feat: add fixed prefix length as an option

README.md

@@ -53,6 +53,38 @@ pull(
 )
 ```
 
+## API
+
+### `encode([opts])`
+
+- `opts: Object`, optional
+  - `fixed: false`:
+  - `bytes: 4`: If `fixed` is `true` this is the amount of bytes used for the prefix.
+
+By default all messages will be prefixed with a varint. If you want to use a fixed length prefix you can specify this through the `opts`.
+
+Returns a pull-stream through.
+
+### `decode([opts])`
+
+- `opts: Object`, optional
+  - `fixed: false`:
+  - `bytes: 4`: If `fixed` is `true` this is the amount of bytes used for the prefix.
+
+By default all messages will be prefixed with a varint. If you want to use a fixed length prefix you can specify this through the `opts`.
+
+
+Returns a pull-stream through.
+
+### `decodeFromReader(reader, [opts], cb)`
+
+- `reader: [pull-reader](https://github.com/dominictarr/pull-reader)`
+- `opts: Object`, optional. Same as for `decode`.
+- `cb: Function`: Callback called with `(err, message)`.
+
+This uses a [pull-reader](https://github.com/dominictarr/pull-reader) instance to reade and decode a single message. Useful when using [pull-handshake](https://github.com/pull-stream/pull-handshake) with length prefixed messages.
+
+
 ## Contribute
 
 PRs and issues gladly accepted! Check out the [issues](//github.com/dignifiedquire/pull-length-prefixed/issues).

src/decode.js

@@ -11,7 +11,7 @@ exports.decodeFromReader = decodeFromReader
 const MSB = 0x80
 const isEndByte = (byte) => !(byte & MSB)
 
-function decode () {
+function decode (opts) {
   let reader = new Reader()
   let p = pushable((err) => {
     reader.abort(err)
@@ -20,7 +20,7 @@ function decode () {
   return (read) => {
     reader(read)
     function next () {
-      decodeFromReader(reader, (err, msg) => {
+      decodeFromReader(reader, opts, (err, msg) => {
         if (err) return p.end(err)
 
         p.push(msg)
@@ -33,7 +33,36 @@ function decode () {
   }
 }
 
-function decodeFromReader (reader, cb) {
+function decodeFromReader (reader, opts, cb) {
+  if (typeof opts === 'function') {
+    cb = opts
+    opts = {}
+  }
+
+  opts = Object.assign({
+    fixed: false,
+    bytes: 4
+  }, opts || {})
+
+  if (opts.fixed) {
+    readFixedMessage(reader, opts.bytes, cb)
+  } else {
+    readVarintMessage(reader, cb)
+  }
+}
+
+function readFixedMessage (reader, byteLength, cb) {
+  reader.read(byteLength, (err, bytes) => {
+    if (err) {
+      return cb(err)
+    }
+
+    const msgSize = bytes.readInt32BE(0)
+    readMessage(reader, msgSize, cb)
+  })
+}
+
+function readVarintMessage (reader, cb) {
   let rawMsgSize = []
   if (rawMsgSize.length === 0) readByte()
 
@@ -48,22 +77,29 @@ function decodeFromReader (reader, cb) {
 
       if (byte && !isEndByte(byte[0])) {
         readByte()
-      } else {
-        readMessage()
+        return
       }
-    })
-  }
 
-  function readMessage () {
-    const msgSize = varint.decode(Buffer.concat(rawMsgSize))
-    reader.read(msgSize, (err, msg) => {
-      if (err) {
-        return cb(err)
-      }
+      const msgSize = varint.decode(Buffer.concat(rawMsgSize))
+      readMessage(reader, msgSize, (err, msg) => {
+        if (err) {
+          return cb(err)
+        }
 
-      rawMsgSize = []
+        rawMsgSize = []
 
-      cb(null, msg)
+        cb(null, msg)
+      })
     })
   }
 }
+
+function readMessage (reader, size, cb) {
+  reader.read(size, (err, msg) => {
+    if (err) {
+      return cb(err)
+    }
+
+    cb(null, msg)
+  })
+}

src/encode.js

@@ -1,14 +1,22 @@
 'use strict'
 
-const varint = require('varint')
 const Buffer = require('safe-buffer').Buffer
 
 module.exports = encode
 
-function encode () {
-  const poolSize = 10 * 1024
-  let pool = Buffer.alloc(poolSize)
+const poolSize = 10 * 1024
+
+function encode (opts) {
+  opts = Object.assign({
+    fixed: false,
+    bytes: 4
+  }, opts || {})
+
+  // Only needed for varint
+  const varint = require('varint')
+  let pool = opts.fixed ? null : createPool()
   let used = 0
+
   let ended = false
 
   return (read) => (end, cb) => {
@@ -24,18 +32,29 @@ function encode () {
         return cb(ended)
       }
 
-      varint.encode(data.length, pool, used)
-      used += varint.encode.bytes
+      let encodedLength
+      if (opts.fixed) {
+        encodedLength = Buffer.alloc(opts.bytes)
+        encodedLength.writeInt32BE(data.length, 0)
+      } else {
+        varint.encode(data.length, pool, used)
+        used += varint.encode.bytes
+        encodedLength = pool.slice(used - varint.encode.bytes, used)
+
+        if (pool.length - used < 100) {
+          pool = createPool()
+          used = 0
+        }
+      }
 
       cb(null, Buffer.concat([
-        pool.slice(used - varint.encode.bytes, used),
+        encodedLength,
         data
       ]))
-
-      if (pool.length - used < 100) {
-        pool = Buffer.alloc(poolSize)
-        used = 0
-      }
     })
   }
 }
+
+function createPool () {
+  return Buffer.alloc(poolSize)
+}

test/fixed.spec.js

@@ -0,0 +1,51 @@
+/* eslint-env mocha */
+'use strict'
+
+const pull = require('pull-stream')
+const expect = require('chai').expect
+
+const lp = require('../src')
+
+describe('pull-length-prefixed', () => {
+  it('basics', (done) => {
+    const input = [
+      new Buffer('hello '),
+      new Buffer('world')
+    ]
+
+    pull(
+      pull.values(input),
+      lp.encode({fixed: true}),
+      pull.collect((err, encoded) => {
+        if (err) throw err
+
+        expect(
+          encoded
+        ).to.be.eql([
+          Buffer.concat([
+            new Buffer('00000006', 'hex'),
+            new Buffer('hello ')
+          ]),
+          Buffer.concat([
+            new Buffer('00000005', 'hex'),
+            new Buffer('world')
+          ])
+        ])
+
+        pull(
+          pull.values(encoded),
+          lp.decode({fixed: true}),
+          pull.collect((err, output) => {
+            if (err) throw err
+            expect(
+              input
+            ).to.be.eql(
+              output
+            )
+            done()
+          })
+        )
+      })
+    )
+  })
+})