class Prism::CodeUnitsCache

A cache that can be used to quickly compute code unit offsets from byte offsets. It purposefully provides only a single [] method to access the cache in order to minimize surface area.

Note that there are some known issues here that may or may not be addressed in the future:

The first is that there are issues when the cache computes values that are not on character boundaries. This can result in subsequent computations being off by one or more code units.
The second is that this cache is currently unbounded. In theory we could introduce some kind of LRU cache to limit the number of entries, but this has not yet been implemented.

Public Class Methods

new (source, encoding)

Initialize a new cache with the given source and encoding.

# File lib/prism/parse_result.rb, line 198
def initialize(source, encoding)
  @source = source
  @counter =
    if encoding == Encoding::UTF_16LE || encoding == Encoding::UTF_16BE
      UTF16Counter.new(source, encoding)
    else
      LengthCounter.new(source, encoding)
    end

  @cache = {}
  @offsets = []
end

Public Instance Methods

[] (byte_offset)

Retrieve the code units offset from the given byte offset.

# File lib/prism/parse_result.rb, line 212
def [](byte_offset)
  @cache[byte_offset] ||=
    if (index = @offsets.bsearch_index { |offset| offset > byte_offset }).nil?
      @offsets << byte_offset
      @counter.count(0, byte_offset)
    elsif index == 0
      @offsets.unshift(byte_offset)
      @counter.count(0, byte_offset)
    else
      @offsets.insert(index, byte_offset)
      offset = @offsets[index - 1]
      @cache[offset] + @counter.count(offset, byte_offset - offset)
    end
end