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:

CodeFrontier#detect_invalid_blocks