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,
         }
     }