update signature validation

app/javascript/submission_form/validate_signature.js

@@ -0,0 +1,38 @@
+function isValidSignatureCanvas (data) {
+  if (data.length === 0) return false
+
+  const strokes = data.filter(stroke => Array.isArray(stroke.points) && stroke.points.length > 2)
+
+  if (strokes.length === 0) return false
+
+  let skippedStraightLine = 0
+
+  const validStrokes = strokes.filter(stroke => {
+    const points = stroke.points
+    const first = points[0]
+    const last = points[points.length - 1]
+    const A = last.y - first.y
+    const B = first.x - last.x
+    const C = last.x * first.y - first.x * last.y
+    const lineLength = Math.sqrt(A * A + B * B)
+
+    const totalDeviation = points.reduce((sum, p) => {
+      const distanceToLine = Math.abs(A * p.x + B * p.y + C) / lineLength
+      return sum + distanceToLine
+    }, 0)
+
+    const avgDeviation = totalDeviation / points.length
+
+    if (avgDeviation < 3 && skippedStraightLine < 2) {
+      skippedStraightLine++
+
+      return false
+    }
+
+    return true
+  })
+
+  return validStrokes.length > 0
+}
+
+export { isValidSignatureCanvas }