Learning Exercise
Working with binary data can be tricky, so Gleam provides a BitArray type and accompanying syntax to construct and to pattern match on binary data.
Bit array literals are defined using the <<>> syntax. When defining a bit array literal, it is defined in segments. Each segment has a value and annotation, separated by a :. The annotation specifies how many bits will be used to encode the value, and can be omitted completely, which will default to a 8-bit integer value.
// This defines a bit array with three segments of a single bit each
<<0:1, 1:1, 0:1>>
// This defines a bit array with three segments of 8 bits each
<<0, 1, 0>>
Specifying the type as :1 is a shorthand for writing :size(1). You need to use the longer syntax if the bit size comes from a variable.
let segment_size = 1
<<0:size(segment_size), 1:size(segment_size), 0:size(segment_size)>>
When writing binary integer literals, we can write them directly in base-2 notation by prefixing the literal with 0b. Note that they will be displayed as decimal numbers when printed in tests or in your program.
<<0b1011:4>> == <<11:4>>
// -> True
If the value of the segment overflows the capacity of the segment's type, it will be truncated from the left.
<<0b1011:3>> == <<0b0011:3>>
// -> True
You can both prepend and append to an existing bit array using the bit array syntax. The :bits annotation must be used for the existing bit array.
let value = <<0b110:3, 0b001:3>>
let new_value = <<0b011:3, value:bits, 0b000:3>>
// -> <<120, 8:size(4)>>
We can concatenate bit arrays stored in variables using the bit array syntax. The :bits annotation must be used when concatenating two bit arrays of variable sizes.
let first = <<0b110:3>>
let second = <<0b001:3>>
let concatenated = <<first:bits, second:bits>>
// -> <<49:size(6)>>
Pattern matching can also be done to obtain values from the bit array. You have to know the number of bits for each segment you want to capture, with one exception: the :bits annotation can be used to pattern match on a bit array of an unknown size, but this can only be used for the last segment.
let assert <<value:4, rest:bits>> = <<0b01101001:8>>
value == 0b0110
// -> True
Bit arrays might be printed in a different format than the format that was used to create them. This often causes confusion when learning bit arrays.
By default, bit arrays are displayed in segments of 8 bits (a byte), even if you created them with segments of a different size.
<<2011:11>>
// -> <<251, 3:size(3)>>
If you create a bit array that represents a printable UTF-8 encoded string, it may displayed as a string by functions such as io.debug. This is due to an implementation detail of how Gleam represents strings internally.
<<>>
// -> ""
<<65, 66, 67>>
// -> "ABC"
In your DNA research lab, you have been working through various ways to compress your research data to save storage space. One teammate suggests converting the DNA data to a binary representation:
| Nucleic Acid | Code |
|---|---|
| Adenine | 00 |
| Cytosine | 01 |
| Guanine | 10 |
| Thymine | 11 |
You ponder this, as it will potentially reduce the required data storage costs, but at the expense of human readability. You decide to write a module to encode and decode your data to benchmark your savings.
Implement encode_nucleotide to accept a nucleotide and return the int value of the encoded code.
encode_nucleotide(Cytosine)
// -> 1
// (which is equal to 0b01)
Implement decode_nucleotide to accept the integer value of the encoded code and return the nucleotide.
decode_nucleotide(0b01)
// -> Ok(Cytosine)
Implement encode to accept a list of nucleotides and return a bit array of the encoded data.
encode([Adenine, Cytosine, Guanine, Thymine])
// -> <<27>>
Implement decode to accept a bit array representing nucleic acid and return the decoded data as a list of nucleotides.
decode(<<27>>)
// -> Ok([Adenine, Cytosine, Guanine, Thymine])
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