Immutable Data Structures（不可变数据结构）, advantages, and applications

1. Immutable Data Structures
Eric Fu
2018-06-15

2. Agenda
• Stack
• Tree
• Vector Trie
• HAMT
• Git
• LSM-Tree
• Practice

3. Immutable Stack
A Clean Example for Starters

4. CopyOnWriteArrayList ?

5. Stack

6. Stack

7. Stack

8. Stack

9. Immutable Tree

10. Tree

11. Tree

12. RB-Tree

13. RB-Tree

14. Vector Trie
Immutable Array List

15. Trie
Also called:
• Dictionary tree
• Radix tree
• Prefix tree
• Digital search tree

16. Trie (compressed)
• With prefix only
"A", "to", "tea", "ted"
"ten", "i", "in", "inn"

17. Vector Trie

18. Vector Trie (Mutable)

19. Vector Trie - Update

20. Vector Trie - Push

21. Vector Trie – Push (1/3)

22. Vector Trie – Push (2/3)

23. Vector Trie – Push (3/3)

24. Vector Trie – Pop (1/3)

25. Vector Trie – Pop (2/3)

26. Vector Trie – Pop (3/3)

27. Complexity
• Mostly....

28. Go Efficient
• More branches

29. Go Efficient
• More branches
• Align to binary bits

30. Go Efficient
• More branches – 32
• Align to binary bits – 5 bits

31. Hash Array Mapped Trie (HAMT)
Immutable Hash Table

32. Rethink Vector Trie
• Must the index be continuous (0, 1, 2...) ?
• Actually not!
• Furthermore: hash(anything) -> integer

33. HAMT - Naive
• Contains {0x00, 0xff}

34. HAMT – Bitmap Compressed
• Compress each node with bitmap

35. HAMT - Pruned
• Leave out unnecessary nodes

36. Some Real-world Examples

37. Git

38. How Git works?
• Git stored all versions of all directories and files
• A Git commit is a snapshot of root directory
• A Git branch/tag is a reference to some commit

52. That's it!

53. Log-Structured Merge-Tree (LSM-
Tree)

59. More Trees

60. Insights
• Modify in memory
• Read merged results
• Newly merged blocks are appended

61. Modern LSM-Tree Implements
• Memtable
• Immutable Memtable
• Level 0 (overlapped)
• Level 1
• Level 2
• ....
Memory
Disk
Write
Read
61

62. Immutable in Practice
In Java

63. A Familiar Example
• Java's string vs. CPP's string

64. Easy to understand
• Same name, same value
Mutable Request
Immutable Request:

65. Always thread-safe

66. Always thread-safe
• Immutable to programming as MVCC to database
• Lock or Snapshot

67. Other Pros
• Friendly for caching
• Atomic Failure
• Avoid null-pointer exception

68. Java Example 1

69. Java Example 2
• Requirement: cache the results of factorizing
• Two variable with setter?
• Two atomic variable?

70. Java Example 2 (cont.)

71. Java Example 2 (cont.)

72. Thanks!
Q&A