feat: Add message validation utility for chat input (#15)

frontend/src/lib/messageValidation.ts

@@ -0,0 +1,104 @@
+// Message Validation Utility for OpenMindWell
+// Handles input validation, character limits, and user feedback
+
+const MAX_MESSAGE_LENGTH = 500;
+const WARNING_THRESHOLD_PERCENT = 80;
+
+export interface MessageValidationResult {
+  isValid: boolean;
+  error?: string;
+  characterCount: number;
+  remainingCharacters: number;
+  isNearLimit: boolean;
+}
+
+/**
+ * Validates a chat message for sending
+ * @param message - The message text to validate
+ * @returns Validation result with error message if invalid
+ */
+export const validateMessage = (message: string): MessageValidationResult => {
+  // Trim whitespace
+  const trimmedMessage = message.trim();
+
+  // Check if message is empty
+  if (!trimmedMessage || trimmedMessage.length === 0) {
+    return {
+      isValid: false,
+      error: 'Message cannot be empty',
+      characterCount: 0,
+      remainingCharacters: MAX_MESSAGE_LENGTH,
+      isNearLimit: false,
+    };
+  }
+
+  // Check if message exceeds maximum length
+  if (trimmedMessage.length > MAX_MESSAGE_LENGTH) {
+    return {
+      isValid: false,
+      error: `Message exceeds maximum length of ${MAX_MESSAGE_LENGTH} characters`,
+      characterCount: trimmedMessage.length,
+      remainingCharacters: 0,
+      isNearLimit: true,
+    };
+  }
+
+  // Calculate if near limit
+  const isNearLimit = (trimmedMessage.length / MAX_MESSAGE_LENGTH) * 100 >= WARNING_THRESHOLD_PERCENT;
+
+  return {
+    isValid: true,
+    characterCount: trimmedMessage.length,
+    remainingCharacters: MAX_MESSAGE_LENGTH - trimmedMessage.length,
+    isNearLimit,
+  };
+};
+
+/**
+ * Gets the character count for a message
+ * @param message - The message text
+ * @returns Character count and remaining characters
+ */
+export const getMessageStats = (message: string) => {
+  const trimmedMessage = message.trim();
+  const characterCount = trimmedMessage.length;
+  const remainingCharacters = MAX_MESSAGE_LENGTH - characterCount;
+  const percentUsed = (characterCount / MAX_MESSAGE_LENGTH) * 100;
+
+  return {
+    characterCount,
+    remainingCharacters,
+    percentUsed: Math.round(percentUsed),
+    maxLength: MAX_MESSAGE_LENGTH,
+    isNearLimit: percentUsed >= WARNING_THRESHOLD_PERCENT,
+  };
+};
+
+/**
+ * Checks if the send button should be disabled
+ * @param message - The message text
+ * @returns True if button should be disabled
+ */
+export const isSendButtonDisabled = (message: string): boolean => {
+  const trimmedMessage = message.trim();
+  return trimmedMessage.length === 0 || trimmedMessage.length > MAX_MESSAGE_LENGTH;
+};
+
+/**
+ * Gets warning message based on character count
+ * @param message - The message text
+ * @returns Warning message or empty string
+ */
+export const getWarningMessage = (message: string): string => {
+  const stats = getMessageStats(message);
+
+  if (stats.percentUsed >= 95) {
+    return `⚠️ Almost at limit: ${stats.remainingCharacters} characters remaining`;
+  }
+
+  if (stats.isNearLimit) {
+    return `📝 ${stats.remainingCharacters} characters remaining`;
+  }
+
+  return '';
+};