fix: rill time ordinal snapping when anchored on smaller grain (#8812)

Author: AdityaHegde

runtime/pkg/rilltime/rilltime.go

@@ -259,8 +259,8 @@ type EvalOptions struct {
 	FirstDay   int
 	FirstMonth int
 
-	ref          time.Time
-	truncatedRef bool
+	ref                 time.Time
+	largestSnappedGrain timeutil.TimeGrain
 }
 
 func Parse(from string, parseOpts ParseOptions) (*Expression, error) {
@@ -347,9 +347,7 @@ func (e *Expression) Eval(evalOpts EvalOptions) (time.Time, time.Time, timeutil.
 	i := len(e.AnchorOverrides) - 1
 	for i >= 0 {
 		evalOpts.ref, _ = e.AnchorOverrides[i].eval(evalOpts, evalOpts.ref, e.tz)
-		if e.AnchorOverrides[i].truncates() {
-			evalOpts.truncatedRef = true
-		}
+		evalOpts.largestSnappedGrain = max(evalOpts.largestSnappedGrain, e.AnchorOverrides[i].largestSnappedGrain())
 		i--
 	}
 
@@ -487,9 +485,10 @@ func (o *OrdinalInterval) eval(evalOpts EvalOptions, start time.Time, tz *time.L
 		return start, end, tg
 	}
 
-	if !evalOpts.truncatedRef {
-		tg = grainMap[o.Ordinals[len(o.Ordinals)-1].Grain]
-		start = truncateWithCorrection(start, higherOrderMap[tg], tz, evalOpts.FirstDay, evalOpts.FirstMonth)
+	lastTg := grainMap[o.Ordinals[len(o.Ordinals)-1].Grain]
+	if lastTg > evalOpts.largestSnappedGrain {
+		tg = lastTg
+		start = truncateWithCorrection(start, higherOrderMap[lastTg], tz, evalOpts.FirstDay, evalOpts.FirstMonth)
 	}
 
 	i := len(o.Ordinals) - 1
@@ -577,8 +576,19 @@ func (p *PointInTime) eval(evalOpts EvalOptions, tm time.Time, tz *time.Location
 	return tm, tg
 }
 
-func (p *PointInTime) truncates() bool {
-	return len(p.Points) > 0 && p.Points[len(p.Points)-1].Snap != nil
+func (p *PointInTime) largestSnappedGrain() timeutil.TimeGrain {
+	largest := timeutil.TimeGrainUnspecified
+	for _, point := range p.Points {
+		if point.Snap == nil {
+			continue
+		}
+
+		tg := grainMap[*point.Snap]
+		if tg > largest {
+			largest = tg
+		}
+	}
+	return largest
 }
 
 func (p *PointInTimeWithSnap) parse() error {

runtime/pkg/rilltime/rilltime_test.go

@@ -235,6 +235,15 @@ func TestEval_OrdinalVariations(t *testing.T) {
 		{"W1", "2025-04-28T00:00:00Z", "2025-05-05T00:00:00Z", timeutil.TimeGrainDay, 1, 1},
 		{"W1 as of -2M", "2025-03-03T00:00:00Z", "2025-03-10T00:00:00Z", timeutil.TimeGrainDay, 1, 1},
 
+		// Ordinals with lower order truncation
+		{"M2 as of watermark/D", "2025-02-01T00:00:00Z", "2025-03-01T00:00:00Z", timeutil.TimeGrainDay, 1, 1},
+		{"M7 as of watermark/D", "2025-07-01T00:00:00Z", "2025-08-01T00:00:00Z", timeutil.TimeGrainDay, 1, 1},
+		{"D4 as of watermark/H", "2025-05-04T00:00:00Z", "2025-05-05T00:00:00Z", timeutil.TimeGrainHour, 1, 1},
+
+		// Ordinals with higher order truncation
+		{"D6 as of watermark/M", "2025-05-06T00:00:00Z", "2025-05-07T00:00:00Z", timeutil.TimeGrainHour, 1, 1},
+		{"D6 as of watermark/W", "2025-05-17T00:00:00Z", "2025-05-18T00:00:00Z", timeutil.TimeGrainHour, 1, 1},
+
 		// Ordinal chaining variations
 		{"s57 of m4 of H2 of D4 as of -1M", "2025-04-04T01:03:56Z", "2025-04-04T01:03:57Z", timeutil.TimeGrainMillisecond, 1, 1},
 	}