fix(prices): preserve coverage around finer price slots

.changeset/keep-uncovered-price-slots.md

@@ -0,0 +1,5 @@
+---
+"ftw": patch
+---
+
+When ENTSO-E returns hourly prices and only some quarter-hour prices, keep the hourly price for the uncovered minutes. A partial quarter-hour series no longer creates gaps in the plan's price data.

go/internal/prices/prices.go

@@ -34,6 +34,7 @@ import (
 	"log/slog"
 	"net/http"
 	"net/url"
+	"sort"
 	"strconv"
 	"strings"
 	"sync"
@@ -431,25 +432,40 @@ func (e *ENTSOEProvider) parseXML(body []byte) ([]RawPrice, error) {
 	return keepFinestRows(out), nil
 }
 
-// keepFinestRows drops each row that overlaps a finer one. One A44 document
-// can publish a delivery day at both PT15M and PT60M; storing both would give
-// the planner overlapping slots.
+// keepFinestRows gives finer rows precedence only over the time they cover.
+// A partial PT15M series must not erase the rest of a PT60M delivery hour.
 func keepFinestRows(rows []RawPrice) []RawPrice {
 	out := make([]RawPrice, 0, len(rows))
 	for _, r := range rows {
-		end := r.SlotStart.Add(time.Duration(r.SlotLenMin) * time.Minute)
-		finer := false
+		parts := []RawPrice{r}
 		for _, o := range rows {
-			if o.SlotLenMin < r.SlotLenMin && o.SlotStart.Before(end) &&
-				r.SlotStart.Before(o.SlotStart.Add(time.Duration(o.SlotLenMin)*time.Minute)) {
-				finer = true
-				break
+			if o.SlotLenMin >= r.SlotLenMin {
+				continue
 			}
+			fineEnd := o.SlotStart.Add(time.Duration(o.SlotLenMin) * time.Minute)
+			var remaining []RawPrice
+			for _, part := range parts {
+				end := part.SlotStart.Add(time.Duration(part.SlotLenMin) * time.Minute)
+				if !o.SlotStart.Before(end) || !part.SlotStart.Before(fineEnd) {
+					remaining = append(remaining, part)
+					continue
+				}
+				if part.SlotStart.Before(o.SlotStart) {
+					left := part
+					left.SlotLenMin = int(o.SlotStart.Sub(part.SlotStart) / time.Minute)
+					remaining = append(remaining, left)
+				}
+				if fineEnd.Before(end) {
+					part.SlotStart = fineEnd
+					part.SlotLenMin = int(end.Sub(fineEnd) / time.Minute)
+					remaining = append(remaining, part)
+				}
+			}
+			parts = remaining
 		}
-		if !finer {
-			out = append(out, r)
-		}
+		out = append(out, parts...)
 	}
+	sort.SliceStable(out, func(i, j int) bool { return out[i].SlotStart.Before(out[j].SlotStart) })
 	return out
 }
 

go/internal/prices/prices_overlap_test.go

@@ -0,0 +1,43 @@
+package prices
+
+import (
+	"reflect"
+	"testing"
+	"time"
+)
+
+func TestMixedPriceResolutionPreservesUncoveredMinutes(t *testing.T) {
+	start := time.Date(2026, 9, 27, 12, 0, 0, 0, time.UTC)
+	row := func(offset, minutes int, price float64) RawPrice {
+		return RawPrice{SlotStart: start.Add(time.Duration(offset) * time.Minute), SlotLenMin: minutes, SEKPerKWh: price}
+	}
+	for _, tc := range []struct {
+		name string
+		fine []RawPrice
+		want []RawPrice
+	}{
+		{"first quarter", []RawPrice{row(0, 15, 2)}, []RawPrice{row(0, 15, 2), row(15, 45, 1)}},
+		{"middle quarter", []RawPrice{row(15, 15, 2)}, []RawPrice{row(0, 15, 1), row(15, 15, 2), row(30, 30, 1)}},
+		{"last quarter", []RawPrice{row(45, 15, 2)}, []RawPrice{row(0, 45, 1), row(45, 15, 2)}},
+		{"two separated quarters", []RawPrice{row(45, 15, 3), row(0, 15, 2)}, []RawPrice{row(0, 15, 2), row(15, 30, 1), row(45, 15, 3)}},
+		{"nested resolutions", []RawPrice{row(0, 30, 2), row(15, 15, 3)}, []RawPrice{row(0, 15, 2), row(15, 15, 3), row(30, 30, 1)}},
+		{"touching boundaries", []RawPrice{row(-15, 15, 2), row(60, 15, 3)}, []RawPrice{row(-15, 15, 2), row(0, 60, 1), row(60, 15, 3)}},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			rows := append([]RawPrice{row(0, 60, 1)}, tc.fine...)
+			original := append([]RawPrice(nil), rows...)
+			if got := keepFinestRows(rows); !reflect.DeepEqual(got, tc.want) {
+				t.Fatalf("prices = %+v, want %+v", got, tc.want)
+			}
+			if !reflect.DeepEqual(rows, original) {
+				t.Fatal("input prices changed")
+			}
+			for i, j := 0, len(rows)-1; i < j; i, j = i+1, j-1 {
+				rows[i], rows[j] = rows[j], rows[i]
+			}
+			if got := keepFinestRows(rows); !reflect.DeepEqual(got, tc.want) {
+				t.Fatalf("reversed prices = %+v, want %+v", got, tc.want)
+			}
+		})
+	}
+}