Gsm7SmsValidator.ts

export type SMSEncoding = 'LATIN' | 'UNICODE';

export interface ValidationResult {
  isValid: boolean;
  characterCount: number;
  segmentCount: number;
  encodingUsed: SMSEncoding;
  error?: string;
}

/**
 * Validates and calculates segment counts for an internal SMS gateway.
 * @param text The actual SMS body string.
 * @param preferredEncoding 'LATIN' (GSM-7) or 'UNICODE' (UCS-2).
 * @param maxSegments Optional cap to prevent runaway billing (Default: 5).
 */
export function validateSMS(
  text: string, 
  preferredEncoding: SMSEncoding, 
  maxSegments: number = 5
): ValidationResult {
  
  if (!text || text.length === 0) {
    return { isValid: false, characterCount: 0, segmentCount: 0, encodingUsed: preferredEncoding, error: 'SMS body cannot be empty.' };
  }

  let finalEncoding: SMSEncoding = preferredEncoding;
  let characterCount = 0;

  // 1. GSM-7 Character Sets
  const gsmBasic = "@£$¥èéùìòÇ\nØø\rÅåΔ_ΦΓΛΩΠΨΣΘΞÆæßÉ !\"#¤%&'()*+,-./0123456789:;<=>?¡ABCDEFGHIJKLMNOPQRSTUVWXYZÄÖÑܧ¿abcdefghijklmnopqrstuvwxyzäöñüà";
  const gsmExtended = "^{}\\[~]|€"; // These take 2 slots in LATIN mode

  // 2. Calculate Count and Auto-Detect Encoding Upgrades
  if (preferredEncoding === 'LATIN') {
    for (let i = 0; i < text.length; i++) {
      const char = text[i];
      
      if (gsmBasic.includes(char)) {
        characterCount += 1;
      } else if (gsmExtended.includes(char)) {
        characterCount += 2; // Extended chars take double space in GSM-7
      } else {
        // Non-GSM character found! Force upgrade to UNICODE.
        finalEncoding = 'UNICODE';
        break;
      }
    }
  }

  // 3. Handle Unicode Mode (or Auto-Upgraded Unicode)
  if (finalEncoding === 'UNICODE') {
    // JS .length natively counts surrogate pairs (emojis) as 2.
    // This perfectly matches UCS-2 byte limits (4 bytes = 2 UCS-2 characters).
    characterCount = text.length;
  }

  // 4. Calculate Segments based on final Encoding
  let segmentCount = 0;

  if (finalEncoding === 'LATIN') {
    if (characterCount <= 160) {
      segmentCount = 1;
    } else {
      segmentCount = Math.ceil(characterCount / 153);
    }
  } else { // UNICODE
    if (characterCount <= 70) {
      segmentCount = 1;
    } else {
      segmentCount = Math.ceil(characterCount / 67);
    }
  }

  // 5. Enforce Corporate Gateway Budget Limits
  if (segmentCount > maxSegments) {
    return {
      isValid: false,
      characterCount,
      segmentCount,
      encodingUsed: finalEncoding,
      error: `Message exceeds maximum allowed segments (${maxSegments}). Calculated segments: ${segmentCount}.`
    };
  }

  return {
    isValid: true,
    characterCount,
    segmentCount,
    encodingUsed: finalEncoding
  };
}