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anatomy of a bug
anatomy of a bug
anatomy of a bug
AVL Trees
Anatomy of a bug
I wrote java code for the avl tree: the data structure, a test/demo programand managed to display it …
I felt … smug
smug
smugsmug
smug
smug
It was written all over me
… so smug that I just HAD to demo it to a friend/colleague …
Dr. Chris “unworthy” Unsworth
… and this is what happened
Calm down … think
1. you have code for btree2. the only difference between btree and avl tree is restoring balance3. disable “restore balance” to confirm that’s were bug lies
Phew!
No duplicates & size is right.
Okay, bug is in restoreBalance
Track it down to one of the rotations
Edit out doubleRotate
Compile and test …
YES!
The bug’s in doubleRotate!
smug
smugsmug
smug
smug
It was written all over me
Let’s get more information out of doubleRotate and fix this baby!
1. Turn doubleRotate back on2. Print out information on before & after doubleRotate
Turn doubleRotate back onPrint out the rotation made
Before & after
Print out information on nodesk1, k2, k3 before & after doubleRotate
Compile & run …
arrrrrrrgh!
Too much information!
Keep it simple!
Get smallest data set possible to demonstrate bug!
ll pp mm
ll
pp
mm
RLRotate
Heck! It looks okay.
What’s up?
Maybe you should show the whole
tree dude.
It’s back! It’s back! It’s back
Is it just a “right thing”?
Arrrrrrgh!It happens on the left too!
Make it go away!!!
A brief intermission is now called for as our hero calms down
Hero re-writes code for rotate …
Hero wonders if there is something flawed in java
Hero wonders if there is something to do with call by reference, call by value …
Our hero clutches at straws …
Hero even reads the book again!!!
Hero has lunch at 12.30
Hero returns from lunch and tries again …
Holy smoke! Size is 4! I only entered 3 strings!!
How the heck can that be?Where is size updated?
twat
twat
twat
twattwat
twat
It was written all over him …
It does this loop an extra time!
Compile and run …
YES!
Now do it on the left …
Get rid of comments etc and try on large data set …
Edit out prints …
Go for it fat boy …
smug twat
smug twatsmug twat
smug twat
smug tw
at
What lessons have we learned?
1. if your program works … you have a subtle bug (worry)2. if data structure is complicated write code to test it (I didn’t)3. produce minimal test cases4. produce BIG test cases5. imagine if I didn’t have the btree code … would that be dangerous?
What would your data structure testing program do?• check that we have a tree
• no loops• check we have no orphans (nodes without parents)• no duplicates• height is okay• size is okay• avl balance is present• check EVERYTHING you can think of!!
Lessons we can learn
This took 1 day of my life
