fix block stream looking at same dep multiple times

lib/block-dependency-stream.js

@@ -25,8 +25,7 @@ module.exports = class BlockStream extends Readable {
     if (this._consumed > this.core.dependencies.length) return
 
     const deps = this.core.dependencies
-
-    const index = findDependencyIndex(deps, this.end, this._consumed++, this.reverse)
+    const index = this._findDependencyIndex(deps)
 
     const curr = deps[index]
     const prev = deps[index - 1]
@@ -38,6 +37,19 @@ module.exports = class BlockStream extends Readable {
     this._makeStream(core.block(ptr, start), core.block(ptr, end))
   }
 
+  _findDependencyIndex (deps) {
+    if (!this.reverse) return this._consumed++
+
+    let i = deps.length - this._consumed++
+    while (i > 0) {
+      if (deps[i - 1].length <= this.end) return i
+      i--
+      this._consumed++
+    }
+
+    return 0
+  }
+
   _predestroy () {
     if (this._stream !== null) this._stream.destroy()
   }
@@ -83,7 +95,7 @@ module.exports = class BlockStream extends Readable {
     // empty the current stream
     if (this._onreadable() === true) this._drained = true
 
-    this._stream = this._start = this._end = null
+    this._stream = null
 
     this._update()
     this._maybeDrain()
@@ -98,13 +110,3 @@ module.exports = class BlockStream extends Readable {
 }
 
 function noop () {}
-
-function findDependencyIndex (deps, end, offset, reverse) {
-  if (!reverse) return offset
-
-  for (let i = deps.length - offset; i > 0; i--) {
-    if (deps[i - 1].length <= end) return i
-  }
-
-  return 0
-}