move client from separate repo

client/lib/source/poll-source.ts

@@ -0,0 +1,56 @@
+import { BehaviorSubject, concat, from, Observable, of } from 'rxjs'
+import { catchError, concatMap, delay, skip, switchMap, take, tap } from 'rxjs/operators'
+import { Http } from '../patch-db'
+import { UpdateReal } from '../sequence-store'
+import { Source } from './source'
+
+export type PollConfig = {
+  cooldown: number
+}
+
+export class PollSource<T> implements Source<T> {
+
+  constructor (
+    private readonly pollConfig: PollConfig,
+    private readonly http: Http<T>,
+  ) { }
+
+  watch$ (sequence$: Observable<number>): Observable<UpdateReal<T>> {
+    console.log('POLL_SOURCE - watch$()')
+
+    const polling$ = new BehaviorSubject('')
+
+    const updates$ = of('').pipe(
+      concatMap(_ => sequence$),
+      take(1),
+      tap(_ => console.log('making request')),
+      concatMap(seq => this.http.getRevisions(seq)),
+      catchError(e => {
+        console.error(e)
+        return of([])
+      }),
+      tap(_ => console.log('request complete')),
+    )
+
+    const delay$ = of([]).pipe(
+      tap(_ => console.log('starting cooldown')),
+      delay(this.pollConfig.cooldown),
+      tap(_ => console.log('cooldown finished')),
+      tap(_ => polling$.next('')),
+      skip(1),
+    )
+
+    const poll$ = concat(updates$, delay$)
+
+    return polling$.pipe(
+       switchMap(_ => poll$),
+       concatMap(res => {
+        if (Array.isArray(res)) {
+          return from(res) // takes Revision[] and converts it into Observable<Revision>
+        } else {
+          return of(res) // takes Dump<T> and converts it into Observable<Dump<T>>
+        }
+       }),
+    )
+  }
+}