feat: add _seek support

leveldown.js

@@ -30,6 +30,10 @@ SubIterator.prototype._next = function (cb) {
   })
 }
 
+SubIterator.prototype._seek = function (key) {
+  this.iterator.seek(concat(this.prefix, key))
+}
+
 SubIterator.prototype._end = function (cb) {
   this.iterator.end(cb)
 }

test/index.js

@@ -248,4 +248,39 @@ test('SubDb main function', function (t) {
       })
     })
   })
+  t.test('can arbitrarily seek', function (t) {
+    t.plan(7)
+
+    var db = levelup(memdown())
+    var sub = subdb(db, 'sub')
+
+    db.once('open', function () {
+      var it = sub.iterator({ keyAsBuffer: false, valueAsBuffer: false })
+      sub.batch([
+        { type: 'put', key: 'a', value: 'A' },
+        { type: 'put', key: 'b', value: 'B' },
+        { type: 'put', key: 'c', value: 'C' },
+        { type: 'put', key: 'd', value: 'D' },
+        { type: 'put', key: 'e', value: 'E' }
+      ], (err) => {
+        t.error(err, 'no error')
+        it.seek('c')
+        it.next((err, key, value) => {
+          t.error(err, 'no error')
+          t.same(key, 'c', 'key is as expected')
+          it.seek('d')
+          it.next((err, key, value) => {
+            t.error(err, 'no error')
+            t.same(key, 'd', 'key is as expected')
+            it.seek('a')
+            it.next((err, key, value) => {
+              t.error(err, 'no error')
+              t.same(key, 'a', 'key is as expected')
+              t.end()
+            })
+          })
+        })
+      })
+    })
+  })
 })