Add err scorer

db/scorers/err@10.js

@@ -0,0 +1,30 @@
+let k = 10 // @Rank
+// if less than K results, need to reduce K now or final score is too low
+k = numReturned() < k ? numReturned() : k
+const scores = Array(k);
+const missing_rating = 0; // pessimistic assumption
+let i = 0
+for (i = 0; i < k; i++) {
+  if (hasDocRating(i)) {
+    scores[i] = (docRating(i));
+  } else {
+    scores[i] = missing_rating;
+  }
+}
+
+function gain(grade, maxGrade=3.0) {
+    return (Math.pow(2,grade) - 1.0)/Math.pow(2,maxGrade)
+}
+function err(lst, maxG=3.0) {
+    let ERR = 0.0
+    let trust = 1.0
+    for (i = 0; i < lst.length; i++) {
+        const rank = i + 1.0
+        const pUseful = gain(lst[i], maxG)
+        const disc = pUseful/rank
+        ERR = ERR + trust * disc
+        trust = trust * (1.0 - pUseful)
+    }
+    return ERR
+}
+setScore(err(scores))