feat(content): adds Data Structures to appendiz(#1175)

Brings back description of Data Structures (as part of the Appendix), which for now includes RLE+ Bitset Encoding only.

Co-authored-by: Hugo Dias <hugomrdias@gmail.com>

content/appendix/data_structures.md

@@ -0,0 +1,72 @@
+---
+title: Data Structures
+weight: 3
+dashboardWeight: 0.2
+dashboardState: reliable
+dashboardAudit: n/a
+---
+
+# Data Structures
+
+## RLE+ Bitset Encoding
+
+RLE+ is a lossless compression format based on [RLE](https://en.wikipedia.org/wiki/Run-length_encoding).
+Its primary goal is to reduce the size in the case of many individual bits, where RLE breaks down quickly,
+while keeping the same level of compression for large sets of contiugous bits.
+
+In tests it has shown to be more compact than RLE itself, as well as [Concise](https://arxiv.org/pdf/1004.0403.pdf) and [Roaring](https://roaringbitmap.org/).
+
+### Format
+
+The format consists of a header, followed by a series of blocks, of which there are three different types.
+
+The format can be expressed as the following [BNF](https://en.wikipedia.org/wiki/Backus%E2%80%93Naur_form) grammar.
+
+```bnf
+    <encoding> ::= <header> <blocks>
+      <header> ::= <version> <bit>
+     <version> ::= "00"
+      <blocks> ::= <block> <blocks> | ""
+       <block> ::= <block_single> | <block_short> | <block_long>
+<block_single> ::= "1"
+ <block_short> ::= "01" <bit> <bit> <bit> <bit>
+  <block_long> ::= "00" <unsigned_varint>
+         <bit> ::= "0" | "1"
+```
+
+An `<unsigned_varint>` is defined as specified [here](https://github.com/multiformats/unsigned-varint).
+
+#### Blocks
+
+The blocks represent how many bits, of the current bit type there are. As `0` and `1` alternate in a bit vector
+the inital bit, which is stored in the header, is enough to determine if a length is currently referencing
+a set of `0`s, or `1`s.
+
+##### Block Single
+
+If the running length of the current bit is only `1`, it is encoded as a single set bit.
+
+##### Block Short
+
+If the running length is less than `16`, it can be encoded into up to four bits, which a short block
+represents. The length is encoded into a 4 bits, and prefixed with `01`, to indicate a short block.
+
+##### Block Long
+
+If the running length is `16` or larger, it is encoded into a varint, and then prefixed with `00` to indicate
+a long block.
+
+> **Note:** The encoding is unique, so no matter which algorithm for encoding is used, it should produce
+> the same encoding, given the same input.
+
+##### Bit Numbering
+
+For Filecoin, byte arrays representing RLE+ bitstreams are encoded using [LSB 0](https://en.wikipedia.org/wiki/Bit_numbering#LSB_0_bit_numbering) bit numbering.
+
+## HAMT
+
+See the draft [IPLD hash map spec](https://github.com/ipld/specs/blob/master/data-structures/hashmap.md) for details on implementing the HAMT used for the global state tree map and throughout the actor code.
+
+## Other Considerations
+
+- The maximum size of an Object should be 1MB (2^20 bytes). Objects larger than this are invalid.