From 4b8747a5fdbf2a223404f612b8e7a58d39ef6509 Mon Sep 17 00:00:00 2001
From: daisuke8000 <daisuke12108000@gmail.com>
Date: Mon, 6 Jul 2026 00:16:14 +0900
Subject: [PATCH] Format jq outputs into one String without Vec (DSK-137)

Replace collect/join in Replace and Append modes with a single-pass
write into the destination string.

Co-authored-by: Cursor <cursoragent@cursor.com>
---
 .../rustern-core/src/pipeline/jq_evaluate.rs  | 30 +++++++++++--------
 1 file changed, 17 insertions(+), 13 deletions(-)

diff --git a/crates/rustern-core/src/pipeline/jq_evaluate.rs b/crates/rustern-core/src/pipeline/jq_evaluate.rs
index ddbee0f..40882b2 100644
--- a/crates/rustern-core/src/pipeline/jq_evaluate.rs
+++ b/crates/rustern-core/src/pipeline/jq_evaluate.rs
@@ -1,10 +1,11 @@
-use std::sync::Arc;
-
-use futures::stream::{Stream, StreamExt};
 use jaq_core::data::JustLut;
 use jaq_core::load::{Arena, File, Loader};
 use jaq_core::{Compiler, Ctx, Filter, Vars};
 use jaq_json::Val;
+use std::fmt::Write as _;
+use std::sync::Arc;
+
+use futures::stream::{Stream, StreamExt};
 
 use crate::source::{LogEvent, LogSourceError};
 
@@ -23,6 +24,17 @@ pub enum QueryMode {
     Filter,
 }
 
+fn format_outputs_joined(outputs: &[Val]) -> String {
+    let mut rendered = String::new();
+    for (i, v) in outputs.iter().enumerate() {
+        if i > 0 {
+            rendered.push('\n');
+        }
+        let _ = write!(rendered, "{v}");
+    }
+    rendered
+}
+
 #[derive(Clone)]
 pub struct CompiledFilter {
     inner: Arc<Filter<JustLut<Val>>>,
@@ -105,21 +117,13 @@ where
                         }
                         QueryMode::Replace => {
                             ev.structured = None;
-                            let rendered = outputs
-                                .iter()
-                                .map(|v| format!("{v}"))
-                                .collect::<Vec<_>>()
-                                .join("\n");
+                            let rendered = format_outputs_joined(&outputs);
                             ev.message = Arc::from(rendered.as_str());
                             Some(Ok(ev))
                         }
                         QueryMode::Append => {
                             ev.structured = None;
-                            let rendered = outputs
-                                .iter()
-                                .map(|v| format!("{v}"))
-                                .collect::<Vec<_>>()
-                                .join("\n");
+                            let rendered = format_outputs_joined(&outputs);
                             ev.message =
                                 Arc::from(format!("{} | {}", ev.message, rendered).as_str());
                             Some(Ok(ev))