Prune unused grammars

script/prune-grammars will remove any grammars that aren't needed from
grammars.yml.

script/prune-grammars

@@ -0,0 +1,57 @@
+#!/usr/bin/env ruby
+
+require "json"
+require "linguist"
+require "set"
+require "yaml"
+
+def find_includes(json)
+  case json
+  when Hash
+    result = []
+    if inc = json["include"]
+      result << inc unless inc.start_with?("#", "$")
+    end
+    result + json.values.flat_map { |v| find_includes(v) }
+  when Array
+    json.flat_map { |v| find_includes(v) }
+  else
+    []
+  end
+end
+
+def transitive_includes(scope, includes)
+  scopes = Set.new
+  queue = includes[scope] || []
+  while s = queue.shift
+    next if scopes.include?(s)
+    scopes << s
+    queue += includes[s] || []
+  end
+  scopes
+end
+
+includes = {}
+Dir["grammars/*.json"].each do |path|
+  scope = File.basename(path).sub(/\.json/, '')
+  json = JSON.load(File.read(path))
+  incs = find_includes(json)
+  next if incs.empty?
+  includes[scope] ||= []
+  includes[scope] += incs
+end
+
+yaml = YAML.load(File.read("grammars.yml"))
+language_scopes = Linguist::Language.all.map(&:tm_scope).to_set
+
+# The set of used scopes is the scopes for each language, plus all the scopes
+# they include, transitively.
+used_scopes = language_scopes + language_scopes.flat_map { |s| transitive_includes(s, includes) }.to_set
+
+unused = yaml.reject { |repo, scopes| scopes.any? { |scope| used_scopes.include?(scope) } }
+
+puts "Unused grammar repos"
+puts unused.map { |repo, scopes| sprintf("%-100s %s", repo, scopes.join(", ")) }.sort.join("\n")
+
+yaml.delete_if { |k| unused.key?(k) }
+File.write("grammars.yml", YAML.dump(yaml))