perf: hoist TextEncoder/TextDecoder to module level in readers

Inline `new TextDecoder()` / `new TextEncoder()` allocations replaced
with module-level singletons in reader.ts, csv-reader.ts, json-reader.ts,
and arrow-reader.ts — avoids per-call allocation during ingest.

Author: teamchong

src/reader.ts

@@ -9,6 +9,8 @@
 import type { ColumnMeta, DataType } from "./types.js";
 import type { FragmentSource } from "./operators.js";
 
+const textEncoder = new TextEncoder();
+
 // ---------------------------------------------------------------------------
 // FormatReader — one per file format (CSV, JSON, Arrow IPC, ...)
 // ---------------------------------------------------------------------------
@@ -238,12 +240,11 @@ export function encodeColumnBuffer(
     }
     case "utf8":
     case "binary": {
-      const encoder = new TextEncoder();
       const parts: Uint8Array[] = [];
       let totalLen = 0;
       for (const v of values) {
         const str = v === null ? "" : String(v);
-        const encoded = encoder.encode(str);
+        const encoded = textEncoder.encode(str);
         const header = new Uint8Array(4);
         new DataView(header.buffer).setUint32(0, encoded.length, true);
         parts.push(header, encoded);

src/readers/arrow-reader.ts

@@ -59,12 +59,14 @@ function fbOffset(buf: DataView, tableOffset: number, fieldIdx: number): number
   return tableOffset + fieldOff;
 }
 
+const textDecoder = new TextDecoder();
+
 /** Read a flatbuffer string. */
 function fbString(buf: DataView, strOffset: number): string {
   const len = readI32(buf, strOffset);
   const start = strOffset + 4;
   const bytes = new Uint8Array(buf.buffer, buf.byteOffset + start, len);
-  return new TextDecoder().decode(bytes);
+  return textDecoder.decode(bytes);
 }
 
 /** Read a flatbuffer vector length. */

src/readers/csv-reader.ts

@@ -13,6 +13,8 @@ import { type FormatReader, type DataSource, encodeColumnBuffer } from "../reade
 import type { ColumnMeta, DataType, PageInfo, Row } from "../types.js";
 import type { FragmentSource } from "../operators.js";
 
+const textDecoder = new TextDecoder("utf-8");
+
 // ---------------------------------------------------------------------------
 // CSV Parser Helpers
 // ---------------------------------------------------------------------------
@@ -337,8 +339,7 @@ export class CsvReader implements FormatReader {
     if (first === 0x5b || first === 0x7b) return false;
 
     // Decode a chunk of text and check for a delimiter pattern
-    const decoder = new TextDecoder("utf-8");
-    const sample = decoder.decode(head.slice(0, Math.min(head.length, 2048)));
+    const sample = textDecoder.decode(head.slice(0, Math.min(head.length, 2048)));
     const firstLine = sample.split(/\r?\n/)[0] ?? "";
     if (firstLine.length === 0) return false;
 
@@ -349,15 +350,15 @@ export class CsvReader implements FormatReader {
 
   async readMeta(source: DataSource): Promise<{ columns: ColumnMeta[]; totalRows: number }> {
     const buf = await source.readAll();
-    const text = new TextDecoder().decode(buf);
+    const text = textDecoder.decode(buf);
     const parsed = parseCsvFull(text);
     const columns = buildColumnMeta(parsed);
     return { columns, totalRows: parsed.rowCount };
   }
 
   async createFragments(source: DataSource, projected: ColumnMeta[]): Promise<FragmentSource[]> {
     const buf = await source.readAll();
-    const text = new TextDecoder().decode(buf);
+    const text = textDecoder.decode(buf);
     const parsed = parseCsvFull(text);
     // Build full column meta so the fragment has the full schema
     const allMeta = buildColumnMeta(parsed);

src/readers/json-reader.ts

@@ -12,6 +12,8 @@ import { type FormatReader, type DataSource, encodeColumnBuffer } from "../reade
 import type { ColumnMeta, DataType, PageInfo, Row } from "../types.js";
 import type { FragmentSource } from "../operators.js";
 
+const textDecoder = new TextDecoder();
+
 // ---------------------------------------------------------------------------
 // Helpers
 // ---------------------------------------------------------------------------
@@ -282,15 +284,15 @@ export class JsonReader implements FormatReader {
 
   async readMeta(source: DataSource): Promise<{ columns: ColumnMeta[]; totalRows: number }> {
     const buf = await source.readAll();
-    const text = new TextDecoder().decode(buf);
+    const text = textDecoder.decode(buf);
     const parsed = parseJsonFull(text);
     const columns = buildColumnMeta(parsed);
     return { columns, totalRows: parsed.rowCount };
   }
 
   async createFragments(source: DataSource, projected: ColumnMeta[]): Promise<FragmentSource[]> {
     const buf = await source.readAll();
-    const text = new TextDecoder().decode(buf);
+    const text = textDecoder.decode(buf);
     const parsed = parseJsonFull(text);
     const allMeta = buildColumnMeta(parsed);
     const projectedNames = new Set(projected.map(c => c.name));