perf: optimize prefix search (#25)

* feat: optimise prefix search

Signed-off-by: Carson Farmer <carson.farmer@gmail.com>

* style: fix linter issue

Signed-off-by: Carson Farmer <carson.farmer@gmail.com>

* chore: remove avoid reaching into internal db

Signed-off-by: Carson Farmer <carson.farmer@gmail.com>

src/index.js

@@ -105,12 +105,23 @@ class LevelDatastore {
  values = !q.keysOnly
  }
 
+ const opts = {
+ keys: true,
+ values: values,
+ keyAsBuffer: true
+ }
+
+ // Let the db do the prefix matching
+ if (q.prefix != null) {
+ const prefix = q.prefix.toString()
+ // Match keys greater than or equal to `prefix` and
+ opts.gte = prefix
+ // less than `prefix` + \xFF (hex escape sequence)
+ opts.lt = prefix + '\xFF'
+ }
+
  let it = levelIteratorToIterator(
- this.db.db.iterator({
- keys: true,
- values: values,
- keyAsBuffer: true
- })
+ this.db.iterator(opts)
  )
 
  it = map(it, ({ key, value }) => {
@@ -121,10 +132,6 @@ class LevelDatastore {
  return res
  })
 
- if (q.prefix != null) {
- it = filter(it, e => e.key.toString().startsWith(q.prefix))
- }
-
  if (Array.isArray(q.filters)) {
  it = q.filters.reduce((it, f) => filter(it, f), it)
  }