# 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 meaningful

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:

    CodeFrontier#detect_invalid_blocks

### Public Class Methods

combination (array)
Source

    # File lib/syntax_suggest/code_frontier.rb, line 162
    def self.combination(array)
      guesses = []
      1.upto(array.length).each do |size|
        guesses.concat(array.combination(size).to_a)
      end
      guesses
    end

Example:

    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]]

new (code_lines:, unvisited: UnvisitedLines.new(code_lines: code_lines))
Source

    # File lib/syntax_suggest/code_frontier.rb, line 53
    def initialize(code_lines:, unvisited: UnvisitedLines.new(code_lines: code_lines))
      @code_lines = code_lines
      @unvisited = unvisited
      @queue = PriorityEngulfQueue.new

      @check_next = true
    end

### Public Instance Methods

<< (block)
Source

    # File lib/syntax_suggest/code_frontier.rb, line 148
    def <<(block)
      @unvisited.visit_block(block)

      @queue.push(block)

      @check_next = true if block.invalid?

      self
    end

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.

count ()
Source

    # File lib/syntax_suggest/code_frontier.rb, line 61
    def count
      @queue.length
    end

detect_invalid_blocks ()
Source

    # File lib/syntax_suggest/code_frontier.rb, line 172
    def detect_invalid_blocks
      self.class.combination(@queue.to_a.select(&:invalid?)).detect do |block_array|
        holds_all_syntax_errors?(block_array, can_cache: false)
      end || []
    end

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

expand? ()
Source

    # File lib/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

      if ENV["SYNTAX_SUGGEST_DEBUG"]
        puts "```"
        puts @queue.peek
        puts "```"
        puts "  @frontier indent:  #{frontier_indent}"
        puts "  @unvisited indent: #{unvisited_indent}"
      end

      # Expand all blocks before moving to unvisited lines
      frontier_indent >= unvisited_indent
    end

holds_all_syntax_errors? (block_array = @queue, can_cache: true)
Source

    # File lib/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|
        block.lines
      end

      SyntaxSuggest.valid_without?(
        without_lines: without_lines,
        code_lines: @code_lines
      )
    end

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

next_indent_line ()
Source

    # File lib/syntax_suggest/code_frontier.rb, line 107
    def next_indent_line
      @unvisited.peek
    end

pop ()
Source

    # File lib/syntax_suggest/code_frontier.rb, line 103
    def pop
      @queue.pop
    end

Returns a code block with the largest indentation possible

register_engulf_block (block)
Source

    # File lib/syntax_suggest/code_frontier.rb, line 140
    def register_engulf_block(block)
    end

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

register_indent_block (block)
Source

    # File lib/syntax_suggest/code_frontier.rb, line 132
    def register_indent_block(block)
      @unvisited.visit_block(block)
      self
    end

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

### Private Instance Methods

can_skip_check? ()
Source

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

      if check_next
        false
      else
        true
      end
    end

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