From 050bb11e3302840480e57ec734e9033e7398683a Mon Sep 17 00:00:00 2001
From: Brandon Johnson <brandon.johnson@observiq.com>
Date: Thu, 2 May 2024 11:35:15 -0400
Subject: [PATCH] [pkg/stanza] Take into account the `ascending` option when
 sorting by mtime (#32820)

**Description:** <Describe what has changed.>
* Allow ascending sort when sorting by mtime

**Link to tracking Issue:** Closes #32792

**Testing:**
* Unit tests
---
 .chloggen/feat_mtime-ascending-sort.yaml      | 13 +++++++++
 .../matcher/internal/filter/sort.go           | 28 ++++++++++++++-----
 .../matcher/internal/filter/sort_test.go      | 17 ++++++++++-
 pkg/stanza/fileconsumer/matcher/matcher.go    |  2 +-
 4 files changed, 51 insertions(+), 9 deletions(-)
 create mode 100644 .chloggen/feat_mtime-ascending-sort.yaml

diff --git a/.chloggen/feat_mtime-ascending-sort.yaml b/.chloggen/feat_mtime-ascending-sort.yaml
new file mode 100644
index 0000000000000..16a0bd869f3fe
--- /dev/null
+++ b/.chloggen/feat_mtime-ascending-sort.yaml
@@ -0,0 +1,13 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: "pkg/stanza"
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Allow sorting by ascending order when using the mtime sort_type.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [32792]
diff --git a/pkg/stanza/fileconsumer/matcher/internal/filter/sort.go b/pkg/stanza/fileconsumer/matcher/internal/filter/sort.go
index a8da985b5b70a..2ca233bd38b4e 100644
--- a/pkg/stanza/fileconsumer/matcher/internal/filter/sort.go
+++ b/pkg/stanza/fileconsumer/matcher/internal/filter/sort.go
@@ -142,7 +142,9 @@ func (t TopNOption) apply(items []*item) ([]*item, error) {
 	return items[:t], nil
 }
 
-type mtimeSortOption struct{}
+type mtimeSortOption struct {
+	ascending bool
+}
 
 type mtimeItem struct {
 	mtime time.Time
@@ -168,10 +170,20 @@ func (m mtimeSortOption) apply(items []*item) ([]*item, error) {
 		})
 	}
 
-	sort.SliceStable(mtimeItems, func(i, j int) bool {
-		// This checks if item i > j, in order to reverse the sort (most recently modified file is first in the list)
-		return mtimeItems[i].mtime.After(mtimeItems[j].mtime)
-	})
+	var lessFunc func(i, j int) bool
+	if m.ascending {
+		lessFunc = func(i, j int) bool {
+			// This checks if item i < j
+			return mtimeItems[i].mtime.Before(mtimeItems[j].mtime)
+		}
+	} else {
+		lessFunc = func(i, j int) bool {
+			// This checks if item i > j, in order to reverse the sort (most recently modified file is first in the list)
+			return mtimeItems[i].mtime.After(mtimeItems[j].mtime)
+		}
+	}
+
+	sort.SliceStable(mtimeItems, lessFunc)
 
 	filteredValues := make([]*item, 0, len(items))
 	for _, mtimeItem := range mtimeItems {
@@ -181,6 +193,8 @@ func (m mtimeSortOption) apply(items []*item) ([]*item, error) {
 	return filteredValues, errs
 }
 
-func SortMtime() Option {
-	return mtimeSortOption{}
+func SortMtime(ascending bool) Option {
+	return mtimeSortOption{
+		ascending: ascending,
+	}
 }
diff --git a/pkg/stanza/fileconsumer/matcher/internal/filter/sort_test.go b/pkg/stanza/fileconsumer/matcher/internal/filter/sort_test.go
index 9257d8b12c182..e5f0b89c1cafd 100644
--- a/pkg/stanza/fileconsumer/matcher/internal/filter/sort_test.go
+++ b/pkg/stanza/fileconsumer/matcher/internal/filter/sort_test.go
@@ -179,6 +179,7 @@ func TestMTimeFilter(t *testing.T) {
 		name        string
 		files       []string
 		fileMTimes  []time.Time
+		ascending   bool
 		expectedErr string
 		expect      []string
 	}{
@@ -200,6 +201,20 @@ func TestMTimeFilter(t *testing.T) {
 			fileMTimes: []time.Time{epoch, epoch.Add(time.Hour)},
 			expect:     []string{"b.log", "a.log"},
 		},
+		{
+			name:       "Multiple files in order, ascending sort",
+			files:      []string{"a.log", "b.log"},
+			fileMTimes: []time.Time{epoch, epoch.Add(time.Hour)},
+			ascending:  true,
+			expect:     []string{"a.log", "b.log"},
+		},
+		{
+			name:       "Multiple files out of order, ascending sort",
+			files:      []string{"a.log", "b.log"},
+			fileMTimes: []time.Time{epoch.Add(time.Hour), epoch},
+			ascending:  true,
+			expect:     []string{"b.log", "a.log"},
+		},
 	}
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {
@@ -221,7 +236,7 @@ func TestMTimeFilter(t *testing.T) {
 				items = append(items, it)
 			}
 
-			f := SortMtime()
+			f := SortMtime(tc.ascending)
 			result, err := f.apply(items)
 			if tc.expectedErr != "" {
 				require.EqualError(t, err, tc.expectedErr)
diff --git a/pkg/stanza/fileconsumer/matcher/matcher.go b/pkg/stanza/fileconsumer/matcher/matcher.go
index adcf42da6909e..948f18852a98b 100644
--- a/pkg/stanza/fileconsumer/matcher/matcher.go
+++ b/pkg/stanza/fileconsumer/matcher/matcher.go
@@ -131,7 +131,7 @@ func New(c Criteria) (*Matcher, error) {
 			if !mtimeSortTypeFeatureGate.IsEnabled() {
 				return nil, fmt.Errorf("the %q feature gate must be enabled to use %q sort type", mtimeSortTypeFeatureGate.ID(), sortTypeMtime)
 			}
-			m.filterOpts = append(m.filterOpts, filter.SortMtime())
+			m.filterOpts = append(m.filterOpts, filter.SortMtime(sc.Ascending))
 		default:
 			return nil, fmt.Errorf("'sort_type' must be specified")
 		}