factor out common append-content-warning routine for use in both frontend and backend

4 files changed, 219 insertions, 12 deletions

diff --git a/packages/backend/src/misc/append-content-warning.ts b/packages/backend/src/misc/append-content-warning.ts
new file mode 100644
index 0000000000..152cd6760e
--- /dev/null
+++ b/packages/backend/src/misc/append-content-warning.ts
@@ -0,0 +1,62 @@
+/*
+ * SPDX-FileCopyrightText: hazelnoot and other Sharkey contributors
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+/*
+ * Important Note: this file must be kept in sync with packages/frontend-shared/js/append-content-warning.ts
+ */
+
+/**
+ * Appends an additional content warning onto an existing one.
+ * The additional value will not be added if it already exists within the original input.
+ * @param original Existing content warning
+ * @param additional Content warning to append
+ * @param reverse If true, then the additional CW will be prepended instead of appended.
+ */
+export function appendContentWarning(original: string | null | undefined, additional: string, reverse = false): string {
+	// Easy case - if original is empty, then additional replaces it.
+	if (!original) {
+		return additional;
+	}
+
+	// Easy case - if the additional CW is empty, then don't append it.
+	if (!additional) {
+		return original;
+	}
+
+	// If the additional CW already exists in the input, then we *don't* append another copy!
+	if (includesWholeWord(original, additional)) {
+		return original;
+	}
+
+	return reverse
+		? `${additional}, ${original}`
+		: `${original}, ${additional}`;
+}
+
+/**
+ * Emulates a regular expression like /\b(pattern)\b/, but with a raw non-regex pattern.
+ * We're checking to see whether the default CW appears inside the existing CW, but *only* if there's word boundaries on either side.
+ * @param input Input string to search
+ * @param target Target word / phrase to search for
+ */
+function includesWholeWord(input: string, target: string): boolean {
+	const parts = input.split(target);
+
+	// The additional string could appear multiple times within the original input.
+	// We need to check each occurrence, since any of them could potentially match.
+	for (let i = 0; i + 1 < parts.length; i++) {
+		const before = parts[i];
+		const after = parts[i + 1];
+
+		// If either the preceding or following tokens are a "word", then this "match" is actually just part of a longer word.
+		// Likewise, if *neither* token is a word, then this is a real match and the CW already exists in the input.
+		if (!/\w$/.test(before) && !/^\w/.test(after)) {
+			return true;
+		}
+	}
+
+	// If we don't match, then there is no existing CW.
+	return false;
+}
diff --git a/packages/backend/test/unit/misc/append-content-warning.ts b/packages/backend/test/unit/misc/append-content-warning.ts
new file mode 100644
index 0000000000..d25d7c4925
--- /dev/null
+++ b/packages/backend/test/unit/misc/append-content-warning.ts
@@ -0,0 +1,92 @@
+/*
+ * SPDX-FileCopyrightText: hazelnoot and other Sharkey contributors
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { appendContentWarning } from '@/misc/append-content-warning.js';
+
+describe(appendContentWarning, () => {
+	it('should return additional when original is null', () => {
+		const result = appendContentWarning(null, 'additional');
+
+		expect(result).toBe('additional');
+	});
+
+	it('should return additional when original is undefined', () => {
+		const result = appendContentWarning(undefined, 'additional');
+
+		expect(result).toBe('additional');
+	});
+
+	it('should return additional when original is empty', () => {
+		const result = appendContentWarning('', 'additional');
+
+		expect(result).toBe('additional');
+	});
+
+	it('should return original when additional is empty', () => {
+		const result = appendContentWarning('original', '');
+
+		expect(result).toBe('original');
+	});
+
+	it('should append additional when it does not exist in original', () => {
+		const result = appendContentWarning('original', 'additional');
+
+		expect(result).toBe('original, additional');
+	});
+
+	it('should append additional when it exists in original but has preceeding word', () => {
+		const result = appendContentWarning('notadditional', 'additional');
+
+		expect(result).toBe('notadditional, additional');
+	});
+
+	it('should append additional when it exists in original but has following word', () => {
+		const result = appendContentWarning('additionalnot', 'additional');
+
+		expect(result).toBe('additionalnot, additional');
+	});
+
+	it('should append additional when it exists in original multiple times but has preceeding or following word', () => {
+		const result = appendContentWarning('notadditional additionalnot', 'additional');
+
+		expect(result).toBe('notadditional additionalnot, additional');
+	});
+
+	it('should not append additional when it exists in original', () => {
+		const result = appendContentWarning('an additional word', 'additional');
+
+		expect(result).toBe('an additional word');
+	});
+
+	it('should not append additional when original starts with it', () => {
+		const result = appendContentWarning('additional word', 'additional');
+
+		expect(result).toBe('additional word');
+	});
+
+	it('should not append additional when original ends with it', () => {
+		const result = appendContentWarning('an additional', 'additional');
+
+		expect(result).toBe('an additional');
+	});
+
+	it('should not append additional when it appears multiple times', () => {
+		const result = appendContentWarning('an additional additional word', 'additional');
+
+		expect(result).toBe('an additional additional word');
+	});
+
+	it('should not append additional when it appears multiple times but some have preceeding or following', () => {
+		const result = appendContentWarning('a notadditional additional additionalnot word', 'additional');
+
+		expect(result).toBe('a notadditional additional additionalnot word');
+	});
+
+	it('should prepend additional when reverse is true', () => {
+		const result = appendContentWarning('original', 'additional', true);
+
+		expect(result).toBe('additional, original');
+	});
+});
diff --git a/packages/frontend-shared/js/append-content-warning.ts b/packages/frontend-shared/js/append-content-warning.ts
new file mode 100644
index 0000000000..7f24a66f23
--- /dev/null
+++ b/packages/frontend-shared/js/append-content-warning.ts
@@ -0,0 +1,62 @@
+/*
+ * SPDX-FileCopyrightText: hazelnoot and other Sharkey contributors
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+/*
+ * Important Note: this file must be kept in sync with packages/backend/src/misc/append-content-warning.ts
+ */
+
+/**
+ * Appends an additional content warning onto an existing one.
+ * The additional value will not be added if it already exists within the original input.
+ * @param original Existing content warning
+ * @param additional Content warning to append
+ * @param reverse If true, then the additional CW will be prepended instead of appended.
+ */
+export function appendContentWarning(original: string | null | undefined, additional: string, reverse = false): string {
+	// Easy case - if original is empty, then additional replaces it.
+	if (!original) {
+		return additional;
+	}
+
+	// Easy case - if the additional CW is empty, then don't append it.
+	if (!additional) {
+		return original;
+	}
+
+	// If the additional CW already exists in the input, then we *don't* append another copy!
+	if (includesWholeWord(original, additional)) {
+		return original;
+	}
+
+	return reverse
+		? `${additional}, ${original}`
+		: `${original}, ${additional}`;
+}
+
+/**
+ * Emulates a regular expression like /\b(pattern)\b/, but with a raw non-regex pattern.
+ * We're checking to see whether the default CW appears inside the existing CW, but *only* if there's word boundaries on either side.
+ * @param input Input string to search
+ * @param target Target word / phrase to search for
+ */
+function includesWholeWord(input: string, target: string): boolean {
+	const parts = input.split(target);
+
+	// The additional string could appear multiple times within the original input.
+	// We need to check each occurrence, since any of them could potentially match.
+	for (let i = 0; i + 1 < parts.length; i++) {
+		const before = parts[i];
+		const after = parts[i + 1];
+
+		// If either the preceding or following tokens are a "word", then this "match" is actually just part of a longer word.
+		// Likewise, if *neither* token is a word, then this is a real match and the CW already exists in the input.
+		if (!/\w$/.test(before) && !/^\w/.test(after)) {
+			return true;
+		}
+	}
+
+	// If we don't match, then there is no existing CW.
+	return false;
+}
diff --git a/packages/frontend/src/components/MkPostForm.vue b/packages/frontend/src/components/MkPostForm.vue
index 059de8011c..c5f5f4514d 100644
--- a/packages/frontend/src/components/MkPostForm.vue
+++ b/packages/frontend/src/components/MkPostForm.vue
@@ -112,6 +112,7 @@ import * as Misskey from 'misskey-js';
 import insertTextAtCursor from 'insert-text-at-cursor';
 import { toASCII } from 'punycode.js';
 import { host, url } from '@@/js/config.js';
+import { appendContentWarning } from '@@/js/append-content-warning.js';
 import type { MenuItem } from '@/types/menu.js';
 import type { PostFormProps } from '@/types/post-form.js';
 import MkNoteSimple from '@/components/MkNoteSimple.vue';
@@ -373,18 +374,8 @@ if ($i.defaultCW) {
 	if (!cw.value || $i.defaultCWPriority === 'default') {
 		cw.value = $i.defaultCW;
 	} else if ($i.defaultCWPriority !== 'parent') {
-		// This is a fancy way of simulating /\bsearch\b/ without a regular expression.
-		// We're checking to see whether the default CW appears inside the existing CW, but *only* if there's word boundaries.
-		const parts = cw.value.split($i.defaultCW);
-		const hasExistingDefaultCW = parts.length === 2 && !/\w$/.test(parts[0]) && !/^\w/.test(parts[1]);
-		if (!hasExistingDefaultCW) {
-			// We need to merge the CWs
-			if ($i.defaultCWPriority === 'defaultParent') {
-				cw.value = `${$i.defaultCW}, ${cw.value}`;
-			} else if ($i.defaultCWPriority === 'parentDefault') {
-				cw.value = `${cw.value}, ${$i.defaultCW}`;
-			}
-		}
+		const putDefaultFirst = $i.defaultCWPriority === 'defaultParent';
+		cw.value = appendContentWarning(cw.value, $i.defaultCW, putDefaultFirst);
 	}
 	// else { do nothing, because existing CW takes priority. }
 }