Add a history stream for apply ticks / views updates

`base.system.createApplyTickHistoryStream()` emits once per `apply` tick
with an object of the form:

```
{
  index: Number, // the index of the 'info' block in the `system.db`
  views: [{ key, appends }], // A list of changed views with the number of blocks appended
  writer: { key, length } // Which writer appended the blocks and what its new length is
}
```

This can be used for debugging how the apply function changed views over
the history of the autobase system.

README.md

@@ -141,6 +141,39 @@ Generate a local core to be used for an Autobase.
 
 Get user data associated with the autobase core.
 
+### `SystemView`
+
+The view of the autobase system.
+
+#### `system.createApplyTickHistoryStream(opts)`
+
+Returns a stream of `apply` 'ticks' history for debugging how apply changed
+the view(s) and what writer wrote the batch of blocks processed. Each update has
+the following structure:
+
+```js
+{
+  index: Number, // the index of the 'info' block in the `system.db`
+  views: [{ key, appends }], // A list of changed views with the number of blocks appended
+  writer: { key, length } // Which writer appended the blocks and what its new length is
+}
+```
+
+Supported options are the same as `hyperbee`'s
+[`.createHistoryStream()`](https://github.com/holepunchto/hyperbee?tab=readme-ov-file#const-stream--dbcreatehistorystreamoptions)
+with two exceptions being `reverse` & `encoding`, so:
+
+```js
+{
+  live: false, // If true the stream will wait for new data and never end
+  gte: seq, // Start with this seq (inclusive)
+  gt: seq, // Start after this index
+  lte: seq, // Stop after this index
+  lt: seq, // Stop before this index
+  limit: -1 // Set to the max number of entries you want
+}
+```
+
 ### `AutoStore`
 
 Each autobase creates a `AutoStore` which is used to create views. The store is passed to the `open` function.

lib/system.js

@@ -3,6 +3,7 @@ const SubEncoder = require('sub-encoder')
 const ReadyResource = require('ready-resource')
 const b4a = require('b4a')
 const c = require('compact-encoding')
+const { pipeline, Transform } = require('streamx')
 
 const { Info, Member } = require('./messages')
 
@@ -402,6 +403,55 @@ module.exports = class SystemView extends ReadyResource {
       await co.close()
     }
   }
+
+  createApplyTickHistoryStream (opts = {}) {
+    const lastSeenViews = []
+    let writerSinceInfoBlock = null
+
+    const infoKey = b4a.from('info')
+
+    return pipeline(
+      this.db.createHistoryStream({
+        ...opts,
+        reverse: false, // force to run forward
+        encoding: { key: DIGEST }
+      }),
+      new Transform({
+        transform (node, cb) {
+          if (b4a.equals(node.key, infoKey)) {
+            const value = c.decode(Info, node.value)
+
+            // Get view changes
+            const views = []
+            for (let i = 0; i < value.views.length; i++) {
+              const view = value.views[i]
+              const lastLength = lastSeenViews[i] || 0
+              if (view.length !== lastLength) {
+                views.push({
+                  key: view.key,
+                  appends: view.length - lastLength
+                })
+              }
+
+              lastSeenViews[i] = view.length
+            }
+
+            const event = { index: node.seq, views, writer: writerSinceInfoBlock }
+            writerSinceInfoBlock = null
+
+            return cb(null, event)
+          }
+
+          const value = c.decode(Member, node.value)
+          const writerLength = value.length
+          if (writerLength <= 0) return cb(null)
+
+          writerSinceInfoBlock = { key: node.key, length: value.length }
+          return cb(null)
+        }
+      })
+    )
+  }
 }
 
 function hasDependency (node, dep) {