diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
index b7a5e6fa..ad5afab1 100644
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -1,145 +1,146 @@
 <?php
 
 /**
  * This file is automatically generated. Use 'phutil_mapper.php' to rebuild it.
  * @generated
  */
 
 phutil_register_library_map(array(
   'class' =>
   array(
     'CommandException' => 'future/exec',
     'ConduitClient' => 'conduit/client',
     'ConduitClientException' => 'conduit/client',
     'ConduitFuture' => 'conduit/client',
     'ExecFuture' => 'future/exec',
     'FileFinder' => 'filesystem/filefinder',
     'FileList' => 'filesystem/filelist',
     'Filesystem' => 'filesystem',
     'FilesystemException' => 'filesystem',
     'Future' => 'future',
     'FutureIterator' => 'future',
     'FutureProxy' => 'future/proxy',
     'HTTPFuture' => 'future/http',
     'HTTPSFuture' => 'future/https',
+    'LinesOfALargeFile' => 'filesystem/linesofalargefile',
     'PhutilConsoleFormatter' => 'console',
     'PhutilDaemon' => 'daemon/base',
     'PhutilDaemonOverseer' => 'daemon/overseer',
     'PhutilDefaultSyntaxHighlighterEngine' => 'markup/syntax/engine/default',
     'PhutilDocblockParser' => 'parser/docblock',
     'PhutilDocblockParserTestCase' => 'parser/docblock/__tests__',
     'PhutilFatalDaemon' => 'daemon/torture/fatal',
     'PhutilHangForeverDaemon' => 'daemon/torture/hangforever',
     'PhutilInteractiveEditor' => 'console/editor',
     'PhutilMarkupEngine' => 'markup/engine',
     'PhutilMissingSymbolException' => 'symbols/exception/missing',
     'PhutilNiceDaemon' => 'daemon/torture/nice',
     'PhutilProcessGroupDaemon' => 'daemon/torture/processgroup',
     'PhutilRemarkupBlockStorage' => 'markup/engine/remarkup/blockstorage',
     'PhutilRemarkupEngine' => 'markup/engine/remarkup',
     'PhutilRemarkupEngineBlockRule' => 'markup/engine/remarkup/blockrule/base',
     'PhutilRemarkupEngineRemarkupCodeBlockRule' => 'markup/engine/remarkup/blockrule/remarkupcode',
     'PhutilRemarkupEngineRemarkupDefaultBlockRule' => 'markup/engine/remarkup/blockrule/remarkupdefault',
     'PhutilRemarkupEngineRemarkupHeaderBlockRule' => 'markup/engine/remarkup/blockrule/remarkupheader',
     'PhutilRemarkupEngineRemarkupInlineBlockRule' => 'markup/engine/remarkup/blockrule/remarkupinline',
     'PhutilRemarkupEngineRemarkupListBlockRule' => 'markup/engine/remarkup/blockrule/remarkuplist',
     'PhutilRemarkupRule' => 'markup/engine/remarkup/markuprule/base',
     'PhutilRemarkupRuleBold' => 'markup/engine/remarkup/markuprule/bold',
     'PhutilRemarkupRuleEscapeHTML' => 'markup/engine/remarkup/markuprule/escapehtml',
     'PhutilRemarkupRuleEscapeRemarkup' => 'markup/engine/remarkup/markuprule/escaperemarkup',
     'PhutilRemarkupRuleHyperlink' => 'markup/engine/remarkup/markuprule/hyperlink',
     'PhutilRemarkupRuleItalic' => 'markup/engine/remarkup/markuprule/italics',
     'PhutilRemarkupRuleLinebreaks' => 'markup/engine/remarkup/markuprule/linebreaks',
     'PhutilRemarkupRuleMonospace' => 'markup/engine/remarkup/markuprule/monospace',
     'PhutilSaturateStdoutDaemon' => 'daemon/torture/saturatestdout',
     'PhutilSymbolLoader' => 'symbols',
     'PhutilSyntaxHighlighter' => 'markup/syntax/highlighter/base',
     'PhutilSyntaxHighlighterEngine' => 'markup/syntax/engine/base',
     'PhutilTortureTestDaemon' => 'daemon/torture/base',
     'PhutilURI' => 'parser/uri',
     'PhutilXHPASTSyntaxHighlighter' => 'markup/syntax/highlighter/xhpast',
     'TempFile' => 'filesystem/tempfile',
     'XHPASTNode' => 'parser/xhpast/api/node',
     'XHPASTNodeList' => 'parser/xhpast/api/list',
     'XHPASTSyntaxErrorException' => 'parser/xhpast/api/exception',
     'XHPASTToken' => 'parser/xhpast/api/token',
     'XHPASTTree' => 'parser/xhpast/api/tree',
     'XHPASTTreeTestCase' => 'parser/xhpast/api/tree/__tests__',
   ),
   'function' =>
   array(
     'Futures' => 'future',
     'array_select_keys' => 'utils',
     'coalesce' => 'utils',
     'csprintf' => 'xsprintf/csprintf',
     'exec_manual' => 'future/exec',
     'execx' => 'future/exec',
     'id' => 'utils',
     'idx' => 'utils',
     'igroup' => 'utils',
     'ipull' => 'utils',
     'isort' => 'utils',
     'jsprintf' => 'xsprintf/jsprintf',
     'mgroup' => 'utils',
     'mpull' => 'utils',
     'msort' => 'utils',
     'newv' => 'utils',
     'nonempty' => 'utils',
     'phutil_autoload_class' => 'autoload',
     'phutil_console_confirm' => 'console',
     'phutil_console_format' => 'console',
     'phutil_console_prompt' => 'console',
     'phutil_console_wrap' => 'console',
     'phutil_escape_html' => 'markup',
     'phutil_escape_uri' => 'markup',
     'phutil_get_library_name_for_root' => 'moduleutils',
     'phutil_get_library_root' => 'moduleutils',
     'phutil_get_library_root_for_path' => 'moduleutils',
     'phutil_render_tag' => 'markup',
     'vcsprintf' => 'xsprintf/csprintf',
     'vjsprintf' => 'xsprintf/jsprintf',
     'xhp_parser_node_constants' => 'parser/xhpast/constants',
     'xhpast_get_binary_path' => 'parser/xhpast/bin',
     'xhpast_get_build_instructions' => 'parser/xhpast/bin',
     'xhpast_get_parser_future' => 'parser/xhpast/bin',
     'xhpast_is_available' => 'parser/xhpast/bin',
     'xhpast_parser_token_constants' => 'parser/xhpast/constants',
     'xsprintf' => 'xsprintf',
     'xsprintf_callback_example' => 'xsprintf',
     'xsprintf_command' => 'xsprintf/csprintf',
     'xsprintf_javascript' => 'xsprintf/jsprintf',
   ),
   'requires_class' =>
   array(
     'ConduitFuture' => 'FutureProxy',
     'ExecFuture' => 'Future',
     'FutureProxy' => 'Future',
     'HTTPFuture' => 'Future',
     'HTTPSFuture' => 'Future',
     'PhutilDefaultSyntaxHighlighterEngine' => 'PhutilSyntaxHighlighterEngine',
     'PhutilDocblockParserTestCase' => 'ArcanistPhutilTestCase',
     'PhutilFatalDaemon' => 'PhutilTortureTestDaemon',
     'PhutilHangForeverDaemon' => 'PhutilTortureTestDaemon',
     'PhutilNiceDaemon' => 'PhutilTortureTestDaemon',
     'PhutilProcessGroupDaemon' => 'PhutilTortureTestDaemon',
     'PhutilRemarkupEngine' => 'PhutilMarkupEngine',
     'PhutilRemarkupEngineRemarkupCodeBlockRule' => 'PhutilRemarkupEngineBlockRule',
     'PhutilRemarkupEngineRemarkupDefaultBlockRule' => 'PhutilRemarkupEngineBlockRule',
     'PhutilRemarkupEngineRemarkupHeaderBlockRule' => 'PhutilRemarkupEngineBlockRule',
     'PhutilRemarkupEngineRemarkupInlineBlockRule' => 'PhutilRemarkupEngineBlockRule',
     'PhutilRemarkupEngineRemarkupListBlockRule' => 'PhutilRemarkupEngineBlockRule',
     'PhutilRemarkupRuleBold' => 'PhutilRemarkupRule',
     'PhutilRemarkupRuleEscapeHTML' => 'PhutilRemarkupRule',
     'PhutilRemarkupRuleEscapeRemarkup' => 'PhutilRemarkupRule',
     'PhutilRemarkupRuleHyperlink' => 'PhutilRemarkupRule',
     'PhutilRemarkupRuleItalic' => 'PhutilRemarkupRule',
     'PhutilRemarkupRuleLinebreaks' => 'PhutilRemarkupRule',
     'PhutilRemarkupRuleMonospace' => 'PhutilRemarkupRule',
     'PhutilSaturateStdoutDaemon' => 'PhutilTortureTestDaemon',
     'PhutilTortureTestDaemon' => 'PhutilDaemon',
     'XHPASTTreeTestCase' => 'ArcanistPhutilTestCase',
   ),
   'requires_interface' =>
   array(
   ),
 ));
diff --git a/src/filesystem/linesofalargefile/LinesOfALargeFile.php b/src/filesystem/linesofalargefile/LinesOfALargeFile.php
new file mode 100644
index 00000000..01552773
--- /dev/null
+++ b/src/filesystem/linesofalargefile/LinesOfALargeFile.php
@@ -0,0 +1,107 @@
+<?php
+
+/*
+ * Copyright 2011 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * Read the lines of a large file. Experimental and probably buggy.
+ *
+ * @group filesystem
+ */
+final class LinesOfALargeFile implements Iterator {
+
+  private $file;
+  private $handle;
+
+  private $pos;
+  private $buf;
+  private $num;
+  private $line;
+  private $valid;
+
+  public function __construct($file) {
+    $this->file = $file;
+  }
+
+  public function __destruct() {
+    $this->closeHandle();
+  }
+
+  private function closeHandle() {
+    if ($this->handle) {
+      fclose($this->handle);
+      $this->handle = null;
+    }
+  }
+
+  public function rewind() {
+    $this->closeHandle();
+    $this->handle = @fopen($this->file, 'r');
+    if (!$this->handle) {
+      throw new FilesystemException(
+        $this->file,
+        "Failed to open file!");
+    }
+
+    $this->buf = '';
+    $this->pos = 0;
+    $this->num = 0;
+    $this->valid = true;
+
+    $this->next();
+  }
+
+  public function key() {
+    return $this->num;
+  }
+
+  public function current() {
+    return $this->line;
+  }
+
+  public function valid() {
+    return $this->valid;
+  }
+
+  public function next() {
+    if (strlen($this->buf)) {
+      $endl = strpos($this->buf, "\n", $this->pos);
+      if ($endl !== false) {
+        $this->num++;
+        $this->line = substr($this->buf, $this->pos, ($endl - $this->pos));
+        $this->pos = $endl + 1;
+        return;
+      }
+      $this->buf = substr($this->buf, $this->pos);
+      $this->pos = 0;
+    }
+
+    $more = fread($this->handle, 2048);
+    if (strlen($more)) {
+      $this->buf .= $more;
+      $this->next();
+    } else {
+      if (strlen($this->buf)) {
+        $this->num++;
+        $this->line = $this->buf;
+        $this->buf = null;
+      } else {
+        $this->valid = false;
+      }
+    }
+  }
+
+}
diff --git a/src/filesystem/linesofalargefile/__init__.php b/src/filesystem/linesofalargefile/__init__.php
new file mode 100644
index 00000000..6b9ee344
--- /dev/null
+++ b/src/filesystem/linesofalargefile/__init__.php
@@ -0,0 +1,12 @@
+<?php
+/**
+ * This file is automatically generated. Lint this module to rebuild it.
+ * @generated
+ */
+
+
+
+phutil_require_module('phutil', 'filesystem');
+
+
+phutil_require_source('LinesOfALargeFile.php');