diff --git a/lib/dead_end/api.rb b/lib/dead_end/api.rb
index f457862..0a52d28 100644
--- a/lib/dead_end/api.rb
+++ b/lib/dead_end/api.rb
@@ -190,7 +190,9 @@ def self.valid?(source)
 require_relative "block_expand"
 require_relative "ripper_errors"
 require_relative "priority_queue"
+require_relative "unvisited_lines"
 require_relative "around_block_scan"
+require_relative "priority_engulf_queue"
 require_relative "pathname_from_message"
 require_relative "display_invalid_blocks"
 require_relative "parse_blocks_from_indent_line"
diff --git a/lib/dead_end/code_frontier.rb b/lib/dead_end/code_frontier.rb
index 920ffcb..f9e6920 100644
--- a/lib/dead_end/code_frontier.rb
+++ b/lib/dead_end/code_frontier.rb
@@ -50,18 +50,16 @@ module DeadEnd
   #   CodeFrontier#detect_invalid_blocks
   #
   class CodeFrontier
-    def initialize(code_lines:)
+    def initialize(code_lines:, unvisited: UnvisitedLines.new(code_lines: code_lines))
       @code_lines = code_lines
-      @frontier = PriorityQueue.new
-      @unvisited_lines = @code_lines.sort_by(&:indent_index)
-      @visited_lines = {}
+      @unvisited = unvisited
+      @queue = PriorityEngulfQueue.new
 
-      @has_run = false
       @check_next = true
     end
 
     def count
-      @frontier.length
+      @queue.length
     end
 
     # Performance optimization
@@ -88,7 +86,7 @@ def count
     # removed. By default it checks all blocks in present in
     # the frontier array, but can be used for arbitrary arrays
     # of codeblocks as well
-    def holds_all_syntax_errors?(block_array = @frontier, can_cache: true)
+    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|
@@ -103,23 +101,23 @@ def holds_all_syntax_errors?(block_array = @frontier, can_cache: true)
 
     # Returns a code block with the largest indentation possible
     def pop
-      @frontier.pop
+      @queue.pop
     end
 
     def next_indent_line
-      @unvisited_lines.last
+      @unvisited.peek
     end
 
     def expand?
-      return false if @frontier.empty?
-      return true if @unvisited_lines.to_a.empty?
+      return false if @queue.empty?
+      return true if @unvisited.empty?
 
-      frontier_indent = @frontier.peek.current_indent
+      frontier_indent = @queue.peek.current_indent
       unvisited_indent = next_indent_line.indent
 
       if ENV["DEBUG"]
         puts "```"
-        puts @frontier.peek.to_s
+        puts @queue.peek.to_s
         puts "```"
         puts "  @frontier indent:  #{frontier_indent}"
         puts "  @unvisited indent: #{unvisited_indent}"
@@ -132,13 +130,7 @@ def expand?
     # Keeps track of what lines have been added to blocks and which are not yet
     # visited.
     def register_indent_block(block)
-      block.lines.each do |line|
-        next if @visited_lines[line]
-        @visited_lines[line] = true
-
-        index = @unvisited_lines.bsearch_index { |l| line.indent_index <=> l.indent_index }
-        @unvisited_lines.delete_at(index)
-      end
+      @unvisited.visit_block(block)
       self
     end
 
@@ -146,23 +138,6 @@ def register_indent_block(block)
     # block from the frontier. This prevents double expansions and all-around
     # weird behavior. However this guarantee is quite expensive to maintain
     def register_engulf_block(block)
-      # If we're about to pop off the same block, we can skip deleting
-      # things from the frontier this iteration since we'll get it
-      # on the next iteration
-      return if @frontier.peek && (block <=> @frontier.peek) == 1
-
-      if block.starts_at != block.ends_at # A block of size 1 cannot engulf another
-        @frontier.to_a.each { |b|
-          if b.starts_at >= block.starts_at && b.ends_at <= block.ends_at
-            b.delete
-            true
-          end
-        }
-      end
-
-      while (last = @frontier.peek) && last.deleted?
-        @frontier.pop
-      end
     end
 
     # Add a block to the frontier
@@ -171,11 +146,11 @@ def register_engulf_block(block)
     # and that each code block's lines are removed from the indentation hash so we
     # don't re-evaluate the same line multiple times.
     def <<(block)
-      register_indent_block(block)
-      register_engulf_block(block)
+      @unvisited.visit_block(block)
+
+      @queue.push(block)
 
       @check_next = true if block.invalid?
-      @frontier << block
 
       self
     end
@@ -195,7 +170,7 @@ def self.combination(array)
     # 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
     def detect_invalid_blocks
-      self.class.combination(@frontier.to_a.select(&:invalid?)).detect do |block_array|
+      self.class.combination(@queue.to_a.select(&:invalid?)).detect do |block_array|
         holds_all_syntax_errors?(block_array, can_cache: false)
       end || []
     end
diff --git a/lib/dead_end/priority_engulf_queue.rb b/lib/dead_end/priority_engulf_queue.rb
new file mode 100644
index 0000000..6254609
--- /dev/null
+++ b/lib/dead_end/priority_engulf_queue.rb
@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+module DeadEnd
+  # Keeps track of what elements are in the queue in
+  # priority and also ensures that when one element
+  # engulfs/covers/eats another that the larger element
+  # evicts the smaller element
+  class PriorityEngulfQueue
+    def initialize
+      @queue = PriorityQueue.new
+    end
+
+    def to_a
+      @queue.to_a
+    end
+
+    def empty?
+      @queue.empty?
+    end
+
+    def length
+      @queue.length
+    end
+
+    def peek
+      @queue.peek
+    end
+
+    def pop
+      @queue.pop
+    end
+
+    def push(block)
+      prune_engulf(block)
+      @queue << block
+      flush_deleted
+
+      self
+    end
+
+    private def flush_deleted
+      while @queue&.peek&.deleted?
+        @queue.pop
+      end
+    end
+
+    private def prune_engulf(block)
+      # If we're about to pop off the same block, we can skip deleting
+      # things from the frontier this iteration since we'll get it
+      # on the next iteration
+      return if @queue.peek && (block <=> @queue.peek) == 1
+
+      if block.starts_at != block.ends_at # A block of size 1 cannot engulf another
+        @queue.to_a.each { |b|
+          if b.starts_at >= block.starts_at && b.ends_at <= block.ends_at
+            b.delete
+            true
+          end
+        }
+      end
+    end
+  end
+end
diff --git a/lib/dead_end/unvisited_lines.rb b/lib/dead_end/unvisited_lines.rb
new file mode 100644
index 0000000..3f5b496
--- /dev/null
+++ b/lib/dead_end/unvisited_lines.rb
@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+module DeadEnd
+  # Tracks which lines various code blocks have expanded to
+  # and which are still unexplored
+  class UnvisitedLines
+    def initialize(code_lines:)
+      @unvisited = code_lines.sort_by(&:indent_index)
+      @visited_lines = {}
+      @visited_lines.compare_by_identity
+    end
+
+    def empty?
+      @unvisited.empty?
+    end
+
+    def peek
+      @unvisited.last
+    end
+
+    def pop
+      @unvisited.pop
+    end
+
+    def visit_block(block)
+      block.lines.each do |line|
+        next if @visited_lines[line]
+        @visited_lines[line] = true
+      end
+
+      while @visited_lines[@unvisited.last]
+        @unvisited.pop
+      end
+    end
+  end
+end