small wants compat mode (#763)

* auto align want reply

* compat mode for old peers

* lint

Author: mafintosh

lib/bitfield.js

@@ -495,6 +495,12 @@ module.exports = class Bitfield {
   }
 
   *want(start, length) {
+    if (start & 31) {
+      const d = start & 31
+      start -= d
+      length += d
+    }
+
     let j = start & (BITS_PER_SEGMENT - 1)
     let i = (start - j) / BITS_PER_SEGMENT
 

lib/wants.js

@@ -1,4 +1,5 @@
-const BATCH_UINTS = 128
+const COMPAT = true // should be flipped once this is widely deployed
+const BATCH_UINTS = COMPAT ? 65536 : 128
 const BATCH_BYTES = BATCH_UINTS * 4
 const BATCH = BATCH_BYTES * 8 // in bits
 const MAX_REMOTE_BATCHES = 4
@@ -25,6 +26,14 @@ class LocalWants {
     if (this.destroyed) return null
     if (this.any === null) this.any = []
 
+    if (COMPAT) {
+      let r = start & (BATCH - 1)
+      start -= r
+      length += r
+      r = length & (BATCH - 1)
+      if (r) length = length - r + BATCH
+    }
+
     for (let i = 0; i < this.any.length; i++) {
       const w = this.any[i]
       if (w.start === start && w.length === length) {
@@ -38,7 +47,7 @@ class LocalWants {
     this.any.push(w)
     handle.addWant(w)
 
-    return { start, length, any: true }
+    return { start, length, any: !COMPAT }
   }
 
   removeAnyRange(start, length, handle) {
@@ -54,7 +63,7 @@ class LocalWants {
 
       const head = this.any.pop()
       if (head !== w) this.any[i] = head
-      return { start, length, any: true }
+      return { start, length, any: !COMPAT }
     }
 
     return null