Load earlier without losing your place

"use client"

import { useEffect, useRef, useState } from "react"
import { Badge } from "@/components/reui/badge"

import {
  Avatar,
  AvatarFallback,
  AvatarImage,
} from "@/components/ui/avatar"
import { Bubble, BubbleContent } from "@/components/ui/bubble"
import { Button } from "@/components/ui/button"
import {
  Card,
  CardAction,
  CardContent,
  CardDescription,
  CardHeader,
  CardTitle,
} from "@/components/ui/card"
import { Marker, MarkerContent } from "@/components/ui/marker"
import {
  Message,
  MessageAvatar,
  MessageContent,
} from "@/components/ui/message"
import {
  MessageScroller,
  MessageScrollerButton,
  MessageScrollerContent,
  MessageScrollerItem,
  MessageScrollerProvider,
  MessageScrollerViewport,
  useMessageScrollerScrollable,
} from "@/components/ui/message-scroller"
import { Spinner } from "@/components/ui/spinner"
import { IconPlaceholder } from "@/app/(create)/components/icon-placeholder"

type Turn = {
  id: string
  role: "user" | "assistant"
  author: string
  text: string
}

const VIEWER_AVATAR =
  "https://images.unsplash.com/photo-1535713875002-d1d0cf377fde?w=96&h=96&dpr=2&q=80"

const INITIAL_VISIBLE = 6

/*
 * Sixteen rows against a six row window, so there is real history to reveal
 * and the restored position is somewhere in the middle rather than at an edge
 * where any implementation would look correct.
 */
const HISTORY: Turn[] = [
  {
    id: "th_5p3w9k_1",
    role: "user",
    author: "Jonas Weber",
    text: "Pulling up ticket 4821. The customer says their exports have been empty since Friday.",
  },
  {
    id: "th_5p3w9k_2",
    role: "assistant",
    author: "Nimbus",
    text: "Empty, not failing. Every job in that window finished with a zero row result.",
  },
  {
    id: "th_5p3w9k_3",
    role: "user",
    author: "Jonas Weber",
    text: "Same workspace every time?",
  },
  {
    id: "th_5p3w9k_4",
    role: "assistant",
    author: "Nimbus",
    text: "One workspace, ws_3311. No other account shows the pattern.",
  },
  {
    id: "th_5p3w9k_5",
    role: "user",
    author: "Jonas Weber",
    text: "What changed for them on Friday?",
  },
  {
    id: "th_5p3w9k_6",
    role: "assistant",
    author: "Nimbus",
    text: "They moved their default view onto a saved filter whose date range ends last Thursday.",
  },
  {
    id: "th_5p3w9k_7",
    role: "user",
    author: "Jonas Weber",
    text: "So the export is right and the filter is wrong.",
  },
  {
    id: "th_5p3w9k_8",
    role: "assistant",
    author: "Nimbus",
    text: "Right for the filter, yes. The export inherits the active view, and that view now excludes everything recent.",
  },
  {
    id: "th_5p3w9k_9",
    role: "user",
    author: "Jonas Weber",
    text: "Does the interface show them that range anywhere?",
  },
  {
    id: "th_5p3w9k_10",
    role: "assistant",
    author: "Nimbus",
    text: "Only inside the filter chip, which is collapsed by default at their screen width.",
  },
  {
    id: "th_5p3w9k_11",
    role: "user",
    author: "Jonas Weber",
    text: "That is the actual bug then.",
  },
  {
    id: "th_5p3w9k_12",
    role: "assistant",
    author: "Nimbus",
    text: "It is at least why nobody caught it. The export dialog does show a row count, but it renders after the request, so a zero reads as loading.",
  },
  {
    id: "th_5p3w9k_13",
    role: "user",
    author: "Jonas Weber",
    text: "Can we tell them what to change today?",
  },
  {
    id: "th_5p3w9k_14",
    role: "assistant",
    author: "Nimbus",
    text: "Clear the saved range on the view, or export from the unfiltered table. Either one gives them Friday onward straight away.",
  },
  {
    id: "th_5p3w9k_15",
    role: "user",
    author: "Jonas Weber",
    text: "Send that, then flag the dialog for the product review.",
  },
  {
    id: "th_5p3w9k_16",
    role: "assistant",
    author: "Nimbus",
    text: "Sent, and the dialog is on the review list with a screenshot attached.\n\nI left a note on the ticket too, so the next person reads the filter before they open the worker logs.",
  },
]

/*
 * The chip sits outside the scroll frame but inside the Provider. Every hook
 * resolves through that context, so a control in the header still has to be a
 * descendant of it.
 */
function HistoryChip() {
  const { start } = useMessageScrollerScrollable()

  return (
    <Badge variant="info-light">
      {start ? "Earlier messages above" : "Top of thread"}
    </Badge>
  )
}

export function Pattern() {
  const [visibleCount, setVisibleCount] = useState(INITIAL_VISIBLE)
  const [loading, setLoading] = useState(false)
  const loadTimer = useRef<number | null>(null)

  useEffect(() => {
    return () => {
      if (loadTimer.current !== null) window.clearTimeout(loadTimer.current)
    }
  }, [])

  /*
   * The window is a slice taken during render, so loading earlier messages is
   * only a longer slice. That is the same shape a cursor paginated endpoint
   * produces, and it keeps the already mounted rows identical.
   */
  const visible = HISTORY.slice(-visibleCount)
  const allLoaded = visibleCount >= HISTORY.length

  /* The delay is a real beat rather than decoration, so the restored scroll
     position is measured against rows that arrived after a paint. */
  function loadEarlier() {
    if (loading || allLoaded) return

    setLoading(true)
    loadTimer.current = window.setTimeout(() => {
      loadTimer.current = null
      setVisibleCount((count) => Math.min(count + 5, HISTORY.length))
      setLoading(false)
    }, 700)
  }

  return (
    /*
     * The frame owns the height. Restoring a scroll position only means
     * something while the reader is inside a bounded region, so a card that
     * grew with its content would have nothing to preserve.
     */
    <Card className="h-110 w-full max-w-2xl gap-0 overflow-hidden py-0">
      <MessageScrollerProvider>
        {/*
         * The load control lives in the header, never as the first row of the
         * content. A prepend is detected by the previously first row moving to
         * an index above zero, so a permanent loader row at the top never moves
         * and the restore would never fire.
         */}
        <CardHeader className="shrink-0 border-b py-4">
          <CardTitle>Support escalation</CardTitle>
          <CardDescription className="flex items-center gap-2">
            <HistoryChip />
            <span>
              {visible.length} of {HISTORY.length} messages
            </span>
          </CardDescription>
          <CardAction>
            <Button
              variant="outline"
              size="sm"
              disabled={loading || allLoaded}
              onClick={loadEarlier}
            >
              {loading ? (
                <Spinner className="size-3" aria-hidden="true" />
              ) : null}
              {allLoaded ? "All loaded" : "Load earlier"}
            </Button>
          </CardAction>
        </CardHeader>
        <CardContent className="min-h-0 flex-1 p-0">
          <MessageScroller>
            {/*
             * preserveScrollOnPrepend is on by default. Stable ids are what
             * give it a specific row to hold under the reader, so a list keyed
             * by index loses the behavior without any error.
             */}
            <MessageScrollerViewport aria-label="Support escalation transcript">
              <MessageScrollerContent className="gap-5 p-(--card-spacing)">
                {visible.map((turn) => {
                  const isUser = turn.role === "user"

                  return (
                    <MessageScrollerItem
                      key={turn.id}
                      messageId={turn.id}
                      scrollAnchor={isUser}
                    >
                      <Message align={isUser ? "end" : "start"}>
                        <MessageAvatar className="size-8">
                          {isUser ? (
                            <Avatar>
                              <AvatarImage
                                src={VIEWER_AVATAR}
                                alt={turn.author}
                              />
                              <AvatarFallback>JW</AvatarFallback>
                            </Avatar>
                          ) : (
                            <>
                              <IconPlaceholder
                                lucide="SparklesIcon"
                                tabler="IconSparkles"
                                hugeicons="SparklesIcon"
                                phosphor="SparkleIcon"
                                remixicon="RiSparklingLine"
                                aria-hidden="true"
                                className="text-muted-foreground size-4"
                              />
                              <span className="sr-only">{turn.author}</span>
                            </>
                          )}
                        </MessageAvatar>
                        <MessageContent>
                          <Bubble
                            variant={isUser ? "muted" : "ghost"}
                            align={isUser ? "end" : "start"}
                          >
                            <BubbleContent className="space-y-2">
                              {turn.text
                                .split("\n\n")
                                .map((paragraph, index) => (
                                  <p
                                    key={index}
                                    className="whitespace-pre-wrap"
                                  >
                                    {paragraph}
                                  </p>
                                ))}
                            </BubbleContent>
                          </Bubble>
                        </MessageContent>
                      </Message>
                    </MessageScrollerItem>
                  )
                })}
                {/*
                 * A row with no messageId still counts for prepend detection
                 * and for anchoring, it is only invisible to scrollToMessage
                 * and to visibility tracking. A divider wants exactly that.
                 */}
                <MessageScrollerItem scrollAnchor={false}>
                  <Marker variant="separator">
                    <MarkerContent>Escalated to Sam Okafor</MarkerContent>
                  </Marker>
                </MessageScrollerItem>
              </MessageScrollerContent>
            </MessageScrollerViewport>
            <MessageScrollerButton
              variant="outline"
              className="rounded-full shadow-sm"
            />
          </MessageScroller>
        </CardContent>
      </MessageScrollerProvider>
    </Card>
  )
}