design-system/src/design-system/composites/LogViewer/LogViewer.tsx

import { useRef, useEffect, useCallback } from 'react'
import styles from './LogViewer.module.css'

export interface LogEntry {
  timestamp: string
  level: 'info' | 'warn' | 'error' | 'debug' | 'trace'
  message: string
}

export interface LogViewerProps {
  entries: LogEntry[]
  maxHeight?: number | string
  className?: string
}

const LEVEL_CLASS: Record<LogEntry['level'], string> = {
  info: styles.levelInfo,
  warn: styles.levelWarn,
  error: styles.levelError,
  debug: styles.levelDebug,
  trace: styles.levelTrace,
}

function formatTime(iso: string): string {
  try {
    return new Date(iso).toLocaleTimeString('en-GB', {
      hour: '2-digit',
      minute: '2-digit',
      second: '2-digit',
      hour12: false,
    })
  } catch {
    return iso
  }
}

export function LogViewer({ entries, maxHeight = 400, className }: LogViewerProps) {
  const scrollRef = useRef<HTMLDivElement>(null)
  const isAtTopRef = useRef(true)

  const handleScroll = useCallback(() => {
    const el = scrollRef.current
    if (!el) return
    isAtTopRef.current = el.scrollTop < 20
  }, [])

  useEffect(() => {
    const el = scrollRef.current
    if (el && isAtTopRef.current) {
      el.scrollTop = 0
    }
  }, [entries])

  const heightStyle = typeof maxHeight === 'number' ? `${maxHeight}px` : maxHeight

  return (
    <div
      ref={scrollRef}
      className={[styles.container, className].filter(Boolean).join(' ')}
      style={{ maxHeight: heightStyle }}
      onScroll={handleScroll}
      role="log"
    >
      {entries.map((entry, i) => (
        <div key={i} className={styles.line}>
          <span className={styles.timestamp}>{formatTime(entry.timestamp)}</span>
          <span className={[styles.levelBadge, LEVEL_CLASS[entry.level]].join(' ')}>
            {entry.level.toUpperCase()}
          </span>
          <span className={styles.message}>{entry.message}</span>
        </div>
      ))}
      {entries.length === 0 && (
        <div className={styles.empty}>No log entries.</div>
      )}
    </div>
  )
}