app/frontend/src/components/Dashboard/ChartWidget.tsx

import { useQueries } from "@tanstack/react-query"
import { useEffect, useRef, useState } from "react"

import type { HistoryPoint } from "@/client"
import {
  DEFAULT_RANGE,
  RANGES,
  type Range,
  RangePicker,
} from "@/components/Common/RangePicker"
import { MAX_SERIES, UplotChart } from "@/components/Common/UplotChart"
import { useLiveValue } from "@/components/Flow/liveStore"
import { messageHistoryQueryOptions, usePublishMessage } from "./queries"
import { type Series, useHeaderSlot, type WidgetProps } from "./widgets"

// The five-series ceiling is the chart host's, and the panel reads it here.
export { MAX_SERIES }

/** The window a chart draws, unless it asks for more. */
const DEFAULT_POINTS = 300

/**
 * How often a chart asks again, for the window it is showing.
 *
 * A window is drawn in buckets, and nothing the chart can show changes until
 * the bucket it is drawing closes — so the resolution sets the pace. A week at
 * quarter-hour buckets asks four times an hour instead of twice a minute, for
 * the same picture. A slower refresh than that is honoured; a faster one only
 * buys the same answer again, so it is clamped.
 */
export const refreshFor = (range: Range, configured: unknown) =>
  Math.max(range.bucketS, Number(configured) || 0)

/** How long an unanswered request blocks an identical one. */
const INFLIGHT_MS = 15_000

/**
 * Requests waiting for an answer, across every chart in this tab.
 *
 * Two charts on the same node, or two panels of one dashboard, would otherwise
 * ask the same question at the same moment and make the flow run twice.
 *
 * ponytail: per-tab. Two browsers still duplicate a query — an `interval` on
 * the request port is what stops that, and it belongs to the flow that serves
 * it rather than to the widget asking.
 */
const inflight = new Map<string, number>()

/** What a data flow answers with: the lines, and what they were computed for. */
type SeriesPayload = {
  range_s?: number
  interval_s?: number
  lines: { label: string; points: [number, number][] }[]
}

function asPayload(value: unknown): SeriesPayload | null {
  const payload = value as SeriesPayload | null
  return payload && Array.isArray(payload.lines) ? payload : null
}

const config = (widget: WidgetProps["widget"]) =>
  (widget.config ?? {}) as Record<string, unknown>

/** The unit, the fixed axis and the line shape, in either mode. */
function presentation(cfg: Record<string, unknown>) {
  const low = Number(cfg.y_min)
  const high = Number(cfg.y_max)
  return {
    unit: cfg.unit ? String(cfg.unit) : undefined,
    yLabel: cfg.y_label ? String(cfg.y_label) : undefined,
    smooth: Boolean(cfg.smooth),
    yRange:
      Number.isFinite(low) && Number.isFinite(high) && low < high
        ? ([low, high] as [number, number])
        : undefined,
  }
}

/** The points a chart actually plots: the stored past, then the live tail. */
function merge(fetched: HistoryPoint[], tail: HistoryPoint[], cap: number) {
  const since = fetched[fetched.length - 1]?.ts ?? 0
  return [...fetched, ...tail.filter((point) => point.ts > since)].slice(-cap)
}

/**
 * A chart, drawn from whichever source it was pointed at.
 *
 * The two modes are separate components rather than branches: each calls a
 * different set of hooks, and React counts them.
 */
export function ChartWidget(props: WidgetProps) {
  return config(props.widget).source === "query" ? (
    <QueryChart {...props} />
  ) : (
    <LiveChart {...props} />
  )
}

/**
 * Several messages over time, drawn on one axis.
 *
 * uPlot rather than SVG: a chart may hold five series of up to `HISTORY_CAP`
 * points each, which is more path data than React should be rebuilding on every
 * value that arrives. Its own legend doubles as the hover readout, so the
 * cursor tells you what each line was worth at that moment.
 */
function LiveChart({ widget }: WidgetProps) {
  // Read inline rather than through `widgets.tsx`: that module renders this
  // one, and a runtime import back would close the circle.
  const series = (((widget.config ?? {}) as { series?: Series[] }).series ?? [])
    .filter((entry): entry is Series & { message: string } =>
      Boolean(entry.message),
    )
    .slice(0, MAX_SERIES)
  const names = series.map((entry) => entry.message)
  const cap = Math.max(
    2,
    Number(
      (widget.config as { history?: { points?: number } })?.history?.points,
    ) || DEFAULT_POINTS,
  )

  const histories = useQueries({
    queries: names.map((name) => messageHistoryQueryOptions(name)),
  })

  // A fixed set: hooks cannot be called in a loop, and five is the ceiling.
  const live = [
    useLiveValue(names[0]),
    useLiveValue(names[1]),
    useLiveValue(names[2]),
    useLiveValue(names[3]),
    useLiveValue(names[4]),
  ]

  // What arrived over the socket since the history was fetched. Appending beats
  // refetching the whole series every time a value lands.
  const [tails, setTails] = useState<Record<string, HistoryPoint[]>>({})
  const key = names.join(" ")
  const stamps = live.map((value) => value?.ts ?? 0).join(",")
  // biome-ignore lint/correctness/useExhaustiveDependencies: the timestamps are what makes a tail longer; `live` is a fresh array every render.
  useEffect(() => {
    setTails((previous) => {
      const next: Record<string, HistoryPoint[]> = {}
      let changed = false
      names.forEach((name, index) => {
        const point = live[index]
        const kept = previous[name] ?? []
        const ts = point?.ts
        if (
          typeof point?.value !== "number" ||
          !ts ||
          kept[kept.length - 1]?.ts === ts
        ) {
          next[name] = kept
          return
        }
        next[name] = [...kept, { ts, value: point.value }].slice(-cap)
        changed = true
      })
      // Dropping a series is a change too, even without a new reading.
      return changed || Object.keys(previous).length !== names.length
        ? next
        : previous
    })
  }, [key, stamps, cap])

  const plots = names.map((name, index) =>
    merge(
      (histories[index]?.data?.points ?? []) as HistoryPoint[],
      tails[name] ?? [],
      cap,
    ),
  )
  const labels = series.map((entry, index) => entry.label || names[index])

  if (names.length === 0) {
    return <p className="text-sm text-muted-foreground">Pick a message.</p>
  }

  return (
    <UplotChart
      labels={labels}
      plots={plots}
      {...presentation(widget.config as Record<string, unknown>)}
    />
  )
}

/**
 * A chart that asks for what it draws.
 *
 * It publishes a request — the window and the resolution it wants — exactly as
 * a slider publishes a value, and draws the `series` a flow answers with. What
 * serves that request is the flow's business: a database node with a function
 * either side of it, and the widget never learns which database it was.
 *
 * The answer says what it was computed for, and one computed for a different
 * window is ignored rather than drawn. Two charts on one node therefore cost a
 * duplicate query instead of overwriting each other's picture.
 */
function QueryChart({ widget, dashboard }: WidgetProps) {
  const cfg = config(widget)
  const request = String(cfg.request ?? "")
  const message = String(cfg.message ?? "")
  const [range, setRange] = useState<Range>(
    () =>
      RANGES.find((r) => r.hours * 3600 === Number(cfg.range_s)) ??
      DEFAULT_RANGE,
  )
  const rangeS = range.hours * 3600
  const intervalS = range.bucketS
  // Follows the window: a wider one is drawn coarser, so it is worth asking
  // about less often.
  const refreshS = refreshFor(range, cfg.refresh_s)

  const publish = usePublishMessage()
  const live = useLiveValue(message || undefined)
  const [answer, setAnswer] = useState<SeriesPayload | null>(null)

  // The request is a value like any other, so it goes out the way a control's
  // does — and the engine type-checks it against the port that declared it.
  const ask = useRef<() => void>(() => {})
  ask.current = () => {
    if (!request) return
    const key = `${request}|${rangeS}|${intervalS}`
    const since = inflight.get(key)
    if (since !== undefined && Date.now() - since < INFLIGHT_MS) return
    inflight.set(key, Date.now())
    publish.mutate({
      name: request,
      value: { range_s: rangeS, interval_s: intervalS },
      dashboard,
      widget: widget.id,
      label: widget.title || widget.id,
      kind: widget.type,
    })
  }

  // Ask now, then keep asking. A new window tears the timer down and asks
  // again at once rather than waiting out the old one.
  // biome-ignore lint/correctness/useExhaustiveDependencies: the window is read through the ref, but changing it is exactly what must restart the timer.
  useEffect(() => {
    if (!request) return
    ask.current()
    const timer = setInterval(() => ask.current(), refreshS * 1000)
    return () => clearInterval(timer)
  }, [request, rangeS, intervalS, refreshS])

  // The window changed, so what is on screen is no longer an answer to it.
  // biome-ignore lint/correctness/useExhaustiveDependencies: the window is the signal; clearing is not derived from the answer.
  useEffect(() => setAnswer(null), [rangeS, intervalS, message])

  // Keep only an answer computed for what we asked. Anything else belongs to
  // another chart's window and would draw the wrong picture under our label.
  useEffect(() => {
    const payload = asPayload(live?.value)
    if (!payload) return
    if (payload.range_s !== rangeS || payload.interval_s !== intervalS) return
    setAnswer(payload)
    inflight.delete(`${request}|${rangeS}|${intervalS}`)
    // The store hands out a fresh object per value, so its identity is the
    // signal that something arrived — no timestamp needed beside it.
  }, [live?.value, request, rangeS, intervalS])

  // Drawn on the title's line rather than above the plot: a tile is short, and
  // a row of its own costs the chart about a tenth of its height. Nothing to
  // pick a window for until the chart is wired, so a half-configured tile says
  // that and nothing else.
  const bound = Boolean(request && message)
  useHeaderSlot(
    bound ? <RangePicker value={range} onChange={setRange} /> : null,
    [range, bound],
  )

  if (!request || !message) {
    return <p className="text-sm text-muted-foreground">Pick a message.</p>
  }

  const lines = (answer?.lines ?? []).slice(0, MAX_SERIES)
  return (
    <UplotChart
      labels={lines.map((line, index) => line.label || `Series ${index + 1}`)}
      plots={lines.map((line) =>
        (line.points ?? []).map(([ts, value]) => ({ ts, value })),
      )}
      empty="Waiting for an answer."
      {...presentation(cfg)}
    />
  )
}