feat: better PDF

report/invoice/reference.go

@@ -0,0 +1,68 @@
+package invoice
+
+import (
+	"crypto/rand"
+	"fmt"
+	"time"
+)
+
+func generateReference() string {
+	// 1) Build the 26-digit payload from time + random suffix
+	now := time.Now().UTC()
+	ts := now.Format("20060102150405") // YYYYMMDDHHMMSS -> 14 chars
+	ns := now.Nanosecond()             // 0..999999999
+	// format nanoseconds as 9 digits, zero padded
+	nsStr := fmt.Sprintf("%09d", ns)
+
+	// 3-digit random suffix (000..999) to reach 26 digits
+	randBytes := make([]byte, 2) // we'll read 2 bytes and map to 0..999
+	rand.Read(randBytes)
+
+	// convert to number 0..65535 then mod 1000
+	rnum := int(randBytes[0])<<8 | int(randBytes[1])
+	rnum = rnum % 1000
+	randStr := fmt.Sprintf("%03d", rnum)
+
+	// compose 26-digit payload: 14 + 9 + 3 = 26
+	payload := ts + nsStr + randStr
+
+	// Safety: ensure payload is 26 digits (should always be true)
+	if len(payload) != 26 {
+		panic(fmt.Errorf("internal error: payload length %d != 26", len(payload)))
+	}
+
+	// 2) Compute Modulo-10 recursive check digit
+	check := mod10RecursiveChecksum(payload)
+
+	// 3) Build full 27-digit reference
+	full := payload + fmt.Sprintf("%d", check)
+
+	// 4) Format with grouping: 2-5-5-5-5-5
+	formatted := fmt.Sprintf("%s %s %s %s %s %s",
+		full[0:2],
+		full[2:7],
+		full[7:12],
+		full[12:17],
+		full[17:22],
+		full[22:27],
+	)
+	return formatted
+}
+
+// mod10RecursiveChecksum computes the Mod10 recursive check digit for a numeric string.
+// Implements the table-based recursive mod10 described in Swiss Annex B / ISR implementations.
+func mod10RecursiveChecksum(digits string) int {
+	// table as used in ISR/QR implementations
+	arrTable := [10]int{0, 9, 4, 6, 8, 2, 7, 1, 3, 5}
+	carry := 0
+	for i := 0; i < len(digits); i++ {
+		ch := digits[i]
+		// assume digits are ASCII '0'..'9'
+		n := int(ch - '0')
+		// update carry
+		idx := (carry + n) % 10
+		carry = arrTable[idx]
+	}
+	// final check digit
+	return (10 - carry) % 10
+}