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node_modules/tar/lib/get-write-flag.js

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+// Get the appropriate flag to use for creating files
+// We use fmap on Windows platforms for files less than
+// 512kb.  This is a fairly low limit, but avoids making
+// things slower in some cases.  Since most of what this
+// library is used for is extracting tarballs of many
+// relatively small files in npm packages and the like,
+// it can be a big boost on Windows platforms.
+// Only supported in Node v12.9.0 and above.
+const platform = process.env.__FAKE_PLATFORM__ || process.platform
+const isWindows = platform === 'win32'
+const fs = global.__FAKE_TESTING_FS__ || require('fs')
+
+/* istanbul ignore next */
+const { O_CREAT, O_TRUNC, O_WRONLY, UV_FS_O_FILEMAP = 0 } = fs.constants
+
+const fMapEnabled = isWindows && !!UV_FS_O_FILEMAP
+const fMapLimit = 512 * 1024
+const fMapFlag = UV_FS_O_FILEMAP | O_TRUNC | O_CREAT | O_WRONLY
+module.exports = !fMapEnabled ? () => 'w'
+  : size => size < fMapLimit ? fMapFlag : 'w'