
$ git branch add-frotz-to-1.0
$ git merge master

When you start using the topic branch workflow, you would merge topics often into a throw-away testing branch, and from time to time, end up performing the same conflict resolution over and over again. Git has a mechanism called rerere to help you in such a situation.

Even people who use rerere often do not realize one interesting aspect of the command, and this article is about showing off that part.

Let's pretend that you start from this base version.

$ mkdir /var/tmp/practice-rerere
$ cd /var/tmp/practice-rerere
$ git init
$ cat >hello.c <<\EOF
#include <stdio.h>
#include <string.h>

/*
 * Say the message
 */
void hello(char *message)
{
        printf("%s\n", message);
}

/*
 * Show greetings
 */
void greetings(char *message)
{
        hello(message);
        printf("Your message is %d bytes long\n",
               (int) strlen(message));
}

/*
 * Main program
 */
int main(int ac, char **av)
{
        greetings(av[1]);
        return 0;
}
EOF

$ cc -o hello ./hello.c && ./hello "hello world"
hello world
Your message is 11 bytes long

$ git add hello.c
$ git commit -m initial

$ git checkout -b upcase-first
$ git apply <<\EOF && git commit -a -m "upcase first word"
diff --git a/hello.c b/hello.c
index e0dbe10..7fcb4f3 100644
--- a/hello.c
+++ b/hello.c
@@ -1,11 +1,16 @@
+#include <ctype.h>
 #include <stdio.h>
 #include <string.h>

 /*
  * Say the message
  */
-void hello(char *message)
+void hello(char *message, int upcase_first)
 {
+       char *cp;
+       for (cp = message; *cp && !isspace(*cp); cp++)
+               *cp = toupper(*cp);
+
        printf("%s\n", message);
 }

@@ -14,7 +19,7 @@ void hello(char *message)
  */
 void greetings(char *message)
 {
-       hello(message);
+       hello(message, 1);
        printf("Your message is %d bytes long\n",
               (int) strlen(message));
 }
EOF
$ cc -o hello ./hello.c && ./hello "hello world"
HELLO world
Your message is 11 bytes long

$ git checkout -b const-fix master
$ git apply <<\EOF && git commit -a -m "constness fix"
diff --git a/hello.c b/hello.c
index e0dbe10..9970faa 100644
--- a/hello.c
+++ b/hello.c
@@ -4,7 +4,7 @@
 /*
  * Say the message
  */
-void hello(char *message)
+void hello(const char *message)
 {
        printf("%s\n", message);
 }
@@ -12,7 +12,7 @@ void hello(char *message)
 /*
  * Show greetings
  */
-void greetings(char *message)
+void greetings(const char *message)
 {
        hello(message);
        printf("Your message is %d bytes long\n",
@@ -22,7 +22,7 @@ void greetings(char *message)
 /*
  * Main program
  */
-int main(int ac, char **av)
+int main(int ac, const char **av)
 {
        greetings(av[1]);
        return 0;
EOF

$ git checkout upcase-first^0
Note: moving to 'upcase-first^0' which isn't a local branch
If you want to create a new branch from this checkout, you may do so
(now or later) by using -b with the checkout command again. Example:
  git checkout -b <new_branch_name>
HEAD is now at 362a9fb... upcase first

$ mkdir .git/rr-cache

$ git merge const-fix
Auto-merging hello.c
CONFLICT (content): Merge conflict in hello.c
Recorded preimage for 'hello.c'
Automatic merge failed; fix conflicts and then commit the result.

$ cat hello.c
#include <ctype.h>
#include <stdio.h>
#include <string.h>

/*
 * Say the message
 */
<<<<<<< HEAD
void hello(char *message, int upcase_first)
=======
void hello(const char *message)
>>>>>>> const-fix
{
        char *cp;
        for (cp = message; *cp && !isspace(*cp); cp++)
                *cp = toupper(*cp);

        printf("%s\n", message);
}

/*
 * Show greetings
...

<<<<<<< HEAD
void hello(char *message, int upcase_first) --- your changes
=======
void hello(const char *message) --- their changes
>>>>>>> const-fix

void hello(const char *message, int upcase_first)

void hello(const char *message, int upcase_first)
{
        const char *cp;
        for (cp = message; *cp && !isspace(*cp); cp++)
                putchar(toupper(*cp));
        message = cp;

        printf("%s\n", message);
}

$ cc -o hello ./hello.c && ./hello "hello world"
HELLO world
Your message is 11 bytes long

$ git commit -a -m 'test resolution'
Recorded resolution for 'hello.c'.
[detached HEAD 2fe010b] test resolution

$ git checkout upcase-first
Previous HEAD position was 2fe010b... test resolution
Switched to branch 'upcase-first'
$ git apply <<\EOF && git commit -a -m "add comment"
diff --git a/hello.c b/hello.c
index 9970faa..53abee2 100644
--- a/hello.c
+++ b/hello.c
@@ -20,7 +20,7 @@ void greetings(const char *message)
 }

 /*
- * Main program
+ * Main program - give greetings with the first word upcased.
  */
 int main(int ac, char **av)
 {
EOF
$ cc -o hello ./hello.c && ./hello "hello world"
HELLO world
Your message is 11 bytes long

$ git checkout HEAD^0
$ git merge const-fix
Auto-merging hello.c
CONFLICT (content): Merge conflict in hello.c
Resolved 'hello.c' using previous resolution.
Automatic merge failed; fix conflicts and then commit the result.

$ git checkout --conflict=merge hello.c
$ cat hello.c

• The branches produced the conflict exactly the same way. The hello() function has different sets of arguments.
• The body of the hello() function did not have any conflict; it merged cleanly at the textual level, but it is wrong as the merge result.

$ git rerere
Resolved 'hello.c' using previous resolution.

• Rerere remembers how you chose to resolve the conflicted regions;
• Rerere also remembers how you touched up outside the conflicted regions to adjust to semantic changes;
• Rerere can reuse previous resolution even though you were merging two branches with different contents than the one you resolved earlier.

• 紀伊國屋さん http://tinyurl.com/kinokuniya-git
• アウル(ジュンク堂)さん http://tinyurl.com/owlbooks-git
• 丸善さん http://tinyurl.com/maruzen-git
• 八重洲ブックセンターさん http://tinyurl.com/yaesu-git

• Customizable UI.  I do not care too much about UI, but at least I wanted to have a customizable wallpaper.  The original firmware offers "wallpaper" option, but it only changes the background color; it does not allow you to use your own image.
• Changing playback speed without affecting pitch.  I mostly use my MP3 players to listen to Podcasts while driving to work, and it is often nice to play at a faster speed.  The original firmware allows you to play faster, but at higher pitch (some people call this "chipmunk mode"), but Rockbox has an option to play faster at the original pitch, which is much nicer.
• The emulator works in Wine.  This is primarily a praise for Wine guys, but I was quite impressed that the Fuze emulator to try custom UI out worked out of the box in Wine (it even plays sound ;-).

• No USB connection support yet.  Plugging the unit over USB will charge and let the original firmware act as a removable disk as usual (i.e. MSC mode), so it is not a showstopper.
• No recording support yet.  Every day, after I park my car, I record where I parked in the parking structure (which is 6 stories tall and often packed, as it is not just the office parking but primarily is used as airport parking); I easily can forget where I parked in the morning without this.
• No embedded album art.  You can extract album art as a separate image file and use it, but then it would become cumbersome to remove it.
• Booting back into the original firmware seems to reset the clock.  This is quite annoying, as connecting with USB to either upload music and podcast tracks, or to charge the unit, will boot into the original firmware.

• You should be able to set the default playback speed per type of contents (e.g. play Audiobooks and Podcasts at 125% speed).  I want to listen to podcasts at faster speed, but want to play music at the original speed.
• The UI to set clock seems to be buggy. When set to October 7th from the UI, it starts ticking on October 8th for whatever reason.
• You can set the display to turn off after inactivity.  When the display goes dark this way, you can set the first UI action to only activate the display without doing anything else.  This is very nice (and is the only mode supported by the original firmware) especially because in Rockbox UI, there is no single safe button to press that wouldn't do anything, when you cannot tell what mode the unit is in.  One glitch of this is that when the unit is on hold (i.e. with Fuze, the power switch can be slided in the opposite direction to put the unit on hold), no action other than the unhold action itself wakes up the display.  This is somewhat irritating.
• The source is in Subversion, although git-svn seems to cope with the trunk version just fine.
• The emulator in Wine does not seem to get long-click (in Windows it reacts).

最近書いた、「入門Git」の最後の方にマニュアル的なリストをつけることも考えなかったわけではありません。でも、お金払った本にマニュアルに書いてあることのコピーがえんえんと載ってたら、なんだか損した気になるでしょ？

入門書である本書のひとつの目的は、マニュアルを効果的に使えるための地固めをすることです。gitメーリングリスト等での新規ユーザの挙動を見ていると、必要な基礎概念を理解する前にいきなりマニュアルを見ようとして、それでかえって混乱していることが往々にしてあります。例えばrevertというコマンド名を思い付いてgit revertのマニュアルを見る。「いままでエディタで色々編集してきたの、みんなでたらめな変更だから捨てたい、どうしたらいいの」という状況に使えるオプションを見たくて、オプションのリストの部分にいきなり行ってもそんな記述がない。

もともとrevertはそういう用途に使うコマンドではないので、そんなオプションがないのは当たり前で、正解は他のコマンドを使うこと、これはrevertマニュアルの先頭にも書いてあるのですが、マニュアルページは基本的に字引きのようなもので、基礎がわかった上でのあちこちつまみ食い、という使い方が効率的にできることを主たる目的とした構成になっていますから、先頭の記述部分をとばしてオプションのリストだけ見て分かるひとにはそれでよいのですが、新規ユーザが全体の構成を理解せずにいきなりマニュアルから入ってもなかなか効果的な習得は望めません。

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