class SyntaxSuggest::CodeFrontier

There are three main phases in the algorithm:

  1. Sanitize/format input source

  2. Search for invalid blocks

  3. Format invalid blocks into something meaninful

The Code frontier is a critical part of the second step

## Knowing where we’ve been

Once a code block is generated it is added onto the frontier. Then it will be sorted by indentation and frontier can be filtered. Large blocks that fully enclose a smaller block will cause the smaller block to be evicted.

CodeFrontier#<<(block) # Adds block to frontier
CodeFrontier#pop # Removes block from frontier

## Knowing where we can go

Internally the frontier keeps track of “unvisited” lines which are exposed via ‘next_indent_line` when called, this method returns, a line of code with the highest indentation.

The returned line of code can be used to build a CodeBlock and then that code block is added back to the frontier. Then, the lines are removed from the “unvisited” so we don’t double-create the same block.

CodeFrontier#next_indent_line # Shows next line
CodeFrontier#register_indent_block(block) # Removes lines from unvisited

## Knowing when to stop

The frontier knows how to check the entire document for a syntax error. When blocks are added onto the frontier, they’re removed from the document. When all code containing syntax errors has been added to the frontier, the document will be parsable without a syntax error and the search can stop.

CodeFrontier#holds_all_syntax_errors? # Returns true when frontier holds all syntax errors

## Filtering false positives

Once the search is completed, the frontier may have multiple blocks that do not contain the syntax error. To limit the result to the smallest subset of “invalid blocks” call:


Public Class Methods

combination(array) click to toggle source


combination([:a, :b, :c, :d])
# => [[:a], [:b], [:c], [:d], [:a, :b], [:a, :c], [:a, :d], [:b, :c], [:b, :d], [:c, :d], [:a, :b, :c], [:a, :b, :d], [:a, :c, :d], [:b, :c, :d], [:a, :b, :c, :d]]
# File syntax_suggest/code_frontier.rb, line 162
def self.combination(array)
  guesses = []
  1.upto(array.length).each do |size|
new(code_lines:, unvisited: code_lines)) click to toggle source
# File syntax_suggest/code_frontier.rb, line 53
def initialize(code_lines:, unvisited: code_lines))
  @code_lines = code_lines
  @unvisited = unvisited
  @queue =

  @check_next = true

Public Instance Methods

<<(block) click to toggle source

Add a block to the frontier

This method ensures the frontier always remains sorted (in indentation order) and that each code block’s lines are removed from the indentation hash so we don’t re-evaluate the same line multiple times.

# File syntax_suggest/code_frontier.rb, line 148
def <<(block)


  @check_next = true if block.invalid?

count() click to toggle source
# File syntax_suggest/code_frontier.rb, line 61
def count
detect_invalid_blocks() click to toggle source

Given that we know our syntax error exists somewhere in our frontier, we want to find the smallest possible set of blocks that contain all the syntax errors

# File syntax_suggest/code_frontier.rb, line 172
def detect_invalid_blocks
  self.class.combination( do |block_array|
    holds_all_syntax_errors?(block_array, can_cache: false)
  end || []
expand?() click to toggle source
# File syntax_suggest/code_frontier.rb, line 111
def expand?
  return false if @queue.empty?
  return true if @unvisited.empty?

  frontier_indent = @queue.peek.current_indent
  unvisited_indent = next_indent_line.indent

    puts "```"
    puts @queue.peek
    puts "```"
    puts "  @frontier indent:  #{frontier_indent}"
    puts "  @unvisited indent: #{unvisited_indent}"

  # Expand all blocks before moving to unvisited lines
  frontier_indent >= unvisited_indent
holds_all_syntax_errors?(block_array = @queue, can_cache: true) click to toggle source

Returns true if the document is valid with all lines removed. By default it checks all blocks in present in the frontier array, but can be used for arbitrary arrays of codeblocks as well

# File syntax_suggest/code_frontier.rb, line 89
def holds_all_syntax_errors?(block_array = @queue, can_cache: true)
  return false if can_cache && can_skip_check?

  without_lines = block_array.to_a.flat_map do |block|

    without_lines: without_lines,
    code_lines: @code_lines
next_indent_line() click to toggle source
# File syntax_suggest/code_frontier.rb, line 107
def next_indent_line
pop() click to toggle source

Returns a code block with the largest indentation possible

# File syntax_suggest/code_frontier.rb, line 103
def pop
register_engulf_block(block) click to toggle source

When one element fully encapsulates another we remove the smaller block from the frontier. This prevents double expansions and all-around weird behavior. However this guarantee is quite expensive to maintain

# File syntax_suggest/code_frontier.rb, line 140
def register_engulf_block(block)
register_indent_block(block) click to toggle source

Keeps track of what lines have been added to blocks and which are not yet visited.

# File syntax_suggest/code_frontier.rb, line 132
def register_indent_block(block)

Private Instance Methods

can_skip_check?() click to toggle source

Performance optimization

Parsing with ripper is expensive If we know we don’t have any blocks with invalid syntax, then we know we cannot have found the incorrect syntax yet.

When an invalid block is added onto the frontier check document state

# File syntax_suggest/code_frontier.rb, line 74
        def can_skip_check?
  check_next = @check_next
  @check_next = false

  if check_next