fix(#74): implement ISO 7064 Mod 37-2 check character for ISBT 128

Add proper check character computation per ISO/IEC 7064. The DIN now
includes a Mod 37-2 check character (0-9, A-Z, or *) as the final
character.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: productdevbook

src/encoders/isbt128.ts

@@ -8,6 +8,38 @@
 
 import { InvalidInputError } from "../errors";
 
+/**
+ * ISO/IEC 7064 Mod 37-2 check character set: 0-9 (values 0-9), A-Z (values 10-35), * (value 36)
+ */
+const MOD37_CHARSET = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ*";
+
+/**
+ * Compute ISO/IEC 7064 Modulo 37-2 check character
+ *
+ * Algorithm (pure system, radix 2, modulus 37):
+ * 1. Start with sum = 0
+ * 2. For each character: sum = ((sum + charValue) * 2) mod 37
+ * 3. Check value = (37 - sum) mod 37
+ * 4. Map back to character set
+ *
+ * @param data - Input string containing only characters from the Mod 37-2 character set
+ * @returns Single check character
+ */
+export function iso7064Mod37_2(data: string): string {
+  let sum = 0;
+  for (let i = 0; i < data.length; i++) {
+    const idx = MOD37_CHARSET.indexOf(data[i]);
+    if (idx === -1) {
+      throw new InvalidInputError(
+        `Invalid character '${data[i]}' for ISO 7064 Mod 37-2 check character computation`,
+      );
+    }
+    sum = ((sum + idx) * 2) % 37;
+  }
+  const check = (37 - sum) % 37;
+  return MOD37_CHARSET[check];
+}
+
 /**
  * Encode ISBT 128 Donation Identification Number (DIN)
  * Format: =FLLLLLYYYNNNNNN where:
@@ -48,7 +80,9 @@ export function encodeISBT128DIN(
 
   const facility = facilityNumber.padStart(5, "0");
   const donation = donationNumber.padStart(6, "0");
-  return `=${countryCode}${facility}${year}${donation}`;
+  const din = `${countryCode}${facility}${year}${donation}`;
+  const check = iso7064Mod37_2(din);
+  return `=${din}${check}`;
 }
 
 /**

src/index.ts

@@ -112,6 +112,7 @@ export {
   encodeISBT128Component,
   encodeISBT128Expiry,
   encodeISBT128BloodGroup,
+  iso7064Mod37_2,
 } from "./encoders/isbt128";
 
 // --- Renderers ---

test/encoders-isbt128.test.ts

@@ -4,18 +4,19 @@ import {
   encodeISBT128Component,
   encodeISBT128Expiry,
   encodeISBT128BloodGroup,
+  iso7064Mod37_2,
 } from "../src/encoders/isbt128";
 import { barcode } from "../src/index";
 
 describe("ISBT 128 DIN", () => {
-  it("formats donation identification number", () => {
+  it("formats donation identification number with check character", () => {
     const result = encodeISBT128DIN("US", "12345", "26", "000001");
-    expect(result).toBe("=US1234526000001");
+    expect(result).toBe("=US12345260000019");
   });
 
-  it("pads facility and donation numbers", () => {
+  it("pads facility and donation numbers with check character", () => {
     const result = encodeISBT128DIN("GB", "1", "26", "1");
-    expect(result).toBe("=GB0000126000001");
+    expect(result).toBe("=GB0000126000001O");
   });
 
   it("throws on invalid country code", () => {
@@ -62,3 +63,27 @@ describe("ISBT 128 Blood Group", () => {
     expect(() => encodeISBT128BloodGroup("")).toThrow();
   });
 });
+
+describe("ISO 7064 Mod 37-2 check character", () => {
+  it("computes correct check for numeric input", () => {
+    expect(iso7064Mod37_2("0000000000")).toBe("0");
+  });
+
+  it("computes correct check for alphanumeric DIN data", () => {
+    expect(iso7064Mod37_2("US1234526000001")).toBe("9");
+  });
+
+  it("computes correct check for another DIN", () => {
+    expect(iso7064Mod37_2("GB0000126000001")).toBe("O");
+  });
+
+  it("throws on invalid character", () => {
+    expect(() => iso7064Mod37_2("us12345")).toThrow();
+  });
+
+  it("handles asterisk in character set", () => {
+    // * has value 36 in the Mod 37-2 character set
+    const result = iso7064Mod37_2("*");
+    expect(result).toMatch(/^[0-9A-Z*]$/);
+  });
+});