From 96b82fe7258d968d32b314cf1b6ffcfd9ebd9235 Mon Sep 17 00:00:00 2001
From: ross-w <ross@inertia.net.nz>
Date: Sat, 4 Oct 2025 17:26:48 +1000
Subject: [PATCH] Amber: use 15min slots (#24084)

---
 tariff/amber.go | 167 ++++++++++++++++++++++++++++++++++--------------
 1 file changed, 118 insertions(+), 49 deletions(-)

diff --git a/tariff/amber.go b/tariff/amber.go
index 2e2b71cf8..f5b97a073 100644
--- a/tariff/amber.go
+++ b/tariff/amber.go
@@ -91,13 +91,12 @@ func (t *Amber) run(done chan error) {
 			continue
 		}
 
-		// Group by hour and average intervals within each hour
-		hourlyData := make(map[time.Time]*struct {
-			totalValue    float64
-			totalDuration time.Duration
-			start         time.Time
-			currentValue  *float64 // Override with current interval if present (for accurate charging session costs)
-		})
+		// Create and sort time-ordered list of all Amber intervals
+		var intervals []struct {
+			start, end time.Time
+			value      float64
+			isCurrent  bool
+		}
 
 		for _, r := range res {
 			if t.channel == strings.ToLower(r.ChannelType) {
@@ -114,58 +113,128 @@ func (t *Amber) run(done chan error) {
 					value = -value
 				}
 
-				localStart := startTime.Local()
-				localEnd := endTime.Local()
-				hourStart := localStart.Truncate(time.Hour) // Preserve date+hour
-				duration := localEnd.Sub(localStart)
-
-				// Initialize hour entry if needed
-				if hourlyData[hourStart] == nil {
-					hourlyData[hourStart] = &struct {
-						totalValue    float64
-						totalDuration time.Duration
-						start         time.Time
-						currentValue  *float64
-					}{start: hourStart}
-				}
-
-				hr := hourlyData[hourStart]
-
-				// If this is the current interval, use its value directly for this hour
-				if r.Type == "CurrentInterval" {
-					hr.currentValue = &value
-				} else {
-					// Add to weighted average for forecast intervals
-					hr.totalValue += value * duration.Seconds()
-					hr.totalDuration += duration
-				}
+				intervals = append(intervals, struct {
+					start, end time.Time
+					value      float64
+					isCurrent  bool
+				}{
+					start:     startTime.Local(),
+					end:       endTime.Local(),
+					value:     value,
+					isCurrent: r.Type == "CurrentInterval",
+				})
 			}
 		}
 
-		// Convert to final hourly rates
-		data := make(api.Rates, 0, len(hourlyData))
-		for _, hr := range hourlyData {
-			var finalValue float64
-			if hr.currentValue != nil {
-				// Use current interval value if available
-				finalValue = *hr.currentValue
-			} else if hr.totalDuration > 0 {
-				// Otherwise use weighted average of forecast intervals
-				finalValue = hr.totalValue / hr.totalDuration.Seconds()
-			}
-
-			data = append(data, api.Rate{
-				Start: hr.start,
-				End:   hr.start.Add(time.Hour),
-				Value: finalValue,
-			})
+		if len(intervals) == 0 {
+			mergeRates(t.data, nil)
+			once.Do(func() { close(done) })
+			continue
 		}
 
+		// Sort intervals by start time to ensure correct processing
+		slices.SortFunc(intervals, func(a, b struct {
+			start, end time.Time
+			value      float64
+			isCurrent  bool
+		}) int {
+			return a.start.Compare(b.start)
+		})
+
+		data := t.buildSlotRates(intervals)
+
 		mergeRates(t.data, data)
 		once.Do(func() { close(done) })
 	}
 }
 
+// buildSlotRates converts Amber intervals into 15-minute slots using bucket sharding
+// to avoid O(slots × intervals) complexity and only create slots with actual data
+func (t *Amber) buildSlotRates(intervals []struct {
+	start, end time.Time
+	value      float64
+	isCurrent  bool
+}) api.Rates {
+	// Build slot buckets using sharding approach
+	type bucket struct {
+		totalSecs   float64
+		weightedSum float64
+		current     *float64
+	}
+	buckets := make(map[time.Time]*bucket)
+
+	for _, iv := range intervals {
+		// Truncate start to slot boundary
+		slot := iv.start.Truncate(SlotDuration)
+		end := iv.end
+
+		for slot.Before(end) {
+			next := slot.Add(SlotDuration)
+
+			// Compute overlap [max(slot, iv.start), min(next, iv.end))
+			overlapStart := slot
+			if iv.start.After(slot) {
+				overlapStart = iv.start
+			}
+
+			overlapEnd := next
+			if iv.end.Before(next) {
+				overlapEnd = iv.end
+			}
+
+			overlapSecs := overlapEnd.Sub(overlapStart).Seconds()
+
+			b, ok := buckets[slot]
+			if !ok {
+				b = &bucket{}
+				buckets[slot] = b
+			}
+
+			if iv.isCurrent {
+				// Current interval overrides the entire slot
+				b.current = &iv.value
+			} else {
+				// Add to weighted average
+				b.weightedSum += iv.value * overlapSecs
+				b.totalSecs += overlapSecs
+			}
+
+			slot = next
+		}
+	}
+
+	// Convert buckets to sorted rates, skipping empty slots
+	var data api.Rates
+	for start, b := range buckets {
+		var finalValue float64
+		hasValue := false
+
+		if b.current != nil {
+			finalValue = *b.current
+			hasValue = true
+		} else if b.totalSecs > 0 {
+			finalValue = b.weightedSum / b.totalSecs
+			hasValue = true
+		}
+
+		// Only add slots with actual data
+		if hasValue {
+			data = append(data, api.Rate{
+				Start: start,
+				End:   start.Add(SlotDuration),
+				Value: finalValue,
+			})
+		}
+	}
+
+	// Sort by start time
+	slices.SortFunc(data, func(a, b api.Rate) int {
+		return a.Start.Compare(b.Start)
+	})
+
+	return data
+}
+
 // Rates implements the api.Tariff interface
 func (t *Amber) Rates() (api.Rates, error) {
 	var res api.Rates