Improve the "gn path" command.

tools/gn/command_path.cc

Index: tools/gn/command_path.cc
diff --git a/tools/gn/command_path.cc b/tools/gn/command_path.cc
index 2368788d581b698e9287510e529297e63f579da8..a17001dd6c684491755dd19f5f38de344cc11a23 100644
--- a/tools/gn/command_path.cc
+++ b/tools/gn/command_path.cc
@@ -17,166 +17,259 @@ namespace commands {
 
 namespace {
 
-enum DepType {
-  DEP_NONE,
-  DEP_PUBLIC,
-  DEP_PRIVATE,
-  DEP_DATA
+enum class DepType {
+  NONE,
+  PUBLIC,
+  PRIVATE,
+  DATA
 };
 
-// As we do a depth-first search, this vector will store the current path
-// the current target for printing when a match is found.
+// The dependency paths are stored in a vector. Assuming the chain:
+//    A --[public]--> B --[private]--> C
+// The stack will look like:
+//    [0] = A, NONE (this has no dep type since nobody depends on it)
+//    [1] = B, PUBLIC
+//    [2] = C, PRIVATE
 using TargetDep = std::pair<const Target*, DepType>;
-using DepStack = std::vector<TargetDep>;
+using PathVector = std::vector<TargetDep>;
 
-// Note that this uses raw pointers. These need to be manually deleted (which
-// we won't normally bother with). This allows the vector to be resized
-// more quickly.
-using DepStackVector = std::vector<DepStack*>;
-
-using DepSet = base::hash_set<const Target*>;
+// How to search.
+enum class PrivateDeps { INCLUDE, EXCLUDE };
+enum class DataDeps { INCLUDE, EXCLUDE };
+enum class PrintWhat { ONE, ALL };
 
 struct Options {
   Options()
-      : all(false),
+      : print_what(PrintWhat::ONE),
         public_only(false),
         with_data(false) {
   }
 
-  bool all;
+  PrintWhat print_what;
   bool public_only;
   bool with_data;
 };
 
-struct State {
-  State() : found_count(0) {
-    // Reserve fairly large buffers for the found vectors.
-    const size_t kReserveSize = 32768;
-    found_public.reserve(kReserveSize);
-    found_other.reserve(kReserveSize);
+typedef std::list<PathVector> WorkQueue;
+
+struct Stats {
+  Stats() : public_paths(0), other_paths(0) {
   }
 
-  // Stores targets that do not have any paths to the destination. This is
-  // an optimization to avoid revisiting useless paths.
-  DepSet rejected;
+  int total_paths() const { return public_paths + other_paths; }
 
-  // Total number of paths found.
-  int found_count;
+  int public_paths;
+  int other_paths;
 
-  // The pointers in these vectors are owned by this object, but are
-  // deliberately leaked. There can be a lot of them which can take a long time
-  // to free, and GN will just exit after this is used anyway.
-  DepStackVector found_public;
-  DepStackVector found_other;
+  // Stores targets that have a path to the destination, and whether that
+  // path is public, private, or data.
+  std::map<const Target*, DepType> found_paths;
 };
 
-void PrintDepStack(const DepStack& stack) {
-  // Don't print toolchains unless they differ from the first target.
-  const Label& default_toolchain = stack[0].first->label().GetToolchainLabel();
-
-  for (const auto& pair : stack) {
-    OutputString(pair.first->label().GetUserVisibleName(default_toolchain));
-    switch (pair.second) {
-      case DEP_NONE:
-        break;
-      case DEP_PUBLIC:
-        OutputString(" --[public]-->", DECORATION_DIM);
-        break;
-      case DEP_PRIVATE:
-        OutputString(" --[private]-->", DECORATION_DIM);
-        break;
-      case DEP_DATA:
-        OutputString(" --[data]-->", DECORATION_DIM);
-        break;
+// If the implicit_last_dep is not "none", this type indicates the
+// classification of the elided last part of path.
+DepType ClassifyPath(const PathVector& path, DepType implicit_last_dep) {
+  DepType result;
+  if (implicit_last_dep != DepType::NONE)
+    result = implicit_last_dep;
+  else
+    result = DepType::PUBLIC;
+
+  // Skip the 0th one since that is always NONE.
+  for (size_t i = 1; i < path.size(); i++) {
+    // PRIVATE overrides PUBLIC, and DATA overrides everything (the idea is
+    // to find the worst link in the path).
+    if (path[i].second == DepType::PRIVATE) {
+      if (result == DepType::PUBLIC)
+        result = DepType::PRIVATE;
+    } else if (path[i].second == DepType::DATA) {
+      result = DepType::DATA;
     }
-    OutputString("\n");
   }
-  OutputString("\n");
+  return result;
 }
 
-bool AreAllPublic(const DepStack& stack) {
-  // Don't check the type of the last one since that doesn't point to anything.
-  for (size_t i = 0; i < stack.size() - 1; i++) {
-    if (stack[i].second != DEP_PUBLIC)
-      return false;
+const char* StringForDepType(DepType type) {
+  switch(type) {
+    case DepType::PUBLIC:
+      return "public";
+    case DepType::PRIVATE:
+      return "private";
+    case DepType::DATA:
+      return "data";
+      break;
+    case DepType::NONE:
+    default:
+      return "";
   }
-  return true;
 }
 
-// Increments *found_count to reflect how many results are found. If print_all
-// is not set, only the first result will be printed.
-//
-// As an optimization, targets that do not have any paths are added to
-// *reject so this function doesn't waste time revisiting them.
-void RecursiveFindPath(const Options& options,
-                       State* state,
-                       const Target* current,
-                       const Target* desired,
-                       DepStack* stack) {
-  if (state->rejected.find(current) != state->rejected.end())
-    return;
-  int initial_found_count = state->found_count;
-
-  if (current == desired) {
-    // Found a path.
-    state->found_count++;
-    stack->push_back(TargetDep(current, DEP_NONE));
-    if (AreAllPublic(*stack))
-      state->found_public.push_back(new DepStack(*stack));
-    else
-      state->found_other.push_back(new DepStack(*stack));
-    stack->pop_back();
+// Prints the given path. If the implicit_last_dep is not "none", the last
+// dependency will show an elided dependency with the given annotation.
+void PrintPath(const PathVector& path, DepType implicit_last_dep) {
+  if (path.empty())
     return;
+
+  // Don't print toolchains unless they differ from the first target.
+  const Label& default_toolchain = path[0].first->label().GetToolchainLabel();
+
+  for (size_t i = 0; i < path.size(); i++) {
+    OutputString(path[i].first->label().GetUserVisibleName(default_toolchain));
+
+    // Output dependency type.
+    if (i == path.size() - 1) {
+      // Last one either gets the implicit last dep type or nothing.
+      if (implicit_last_dep != DepType::NONE) {
+        OutputString(std::string(" --> see ") +
+                     StringForDepType(implicit_last_dep) +
+                     " chain printed above...", DECORATION_DIM);
+      }
+    } else {
+      // Take type from the next entry.
+      OutputString(std::string(" --[") + StringForDepType(path[i + 1].second) +
+                   "]-->", DECORATION_DIM);
+    }
+    OutputString("\n");
   }
 
-  stack->push_back(TargetDep(current, DEP_PUBLIC));
-  for (const auto& pair : current->public_deps())
-    RecursiveFindPath(options, state, pair.ptr, desired, stack);
+  OutputString("\n");
+}
 
-  if (!options.public_only) {
-    stack->back().second = DEP_PRIVATE;
-    for (const auto& pair : current->private_deps())
-      RecursiveFindPath(options, state, pair.ptr, desired, stack);
+void InsertTargetsIntoFoundPaths(const PathVector& path,
+                                 DepType implicit_last_dep,
+                                 Stats* stats) {
+  DepType type = ClassifyPath(path, implicit_last_dep);
+
+  bool inserted = false;
+
+  // Don't try to insert the 0th item in the list which is the "from" target.
+  // The search will be run more than once (for the different path types) and
+  // if the "from" target was in the list, subsequent passes could never run
+  // the starting point is alredy in the list of targets considered).
+  //
+  // One might imagine an alternate implementation where all items are counted
+  // here but the "from" item is erased at the beginning of each search, but
+  // that will mess up the metrics (the private search pass will find the
+  // same public paths as the previous public pass, "inserted" will be true
+  // here since the item wasn't found, and the public path will be
+  // double-counted in the stats.
+  for (size_t i = 1; i < path.size(); i++) {
+    const auto& pair = path[i];
+
+    // Don't overwrite an existing one. The algorithm works by first doing
+    // public, then private, then data, so anything already there is guaranteed
+    // at least as good as our addition.
+    if (stats->found_paths.find(pair.first) == stats->found_paths.end()) {
+      stats->found_paths.insert(std::make_pair(pair.first, type));
+      inserted = true;
+    }
   }
 
-  if (options.with_data) {
-    stack->back().second = DEP_DATA;
-    for (const auto& pair : current->data_deps())
-      RecursiveFindPath(options, state, pair.ptr, desired, stack);
+  if (inserted) {
+    // Only count this path in the stats if any part of it was actually new.
+    if (type == DepType::PUBLIC)
+      stats->public_paths++;
+    else
+      stats->other_paths++;
   }
+}
+
+void BreadthFirstSearch(const Target* from, const Target* to,
+                        PrivateDeps private_deps, DataDeps data_deps,
+                        PrintWhat print_what,
+                        Stats* stats) {
+  // Seed the initial stack with just the "from" target.
+  PathVector initial_stack;
+  initial_stack.emplace_back(from, DepType::NONE);
+  WorkQueue work_queue;
+  work_queue.push_back(initial_stack);
+
+  // Track checked targets to avoid checking the same once more than once.
+  std::set<const Target*> visited;
+
+  while (!work_queue.empty()) {
+    PathVector current_path = work_queue.front();
+    work_queue.pop_front();
+
+    const Target* current_target = current_path.back().first;
+
+    if (current_target == to) {
+      // Found a new path.
+      if (stats->total_paths() == 0 || print_what == PrintWhat::ALL)
+        PrintPath(current_path, DepType::NONE);
+
+      // Insert all nodes on the path into the found paths list. Since we're
+      // doing search breadth first, we know that the current path is the best
+      // path for all nodes on it.
+      InsertTargetsIntoFoundPaths(current_path, DepType::NONE, stats);
+    } else {
+      // Check for a path that connects to an already known-good one. Printing
+      // this here will mean the results aren't strictly in depth-first order
+      // since there could be many items on the found path this connects to.
+      // Doing this here will mean that the output is sorted by length of items
+      // printed (with the redundant parts of the path omitted) rather than
+      // complete path length.
+      const auto& found_current_target =
+          stats->found_paths.find(current_target);
+      if (found_current_target != stats->found_paths.end()) {
+        if (stats->total_paths() == 0 || print_what == PrintWhat::ALL)
+          PrintPath(current_path, found_current_target->second);
+
+        // Insert all nodes on the path into the found paths list since we know
+        // everything along this path also leads to the destination.
+        InsertTargetsIntoFoundPaths(current_path, found_current_target->second,
+                                    stats);
+        continue;
+      }
+    }
 
-  stack->pop_back();
+    // If we've already checked this one, stop. This should be after the above
+    // check for a known-good check, because known-good ones will always have
+    // been previously visited.
+    if (visited.find(current_target) == visited.end())
+      visited.insert(current_target);
+    else
+      continue;
 
-  if (state->found_count == initial_found_count)
-    state->rejected.insert(current);  // Eliminated this target.
-}
+    // Add public deps for this target to the queue.
+    for (const auto& pair : current_target->public_deps()) {
+      work_queue.push_back(current_path);
+      work_queue.back().push_back(TargetDep(pair.ptr, DepType::PUBLIC));
+    }
 
-bool StackLengthLess(const DepStack* a, const DepStack* b) {
-  return a->size() < b->size();
-}
+    if (private_deps == PrivateDeps::INCLUDE) {
+      // Add private deps.
+      for (const auto& pair : current_target->private_deps()) {
+        work_queue.push_back(current_path);
+        work_queue.back().push_back(
+            TargetDep(pair.ptr, DepType::PRIVATE));
+      }
+    }
 
-// Prints one result vector. The vector will be modified.
-void PrintResultVector(const Options& options, DepStackVector* result) {
-  if (!options.all && !result->empty()) {
-    // Just print the smallest one.
-    PrintDepStack(**std::min_element(result->begin(), result->end(),
-                                     &StackLengthLess));
-    return;
+    if (data_deps == DataDeps::INCLUDE) {
+      // Add data deps.
+      for (const auto& pair : current_target->data_deps()) {
+        work_queue.push_back(current_path);
+        work_queue.back().push_back(TargetDep(pair.ptr, DepType::DATA));
+      }
+    }
   }
-
-  // Print all in order of increasing length.
-  std::sort(result->begin(), result->end(), &StackLengthLess);
-  for (const auto& stack : *result)
-    PrintDepStack(*stack);
 }
 
-void PrintResults(const Options& options, State* state) {
-  PrintResultVector(options, &state->found_public);
-
-  // Consider non-public paths only if all paths are requested or there were
-  // no public paths.
-  if (state->found_public.empty() || options.all)
-    PrintResultVector(options, &state->found_other);
+void DoSearch(const Target* from, const Target* to, const Options& options,
+              Stats* stats) {
+  BreadthFirstSearch(from, to, PrivateDeps::EXCLUDE, DataDeps::EXCLUDE,
+                     options.print_what, stats);
+  if (!options.public_only) {
+    // Check private deps.
+    BreadthFirstSearch(from, to, PrivateDeps::INCLUDE,
+                       DataDeps::EXCLUDE, options.print_what, stats);
+    if (options.with_data) {
+      // Check data deps.
+      BreadthFirstSearch(from, to, PrivateDeps::INCLUDE,
+                         DataDeps::INCLUDE, options.print_what, stats);
+    }
+  }
 }
 
 }  // namespace
@@ -189,8 +282,8 @@ const char kPath_Help[] =
     "\n"
     "  Finds paths of dependencies between two targets. Each unique path\n"
     "  will be printed in one group, and groups will be separate by newlines.\n"
-    "  The two targets can appear in either order: paths will be found going\n"
-    "  in either direction.\n"
+    "  The two targets can appear in either order (paths will be found going\n"
+    "  in either direction).\n"
     "\n"
     "  By default, a single path will be printed. If there is a path with\n"
     "  only public dependencies, the shortest public path will be printed.\n"
@@ -199,12 +292,19 @@ const char kPath_Help[] =
     "  will also be considered. If there are multiple shortest paths, an\n"
     "  arbitrary one will be selected.\n"
     "\n"
+    "Interesting paths\n"
+    "\n"
+    "  In a large project, there can be 100's of millions of unique paths\n"
+    "  between a very high level and a common low-level target. To make the\n"
+    "  output more useful (and terminate in a reasonable time), GN will not\n"
+    "  revisit sub-paths previously known to lead to the target.\n"
+    "\n"
     "Options\n"
     "\n"
     "  --all\n"
-    "     Prints all paths found rather than just the first one. Public paths\n"
-    "     will be printed first in order of increasing length, followed by\n"
-    "     non-public paths in order of increasing length.\n"
+    "     Prints all \"interesting\" paths found rather than just the first\n"
+    "     one. Public paths will be printed first in order of increasing\n"
+    "     length, followed by non-public paths in order of increasing length.\n"
     "\n"
     "  --public\n"
     "     Considers only public paths. Can't be used with --with-data.\n"
@@ -239,7 +339,8 @@ int RunPath(const std::vector<std::string>& args) {
     return 1;
 
   Options options;
-  options.all = base::CommandLine::ForCurrentProcess()->HasSwitch("all");
+  options.print_what = base::CommandLine::ForCurrentProcess()->HasSwitch("all")
+      ? PrintWhat::ALL : PrintWhat::ONE;
   options.public_only =
       base::CommandLine::ForCurrentProcess()->HasSwitch("public");
   options.with_data =
@@ -251,21 +352,15 @@ int RunPath(const std::vector<std::string>& args) {
     return 1;
   }
 
-  // If we don't find a path going "forwards", try the reverse direction. Deps
-  // can only go in one direction without having a cycle, which will have
-  // caused a run failure above.
-  State state;
-  DepStack stack;
-  RecursiveFindPath(options, &state, target1, target2, &stack);
-  if (state.found_count == 0) {
-    // Need to reset the rejected set for a new invocation since the reverse
-    // search will revisit the same targets looking for something else.
-    state.rejected.clear();
-    RecursiveFindPath(options, &state, target2, target1, &stack);
+  Stats stats;
+  DoSearch(target1, target2, options, &stats);
+  if (stats.total_paths() == 0) {
+    // If we don't find a path going "forwards", try the reverse direction.
+    // Deps can only go in one direction without having a cycle, which will
+    // have caused a run failure above.
+    DoSearch(target2, target1, options, &stats);
   }
 
-  PrintResults(options, &state);
-
   // This string is inserted in the results to annotate whether the result
   // is only public or includes data deps or not.
   const char* path_annotation = "";
@@ -274,42 +369,42 @@ int RunPath(const std::vector<std::string>& args) {
   else if (!options.with_data)
     path_annotation = "non-data ";
 
-  if (state.found_count == 0) {
+  if (stats.total_paths() == 0) {
     // No results.
     OutputString(base::StringPrintf(
         "No %spaths found between these two targets.\n", path_annotation),
         DECORATION_YELLOW);
-  } else if (state.found_count == 1) {
+  } else if (stats.total_paths() == 1) {
     // Exactly one result.
     OutputString(base::StringPrintf("1 %spath found.", path_annotation),
                  DECORATION_YELLOW);
     if (!options.public_only) {
-      if (state.found_public.empty())
-        OutputString(" It is not public.");
-      else
+      if (stats.public_paths)
         OutputString(" It is public.");
+      else
+        OutputString(" It is not public.");
     }
     OutputString("\n");
   } else {
-    if (options.all) {
+    if (options.print_what == PrintWhat::ALL) {
       // Showing all paths when there are many.
-      OutputString(base::StringPrintf("%d unique %spaths found.",
-                                      state.found_count, path_annotation),
+      OutputString(base::StringPrintf("%d \"interesting\" %spaths found.",
+                                      stats.total_paths(), path_annotation),
                    DECORATION_YELLOW);
       if (!options.public_only) {
         OutputString(base::StringPrintf(" %d of them are public.",
-            static_cast<int>(state.found_public.size())));
+                                        stats.public_paths));
       }
       OutputString("\n");
     } else {
       // Showing one path when there are many.
       OutputString(
-          base::StringPrintf("Showing one of %d unique %spaths.",
-                             state.found_count, path_annotation),
+          base::StringPrintf("Showing one of %d \"interesting\" %spaths.",
+                             stats.total_paths(), path_annotation),
           DECORATION_YELLOW);
       if (!options.public_only) {
         OutputString(base::StringPrintf(" %d of them are public.\n",
-            static_cast<int>(state.found_public.size())));
+                                        stats.public_paths));
       }
       OutputString("Use --all to print all paths.\n");
     }