feat: improve dropped question support

src/analysis/classical.rs

@@ -396,6 +396,39 @@ pub fn analyze(
         ));
     }
 
+    // A drop changes every student's percentage, so the two ways to get it wrong
+    // are worth saying out loud. Both are silent otherwise: the numbers simply
+    // come out different from the platform's.
+    if let Some(record) = record {
+        for placement in record.items.iter().filter(|p| p.dropped) {
+            let rows = set.for_item(placement.number);
+            if rows.is_empty() {
+                continue;
+            }
+            let all_credited = rows.iter().all(|r| r.credit >= 0.999);
+            if placement.dropped_with_credit() && !all_credited {
+                let short = rows.iter().filter(|r| r.credit < 0.999).count();
+                warnings.push(format!(
+                    "question {} is marked `dropped_as: full_credit`, but {short} of {} responses \
+                     carry less than full credit. Either the platform was not regraded or the \
+                     export predates the regrade; until one of those is fixed this report's \
+                     percentages will sit below the grade of record",
+                    placement.number,
+                    rows.len()
+                ));
+            }
+            if !placement.dropped_with_credit() && all_credited {
+                warnings.push(format!(
+                    "question {} is dropped and every response carries full credit, which is what \
+                     crediting every option on the platform looks like. It is being removed from \
+                     the denominator here, so this report will read slightly lower than the \
+                     platform. Set `dropped_as: full_credit` if the platform kept the point",
+                    placement.number
+                ));
+            }
+        }
+    }
+
     let mut items = Vec::new();
     let mut p_values = Vec::new();
     let mut rpbs = Vec::new();
@@ -986,6 +1019,7 @@ mod tests {
             topics: vec![],
             bonus: false,
             dropped: false,
+            dropped_full_credit: false,
         }
     }
 

src/analysis/diagnostic.rs

@@ -172,12 +172,14 @@ pub struct StudentDiagnostic {
     /// One row per question, with no question in it.
     #[serde(skip_serializing_if = "Vec::is_empty")]
     pub questions: Vec<QuestionRow>,
-    /// Questions dropped from scoring, in number order.
+    /// Questions thrown out, in number order.
     ///
     /// Reported separately from the question rows so the document can say once,
-    /// in prose, why the denominator is smaller than the paper was.
+    /// in prose, what happened to them. The two kinds need different sentences:
+    /// a removed question is gone from the denominator, and a full-credit
+    /// question is still in it.
     #[serde(skip_serializing_if = "Vec::is_empty")]
-    pub dropped_questions: Vec<u32>,
+    pub dropped_questions: Vec<DroppedQuestion>,
     /// Which lectures to go back to, the one that would repay the most time
     /// first.
     #[serde(skip_serializing_if = "Vec::is_empty")]
@@ -360,6 +362,17 @@ pub struct QuestionRow {
     pub review: Vec<ItemReading>,
 }
 
+/// One question thrown out after the exam.
+#[derive(Debug, Clone, Serialize)]
+#[serde(rename_all = "kebab-case")]
+pub struct DroppedQuestion {
+    /// The recorded question number.
+    pub number: u32,
+    /// Whether the drop was applied by crediting every option, in which case the
+    /// question is still in the points of record.
+    pub full_credit: bool,
+}
+
 /// One citation to read again after missing a question.
 #[derive(Debug, Clone, Serialize)]
 #[serde(rename_all = "kebab-case")]
@@ -543,13 +556,16 @@ pub fn student(
         Vec::new()
     };
 
-    let mut dropped_questions: Vec<u32> = rows
+    let mut dropped_questions: Vec<DroppedQuestion> = rows
         .iter()
         .filter(|r| r.dropped)
-        .map(|r| r.item_number)
+        .map(|r| DroppedQuestion {
+            number: r.item_number,
+            full_credit: r.dropped_full_credit,
+        })
         .collect();
-    dropped_questions.sort_unstable();
-    dropped_questions.dedup();
+    dropped_questions.sort_by_key(|d| d.number);
+    dropped_questions.dedup_by_key(|d| d.number);
 
     // Built from the response rows rather than from `questions`, so a report with
     // `--no-questions` still says where to go back to; it just does not name the
@@ -952,11 +968,11 @@ pub struct CohortDiagnostic {
     pub lectures: Vec<CohortLectureRow>,
     /// Per-question statistics.
     pub questions: Vec<CohortQuestionRow>,
-    /// Questions dropped from scoring, which are therefore absent from every
-    /// table above. Recorded so the report says why rather than leaving a gap in
-    /// the numbering.
+    /// Questions thrown out, which are therefore absent from every table above.
+    /// Recorded so the report says why rather than leaving a gap in the
+    /// numbering.
     #[serde(skip_serializing_if = "Vec::is_empty")]
-    pub dropped_questions: Vec<u32>,
+    pub dropped_questions: Vec<DroppedQuestion>,
     /// What to do about each question that raised something.
     pub triage: Triage,
     /// How the authored expectations did.
@@ -1509,12 +1525,16 @@ pub fn cohort(
         .filter_map(|item| questions.iter().find(|q| q.number == item.number).cloned())
         .collect();
 
-    let mut dropped_questions: Vec<u32> = set
+    let mut dropped_questions: Vec<DroppedQuestion> = set
         .all_items()
         .into_iter()
         .filter(|n| set.item_dropped(*n))
+        .map(|number| DroppedQuestion {
+            number,
+            full_credit: set.for_item(number).iter().any(|r| r.dropped_full_credit),
+        })
         .collect();
-    dropped_questions.sort_unstable();
+    dropped_questions.sort_by_key(|d| d.number);
 
     let default_options = course.policy.options_per_item;
     let triage = triage(&questions, threshold, default_options);

src/analysis/students.rs

@@ -1112,6 +1112,7 @@ mod tests {
             topics: vec![],
             bonus: false,
             dropped: false,
+            dropped_full_credit: false,
         }
     }