save / load column mapping file for delimiter-separated values (dsv) file

src/lib/ui/common.ts

@@ -4,6 +4,8 @@ import { ThemeType } from '@/core/theme.ts';
 
 import { type AmountColor, PresetAmountColor } from '@/core/color.ts';
 
+import logger from '../logger.ts';
+
 export function getSystemTheme(): ThemeType {
     if (window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches) {
         return ThemeType.Dark;
@@ -84,6 +86,43 @@ export function copyTextToClipboard(text: string, container?: Element | null): v
     });
 }
 
+export function openTextFileContent({ allowedExtensions }: { allowedExtensions: string }): Promise<string> {
+    return new Promise((resolve, reject) => {
+        const fileInput = document.createElement('input');
+
+        fileInput.style.display = 'none';
+        fileInput.type = 'file';
+        fileInput.accept = allowedExtensions;
+
+        fileInput.onchange = (event) => {
+            const el = event.target as HTMLInputElement;
+
+            if (el.files && el.files.length > 0) {
+                const file = el.files[0];
+                const reader = new FileReader();
+
+                reader.onload = (e) => {
+                    const content = e.target?.result;
+
+                    if (typeof content === 'string') {
+                        resolve(content);
+                    } else {
+                        reject(new Error('failed to load file, because file reader result is not string'));
+                    }
+                };
+
+                reader.onerror = (error) => {
+                    logger.error('failed to load file', error);
+                };
+
+                reader.readAsText(file);
+            }
+        };
+
+        fileInput.click();
+    });
+}
+
 export function startDownloadFile(fileName: string, fileData: Blob): void {
     const dataObjectUrl = URL.createObjectURL(fileData);
     const dataLink = document.createElement('a');