From fb1b8ba2c552cf873e7233abd09d1a8970f4ecb8 Mon Sep 17 00:00:00 2001
From: ydah <t.yudai92@gmail.com>
Date: Sat, 28 Mar 2026 22:59:41 +0900
Subject: [PATCH] Fix compute_follow_kernel_items to properly index bool array
 per kernel

Each kernel was incorrectly mapped to the full `Bitmap.to_bool_array`
result (`Array[bool]`) instead of its indexed value. Since arrays are
always truthy in Ruby, `follow_kernel_items[goto][kernel]` evaluated to
true for every kernel, making the filtering in `goto_follow_set`
(Definition 3.39) and `lhs_contributions` (Definition 3.31) ineffective.

The fix computes the bool array once and assigns each kernel its
corresponding bool value via index, which also eliminates redundant
`Bitmap.to_bool_array` calls (previously N calls per goto, now 1).
---
 lib/lrama/states.rb       |  5 +++--
 spec/lrama/states_spec.rb | 28 ++++++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/lib/lrama/states.rb b/lib/lrama/states.rb
index ddce627df..c3fa1735f 100644
--- a/lib/lrama/states.rb
+++ b/lib/lrama/states.rb
@@ -661,8 +661,9 @@ def compute_follow_kernel_items
       base_function = compute_goto_bitmaps
       Digraph.new(set, relation, base_function).compute.each do |goto, follow_kernel_items|
         state = goto.from_state
-        state.follow_kernel_items[goto] = state.kernels.map {|kernel|
-          [kernel, Bitmap.to_bool_array(follow_kernel_items, state.kernels.count)]
+        bool_array = Bitmap.to_bool_array(follow_kernel_items, state.kernels.count)
+        state.follow_kernel_items[goto] = state.kernels.each_with_index.map {|kernel, i|
+          [kernel, bool_array[i]]
         }.to_h
       end
     end
diff --git a/spec/lrama/states_spec.rb b/spec/lrama/states_spec.rb
index 28c217e2a..6aeeb3c77 100644
--- a/spec/lrama/states_spec.rb
+++ b/spec/lrama/states_spec.rb
@@ -397,6 +397,34 @@ class    go to state 5
     end
   end
 
+  describe '#compute_follow_kernel_items' do
+    it 'stores follow kernel items per kernel as booleans' do
+      path = "common/basic.y"
+      y = File.read(fixture_path(path))
+      grammar = Lrama::Parser.new(y, path).parse
+      grammar.prepare
+      grammar.validate!
+      states = Lrama::States.new(grammar, Lrama::Tracer.new(Lrama::Logger.new))
+      states.compute
+
+      state = states.states.find {|s| s.id == 1 }
+      goto = state.nterm_transitions.find {|transition| transition.next_sym.name == "$@1" }
+
+      allow(states).to receive(:nterm_transitions).and_return([goto])
+      allow(states).to receive(:compute_goto_internal_relation).and_return({ goto => [] })
+      allow(states).to receive(:compute_goto_bitmaps).and_return({ goto => Lrama::Bitmap.from_array([0, 2]) })
+
+      state.follow_kernel_items.clear
+      states.send(:compute_follow_kernel_items)
+
+      expect(state.follow_kernel_items[goto]).to eq({
+        state.kernels[0] => true,
+        state.kernels[1] => false,
+        state.kernels[2] => true,
+      })
+    end
+  end
+
   describe '#reads_relation' do
     it do
       path = "states/reads_relation.y"