feat: add interactive matching question component with responsive connection lines and styles

frontend/src/components/questions/Match.tsx

@@ -1,4 +1,4 @@
-import { useState, useMemo } from 'react'
+import { useState, useMemo, useRef, useEffect, useCallback } from 'react'
 import Tex from '../Tex'
 import type { Question } from '../../types'
 
@@ -10,17 +10,102 @@ interface Props {
   graded?: boolean
 }
 
+const PASTEL_COLORS = [
+  'oklch(0.78 0.12 330)',  // pink
+  'oklch(0.78 0.12 200)',  // sky blue
+  'oklch(0.80 0.13 140)',  // mint green
+  'oklch(0.78 0.12 80)',   // peach / orange
+  'oklch(0.78 0.12 270)',  // lavender
+  'oklch(0.82 0.10 60)',   // soft yellow
+  'oklch(0.78 0.12 170)',  // teal
+  'oklch(0.78 0.10 20)',   // salmon
+  'oklch(0.78 0.12 300)',  // orchid
+  'oklch(0.80 0.10 110)',  // lime
+]
+
 export default function Match({ q, value, onChange, locked, graded }: Props) {
   const current = value ?? {}
   const [activeLeft, setActiveLeft] = useState<string | null>(null)
   const answer = (q.answer as Record<string, string>) ?? {}
 
+  const gridRef = useRef<HTMLDivElement>(null)
+  const svgRef = useRef<SVGSVGElement>(null)
+  const leftRefs = useRef<Record<string, HTMLButtonElement | null>>({})
+  const rightRefs = useRef<Record<string, HTMLButtonElement | null>>({})
+
   const reverseMap = useMemo(() => {
     const m: Record<string, string> = {}
     Object.entries(current).forEach(([l, r]) => { m[r] = l })
     return m
   }, [current])
 
+  /* Build a stable color map: each pair index gets a pastel */
+  const pairColorMap = useMemo(() => {
+    const map: Record<string, string> = {}
+    let colorIdx = 0
+    const leftItems = q.left ?? []
+    leftItems.forEach((item) => {
+      if (current[item.id]) {
+        map[item.id] = PASTEL_COLORS[colorIdx % PASTEL_COLORS.length]
+        colorIdx++
+      }
+    })
+    return map
+  }, [current, q.left])
+
+  /* Stringify the current pairings so we can use it as a stable dependency */
+  const currentKey = JSON.stringify(current)
+
+  const drawLines = useCallback(() => {
+    const grid = gridRef.current
+    const svg = svgRef.current
+    if (!grid || !svg) return
+
+    const gridRect = grid.getBoundingClientRect()
+
+    /* Clear existing paths */
+    while (svg.firstChild) svg.removeChild(svg.firstChild)
+
+    Object.entries(current).forEach(([leftId, rightId]) => {
+      const leftEl = leftRefs.current[leftId]
+      const rightEl = rightRefs.current[rightId]
+      if (!leftEl || !rightEl) return
+
+      const lr = leftEl.getBoundingClientRect()
+      const rr = rightEl.getBoundingClientRect()
+
+      const x1 = lr.right - gridRect.left
+      const y1 = lr.top + lr.height / 2 - gridRect.top
+      const x2 = rr.left - gridRect.left
+      const y2 = rr.top + rr.height / 2 - gridRect.top
+
+      const dx = (x2 - x1) * 0.45
+      const color = pairColorMap[leftId] ?? PASTEL_COLORS[0]
+
+      const path = document.createElementNS('http://www.w3.org/2000/svg', 'path')
+      path.setAttribute('d', `M ${x1} ${y1} C ${x1 + dx} ${y1}, ${x2 - dx} ${y2}, ${x2} ${y2}`)
+      path.setAttribute('stroke', color)
+      path.setAttribute('stroke-width', '2.5')
+      path.setAttribute('fill', 'none')
+      path.setAttribute('stroke-linecap', 'round')
+      path.setAttribute('opacity', '0.85')
+      svg.appendChild(path)
+    })
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [currentKey, pairColorMap])
+
+  useEffect(() => {
+    /* Draw lines after render */
+    requestAnimationFrame(drawLines)
+
+    window.addEventListener('resize', drawLines)
+    window.addEventListener('scroll', drawLines, true)
+    return () => {
+      window.removeEventListener('resize', drawLines)
+      window.removeEventListener('scroll', drawLines, true)
+    }
+  }, [drawLines])
+
   const handleRight = (rightId: string) => {
     if (locked) return
     if (activeLeft) {
@@ -48,7 +133,10 @@ export default function Match({ q, value, onChange, locked, graded }: Props) {
   }
 
   return (
-    <div className="match-grid">
+    <div className="match-grid" ref={gridRef}>
+      {/* SVG overlay for connection lines */}
+      <svg className="match-lines-svg" ref={svgRef} />
+
       <div className="match-col">
         <div className="match-col-label">Items</div>
         {(q.left ?? []).map((item, i) => {
@@ -62,14 +150,27 @@ export default function Match({ q, value, onChange, locked, graded }: Props) {
           const pairedRightIdx = current[item.id]
             ? (q.right ?? []).findIndex((x) => x.id === current[item.id])
             : -1
+          const pairColor = pairColorMap[item.id]
           return (
-            <button key={item.id} className={cls} onClick={() => handleLeft(item.id)} disabled={locked}>
+            <button
+              key={item.id}
+              className={cls}
+              onClick={() => handleLeft(item.id)}
+              disabled={locked}
+              ref={(el) => { leftRefs.current[item.id] = el }}
+              style={pairColor && !graded ? { borderColor: pairColor, borderRightWidth: 3 } : undefined}
+            >
               <span style={{ display: 'flex', gap: 10, alignItems: 'center' }}>
                 <span className="mono faint" style={{ fontSize: 11 }}>{i + 1}</span>
                 <Tex>{item.text}</Tex>
               </span>
               {pairedRightIdx >= 0 && (
-                <span className="match-pair-badge">{String.fromCharCode(65 + pairedRightIdx)}</span>
+                <span
+                  className="match-pair-badge"
+                  style={pairColor ? { background: pairColor, color: '#fff' } : undefined}
+                >
+                  {String.fromCharCode(65 + pairedRightIdx)}
+                </span>
               )}
             </button>
           )
@@ -86,19 +187,27 @@ export default function Match({ q, value, onChange, locked, graded }: Props) {
           const pairedLeftIdx = reverseMap[item.id]
             ? (q.left ?? []).findIndex((x) => x.id === reverseMap[item.id])
             : -1
+          const pairColor = reverseMap[item.id] ? pairColorMap[reverseMap[item.id]] : undefined
           return (
             <button
               key={item.id}
               className={cls}
               onClick={() => handleRight(item.id)}
               disabled={locked || (!activeLeft && !reverseMap[item.id])}
+              ref={(el) => { rightRefs.current[item.id] = el }}
+              style={pairColor && !graded ? { borderColor: pairColor, borderLeftWidth: 3 } : undefined}
             >
               <span style={{ display: 'flex', gap: 10, alignItems: 'center' }}>
                 <span className="mono faint" style={{ fontSize: 11 }}>{String.fromCharCode(65 + i)}</span>
                 <Tex>{item.text}</Tex>
               </span>
               {pairedLeftIdx >= 0 && (
-                <span className="match-pair-badge">{pairedLeftIdx + 1}</span>
+                <span
+                  className="match-pair-badge"
+                  style={pairColor ? { background: pairColor, color: '#fff' } : undefined}
+                >
+                  {pairedLeftIdx + 1}
+                </span>
               )}
             </button>
           )