Compute the set of variables that can begin with each terminal symbol

src/compiler/build_tables/symbols_by_first_symbol.cc

@@ -0,0 +1,55 @@
+#include "compiler/build_tables/symbols_by_first_symbol.h"
+#include "compiler/syntax_grammar.h"
+#include "compiler/rules/symbol.h"
+
+namespace tree_sitter {
+namespace build_tables {
+
+using std::map;
+using std::set;
+using rules::Symbol;
+
+map<Symbol, set<Symbol>> symbols_by_first_symbol(const SyntaxGrammar &grammar) {
+  map<Symbol, set<Symbol>> result;
+
+  size_t variable_index = -1;
+  for (const SyntaxVariable &variable : grammar.variables) {
+    variable_index++;
+    Symbol symbol(variable_index);
+    result[symbol].insert(symbol);
+    for (const Production &production : variable.productions)
+      if (!production.empty()) {
+        Symbol first_symbol = production[0].symbol;
+        result[first_symbol].insert(symbol);
+        result[first_symbol].insert(first_symbol);
+      }
+  }
+
+  bool done = false;
+  while (!done) {
+    done = true;
+    for (auto &entry : result) {
+      set<Symbol> new_symbols;
+      for (const Symbol &symbol : entry.second)
+        for (const Symbol &other_symbol : result[symbol])
+          new_symbols.insert(other_symbol);
+
+      for (const Symbol &new_symbol : new_symbols)
+        if (entry.second.insert(new_symbol).second)
+          done = false;
+    }
+  }
+
+  for (auto iter = result.begin(), end = result.end(); iter != end;) {
+    if (!iter->first.is_token) {
+      result.erase(iter++);
+    } else {
+      iter++;
+    }
+  }
+
+  return result;
+}
+
+}  // namespace build_tables
+}  // namespace tree_sitter

src/compiler/build_tables/symbols_by_first_symbol.h

@@ -0,0 +1,19 @@
+#ifndef COMPILER_BUILD_TABLES_SYMBOLS_BY_FIRST_SYMBOL_H_
+#define COMPILER_BUILD_TABLES_SYMBOLS_BY_FIRST_SYMBOL_H_
+
+#include <map>
+#include <set>
+#include "compiler/rules/symbol.h"
+
+namespace tree_sitter {
+
+struct SyntaxGrammar;
+
+namespace build_tables {
+
+std::map<rules::Symbol, std::set<rules::Symbol>> symbols_by_first_symbol(const SyntaxGrammar &);
+
+}  // namespace build_tables
+}  // namespace tree_sitter
+
+#endif  // COMPILER_BUILD_TABLES_SYMBOLS_BY_FIRST_SYMBOL_H_