Sample correction range per prediction point in insulinCorrection

Sources/LoopAlgorithm/Insulin/DoseMath.swift

@@ -193,10 +193,6 @@ extension Array where Element: GlucoseValue {
 
         let endOfAbsorption = date.addingTimeInterval(model.effectDuration)
 
-        guard let correctionRangeItem = correctionRange.closestPrior(to: date) else {
-            preconditionFailure("Correction range must cover date: \(date)")
-        }
-
         // For each prediction above target, determine the amount of insulin necessary to correct glucose based on the modeled effectiveness of the insulin at that time
         for prediction in self {
             guard prediction.startDate >= date else {
@@ -213,6 +209,13 @@ extension Array where Element: GlucoseValue {
             let predictedGlucoseValue = prediction.quantity.doubleValue(for: unit)
             let time = prediction.startDate.timeIntervalSince(date)
 
+            // The correction range in effect at this prediction point's time, so a
+            // forecast that crosses a target-band boundary is corrected to the band
+            // in effect at each point.
+            guard let correctionRangeItem = correctionRange.closestPrior(to: prediction.startDate) else {
+                preconditionFailure("Correction range must cover prediction: \(prediction.startDate)")
+            }
+
             // Compute the target value as a function of time since the dose started
             let targetValue = targetGlucoseValue(
                 percentEffectDuration: time / model.effectDuration,

Tests/LoopAlgorithmTests/CorrectionDosingTests.swift

@@ -570,4 +570,66 @@ class CorrectionDosingTests: XCTestCase {
             XCTFail("Wrong .notice: \(String(describing: manualDose.notice))")
         }
     }
+
+    func testCorrectionRangeSampledPerPredictionPoint() {
+        // A flat high prediction crossing a target-band boundary two hours out.
+        let prediction = [
+            PredictedGlucoseValue(startDate: testDate.addingTimeInterval(.hours(0)), quantity: .glucose(160)),
+            PredictedGlucoseValue(startDate: testDate.addingTimeInterval(.hours(6.2)), quantity: .glucose(160))
+        ]
+
+        // Target switches from 90-120 to 150-170 at t+2h; predicted points beyond
+        // the boundary are already in range, so they should require no correction.
+        let steppedTarget: GlucoseRangeTimeline = [
+            AbsoluteScheduleValue(
+                startDate: testDate.addingTimeInterval(.hours(-24)),
+                endDate: testDate.addingTimeInterval(.hours(2)),
+                value: LoopQuantity.glucose(90)...LoopQuantity.glucose(120)
+            ),
+            AbsoluteScheduleValue(
+                startDate: testDate.addingTimeInterval(.hours(2)),
+                endDate: testDate.addingTimeInterval(.hours(24)),
+                value: LoopQuantity.glucose(150)...LoopQuantity.glucose(170)
+            )
+        ]
+
+        let uniformCorrection = LoopAlgorithm.insulinCorrection(
+            prediction: prediction,
+            at: testDate,
+            target: target,
+            suspendThreshold: suspendThreshold,
+            sensitivity: sensitivity,
+            insulinModel: insulinModel
+        )
+
+        let steppedCorrection = LoopAlgorithm.insulinCorrection(
+            prediction: prediction,
+            at: testDate,
+            target: steppedTarget,
+            suspendThreshold: suspendThreshold,
+            sensitivity: sensitivity,
+            insulinModel: insulinModel
+        )
+
+        let uniformDose = LoopAlgorithm.recommendManualBolus(
+            for: uniformCorrection,
+            maxBolus: 6,
+            currentGlucose: FixtureGlucoseSample(startDate: testDate, quantity: .glucose(160)),
+            target: target
+        )
+
+        let steppedDose = LoopAlgorithm.recommendManualBolus(
+            for: steppedCorrection,
+            maxBolus: 6,
+            currentGlucose: FixtureGlucoseSample(startDate: testDate, quantity: .glucose(160)),
+            target: steppedTarget
+        )
+
+        // With a uniform 90-120 target the whole forecast is above range.
+        XCTAssertGreaterThan(uniformDose.amount, 0.5)
+        // The points beyond t+2h sit inside the 150-170 band in effect there, so
+        // correcting to the per-point band requires less insulin than correcting
+        // the whole forecast to 90-120.
+        XCTAssertLessThan(steppedDose.amount, uniformDose.amount)
+    }
 }