1 files changed, 166 insertions, 0 deletions
diff --git a/packages/core/src/utils/systemEncoding.ts b/packages/core/src/utils/systemEncoding.ts
new file mode 100644
index 00000000..f162c223
--- /dev/null
+++ b/packages/core/src/utils/systemEncoding.ts
@@ -0,0 +1,166 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { execSync } from 'child_process';
+import os from 'os';
+import { detect as chardetDetect } from 'chardet';
+
+// Cache for system encoding to avoid repeated detection
+// Use undefined to indicate "not yet checked" vs null meaning "checked but failed"
+let cachedSystemEncoding: string | null | undefined = undefined;
+
+/**
+ * Reset the encoding cache - useful for testing
+ */
+export function resetEncodingCache(): void {
+  cachedSystemEncoding = undefined;
+}
+
+/**
+ * Returns the system encoding, caching the result to avoid repeated system calls.
+ * If system encoding detection fails, falls back to detecting from the provided buffer.
+ * Note: Only the system encoding is cached - buffer-based detection runs for each buffer
+ * since different buffers may have different encodings.
+ * @param buffer A buffer to use for detecting encoding if system detection fails.
+ */
+export function getCachedEncodingForBuffer(buffer: Buffer): string {
+  // Cache system encoding detection since it's system-wide
+  if (cachedSystemEncoding === undefined) {
+    cachedSystemEncoding = getSystemEncoding();
+  }
+
+  // If we have a cached system encoding, use it
+  if (cachedSystemEncoding) {
+    return cachedSystemEncoding;
+  }
+
+  // Otherwise, detect from this specific buffer (don't cache this result)
+  return detectEncodingFromBuffer(buffer) || 'utf-8';
+}
+
+/**
+ * Detects the system encoding based on the platform.
+ * For Windows, it uses the 'chcp' command to get the current code page.
+ * For Unix-like systems, it checks environment variables like LC_ALL, LC_CTYPE, and LANG.
+ * If those are not set, it tries to run 'locale charmap' to get the encoding.
+ * If detection fails, it returns null.
+ * @returns The system encoding as a string, or null if detection fails.
+ */
+export function getSystemEncoding(): string | null {
+  // Windows
+  if (os.platform() === 'win32') {
+    try {
+      const output = execSync('chcp', { encoding: 'utf8' });
+      const match = output.match(/:\s*(\d+)/);
+      if (match) {
+        const codePage = parseInt(match[1], 10);
+        if (!isNaN(codePage)) {
+          return windowsCodePageToEncoding(codePage);
+        }
+      }
+      // Only warn if we can't parse the output format, not if windowsCodePageToEncoding fails
+      throw new Error(
+        `Unable to parse Windows code page from 'chcp' output "${output.trim()}". `,
+      );
+    } catch (error) {
+      console.warn(
+        `Failed to get Windows code page using 'chcp' command: ${error instanceof Error ? error.message : String(error)}. ` +
+          `Will attempt to detect encoding from command output instead.`,
+      );
+    }
+    return null;
+  }
+
+  // Unix-like
+  // Use environment variables LC_ALL, LC_CTYPE, and LANG to determine the
+  // system encoding. However, these environment variables might not always
+  // be set or accurate. Handle cases where none of these variables are set.
+  const env = process.env;
+  let locale = env.LC_ALL || env.LC_CTYPE || env.LANG || '';
+
+  // Fallback to querying the system directly when environment variables are missing
+  if (!locale) {
+    try {
+      locale = execSync('locale charmap', { encoding: 'utf8' })
+        .toString()
+        .trim();
+    } catch (_e) {
+      console.warn('Failed to get locale charmap.');
+      return null;
+    }
+  }
+
+  const match = locale.match(/\.(.+)/); // e.g., "en_US.UTF-8"
+  if (match && match[1]) {
+    return match[1].toLowerCase();
+  }
+
+  // Handle cases where locale charmap returns just the encoding name (e.g., "UTF-8")
+  if (locale && !locale.includes('.')) {
+    return locale.toLowerCase();
+  }
+
+  return null;
+}
+
+/**
+ * Converts a Windows code page number to a corresponding encoding name.
+ * @param cp The Windows code page number (e.g., 437, 850, etc.)
+ * @returns The corresponding encoding name as a string, or null if no mapping exists.
+ */
+export function windowsCodePageToEncoding(cp: number): string | null {
+  // Most common mappings; extend as needed
+  const map: { [key: number]: string } = {
+    437: 'cp437',
+    850: 'cp850',
+    852: 'cp852',
+    866: 'cp866',
+    874: 'windows-874',
+    932: 'shift_jis',
+    936: 'gb2312',
+    949: 'euc-kr',
+    950: 'big5',
+    1200: 'utf-16le',
+    1201: 'utf-16be',
+    1250: 'windows-1250',
+    1251: 'windows-1251',
+    1252: 'windows-1252',
+    1253: 'windows-1253',
+    1254: 'windows-1254',
+    1255: 'windows-1255',
+    1256: 'windows-1256',
+    1257: 'windows-1257',
+    1258: 'windows-1258',
+    65001: 'utf-8',
+  };
+
+  if (map[cp]) {
+    return map[cp];
+  }
+
+  console.warn(`Unable to determine encoding for windows code page ${cp}.`);
+  return null; // Return null if no mapping found
+}
+
+/**
+ * Attempts to detect encoding from a buffer using chardet.
+ * This is useful when system encoding detection fails.
+ * Returns the detected encoding in lowercase, or null if detection fails.
+ * @param buffer The buffer to analyze for encoding.
+ * @return The detected encoding as a lowercase string, or null if detection fails.
+ */
+export function detectEncodingFromBuffer(buffer: Buffer): string | null {
+  try {
+    const detected = chardetDetect(buffer);
+    if (detected && typeof detected === 'string') {
+      return detected.toLowerCase();
+    }
+  } catch (error) {
+    console.warn('Failed to detect encoding with chardet:', error);
+  }
+
+  return null;
+}