From 06cd1558bb0ead3b03591b111c54011c02314a9f Mon Sep 17 00:00:00 2001
From: Tyler Yahn <codingalias@gmail.com>
Date: Tue, 2 Jan 2024 13:34:13 -0800
Subject: [PATCH] Use filteredToFront to unique

---
 attribute/set.go | 38 +++++++++++++++++---------------------
 1 file changed, 17 insertions(+), 21 deletions(-)

diff --git a/attribute/set.go b/attribute/set.go
index f9df5eb12b74..f9bbdea0590c 100644
--- a/attribute/set.go
+++ b/attribute/set.go
@@ -250,8 +250,8 @@ func NewSetWithFiltered(kvs []KeyValue, filter Filter) (Set, []KeyValue) {
 // The second []KeyValue return value is a list of attributes that were
 // excluded by the Filter (if non-nil).
 func NewSetWithSortableFiltered(kvs []KeyValue, tmp *Sortable, filter Filter) (Set, []KeyValue) {
-	// Check for empty set.
-	if len(kvs) == 0 {
+	n := len(kvs)
+	if n == 0 {
 		return empty(), nil
 	}
 
@@ -263,28 +263,24 @@ func NewSetWithSortableFiltered(kvs []KeyValue, tmp *Sortable, filter Filter) (S
 
 	*tmp = nil
 
-	position := len(kvs) - 1
-	offset := position - 1
-
-	// The requirements stated above require that the stable
-	// result be placed in the end of the input slice, while
-	// overwritten values are swapped to the beginning.
-	//
-	// De-duplicate with last-value-wins semantics.  Preserve
-	// duplicate values at the beginning of the input slice.
-	for ; offset >= 0; offset-- {
-		if kvs[offset].Key == kvs[position].Key {
-			continue
+	// Use filteredToFront here for last-value-wins semantics.
+	prev := kvs[n-1].Key
+	uniq := filteredToFront(kvs[:n-1], func(kv KeyValue) bool {
+		if kv.Key == prev {
+			return false
 		}
-		position--
-		kvs[offset], kvs[position] = kvs[position], kvs[offset]
-	}
+		prev = kv.Key
+		return true
+	})
+	kvs = kvs[uniq:]
+
 	if filter != nil {
-		return filterSet(kvs[position:], filter)
+		// TODO (MrAlias): there is a potential optimization here where the
+		// filtering and uniquing are all done in the same step. For example:
+		// https://github.com/open-telemetry/opentelemetry-go/blob/228cc878a0f5de19290e8b2035dc4d0019f2a249/attribute/set.go#L266-L300
+		return filterSet(kvs, filter)
 	}
-	return Set{
-		equivalent: computeDistinct(kvs[position:]),
-	}, nil
+	return Set{equivalent: computeDistinct(kvs)}, nil
 }
 
 // filterSet reorders kvs so that included keys are contiguous at the end of