fix dependency streams

lib/block-dependency-stream.js

@@ -8,6 +8,7 @@ module.exports = class BlockStream extends Readable {
     this.core = core
     this.db = db
     this.updates = updates
+    this.start = start
     this.end = end
     this.reverse = reverse === true
 
@@ -23,26 +24,18 @@ module.exports = class BlockStream extends Readable {
   _update () {
     if (this._consumed > this.core.dependencies.length) return
 
-    const index = this.reverse ? this.core.dependencies.length - this._consumed : this._consumed
-    const offset = index === 0 ? 0 : this.core.dependencies[index - 1].length
+    const deps = this.core.dependencies
 
-    let end = 0
-    let ptr = 0
+    const index = findDependencyIndex(deps, this.end, this._consumed++, this.reverse)
 
-    if (this._consumed < this.core.dependencies.length) {
-      const dep = this.core.dependencies[this._consumed]
-      end = this.end === -1 ? dep.length : Math.min(this.end, dep.length)
-      ptr = dep.dataPointer
-    } else {
-      end = this.end === -1 ? Infinity : this.end
-      ptr = this.core.dataPointer
-    }
-
-    this._consumed++
+    const curr = deps[index]
+    const prev = deps[index - 1]
 
-    if (end === offset) return
+    const start = prev && prev.length > this.start ? prev.length : this.start
+    const end = curr && curr.length < this.end ? curr.length : this.end
+    const ptr = curr ? curr.dataPointer : this.core.dataPointer
 
-    this._makeStream(core.block(ptr, offset), core.block(ptr, end))
+    this._makeStream(core.block(ptr, start), core.block(ptr, end))
   }
 
   _predestroy () {
@@ -105,3 +98,13 @@ 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
+}