feat(prefetch): added tarball prefetch support

index.js

@@ -2,5 +2,6 @@
 
 module.exports = {
   extract: require('./extract'),
-  manifest: require('./manifest')
+  manifest: require('./manifest'),
+  prefetch: require('./prefetch')
 }

prefetch.js

@@ -0,0 +1,46 @@
+'use strict'
+
+var cache = require('./lib/cache')
+var finished = require('mississippi').finished
+var optCheck = require('./lib/util/opt-check')
+var rps = require('realize-package-specifier')
+
+module.exports = prefetch
+function prefetch (spec, opts, cb) {
+  if (!cb) {
+    cb = opts
+    opts = null
+  }
+  opts = optCheck(opts)
+  if (!opts.cache) {
+    opts.log.info('prefetch', 'skipping prefetch: no cache provided')
+    setImmediate(function () { cb() })
+  }
+  if (opts.digest) {
+    opts.log.silly('prefetch', 'checking if ', spec, ' digest is already cached')
+    cache.get.hasContent(opts.cache, opts.digest, function (err, exists) {
+      if (err) { return cb(err) }
+      if (exists) {
+        opts.log.silly('prefetch', 'content already exists for', spec)
+        return cb(null)
+      } else {
+        return prefetchByManifest(spec, opts, cb)
+      }
+    })
+  } else {
+    opts.log.silly('prefetch', 'no digest provided for ', spec, '- fetching by manifest')
+    prefetchByManifest(spec, opts, cb)
+  }
+}
+
+function prefetchByManifest (spec, opts, cb) {
+  rps(spec, function (err, res) {
+    if (err) { return cb(err) }
+    var stream = require('./lib/handlers/' + res.type + '/tarball')(res, opts)
+    finished(stream, function (err) {
+      opts.log.silly('prefetch', 'prefetch finished for', spec)
+      cb(err)
+    })
+    stream.on('data', function () {})
+  })
+}