rowutil.ToVersionedRows: Inner slices are chronological, outer one ca…

…n be either (sorted with given rowLess func)

rowutil/version.go

@@ -18,8 +18,8 @@ func ToVersionedRows(origRows types.Rows, rowLess func(r1, r2 *types.Row) bool)
 	rows := make(types.Rows, len(origRows))
 	copy(rows, origRows)
 
-	// Sort ~original
-	rows.Sort(rowLess)
+	// Every inner slice will remain in chronological order (ascending)
+	rows.Sort(ByTagPrefix("created:", true))
 
 	// 2D return value
 	var rrows []types.Rows
@@ -46,21 +46,23 @@ func ToVersionedRows(origRows types.Rows, rowLess func(r1, r2 *types.Row) bool)
 
 	// Each version slice is now individually ordered, but we need to
 	// return them in a [][]*Row. What should the ordering of this
-	// slice of slices be? Answer: ordered by the first member of each
-	// version slice.
+	// slice of slices be? Answer: ordered by the last member of each
+	// version slice (in either ascending or descending order, as
+	// determined by rowLess).
 
-	firsts := make(types.Rows, 0, len(mRows))
-	firstToRows := make(map[*types.Row]types.Rows, len(firsts))
+	lasts := make(types.Rows, 0, len(mRows))
+	lastToRows := make(map[*types.Row]types.Rows, len(lasts))
 
 	for _, rowGroup := range mRows {
-		firsts = append(firsts, rowGroup[0])
-		// Map the first element of each slice to its containing slice
-		firstToRows[rowGroup[0]] = rowGroup
+		last := rowGroup[len(rowGroup)-1]
+		lasts = append(lasts, last)
+		// Map the last element of each slice to its containing slice
+		lastToRows[last] = rowGroup
 	}
-	firsts.Sort(rowLess)
+	lasts.Sort(rowLess)
 
-	for _, fr := range firsts {
-		rrows = append(rrows, firstToRows[fr])
+	for _, fr := range lasts {
+		rrows = append(rrows, lastToRows[fr])
 	}
 
 	return rrows

rowutil/version_test.go

@@ -30,8 +30,8 @@ func TestToVersionedRows(t *testing.T) {
 
 	orig0 := types.Rows{r0, r1, r2, r3, r4, r5, r6, r7}
 	want0 := []types.Rows{
-		types.Rows{r0, r1, r3, r4},
 		types.Rows{r2},
+		types.Rows{r0, r1, r3, r4},
 		types.Rows{r5},
 		types.Rows{r6, r7},
 	}
@@ -53,9 +53,9 @@ func TestToVersionedRows(t *testing.T) {
 	// Descending
 
 	want0 = []types.Rows{
-		types.Rows{r7, r6},
+		types.Rows{r6, r7},
 		types.Rows{r5},
-		types.Rows{r4, r3, r1, r0},
+		types.Rows{r0, r1, r3, r4},
 		types.Rows{r2},
 	}
 	want1 = want0