Algorithm

Thai National ID Structure

A Thai National ID consists of 13 digits with the following structure:

┌─┬──┬──┬───────┬─┐
│1│23│45│6789012│3│
└─┴──┴──┴───────┴─┘
 │ │  │    │     └─ Check digit (digit 13)
 │ │  │    └─────── Serial number (digits 6-12)
 │ │  └──────────── District code (digits 4-5)
 │ └─────────────── Province code (digits 2-3)
 └───────────────── Person type (digit 1)

Digit Breakdown

Position	Name	Description
1	Person Type	Category of person (1-8)
2-3	Province	Province code (10-96)
4-5	District	Amphoe code (continues from province)
6-12	Serial	Unique serial number
13	Check Digit	Checksum validation digit

Checksum Algorithm

The check digit (digit 13) is calculated from the first 12 digits using a weighted sum algorithm.

Steps

1. Validate format: Ensure the input is exactly 13 ASCII digits
2. Calculate weighted sum: Multiply each of the first 12 digits by a weight
3. Compute check digit: Apply modulo operations to get the expected check digit
4. Compare: Verify the calculated check digit matches digit 13

Formula

sum = Σ (digit[i] × (13 - i))  for i = 0 to 11

check_digit = (11 - (sum mod 11)) mod 10

Example Calculation

For ID 1112034563562:

Position (i)	Digit	Weight (13-i)	Product
0	1	13	13
1	1	12	12
2	1	11	11
3	2	10	20
4	0	9	0
5	3	8	24
6	4	7	28
7	5	6	30
8	6	5	30
9	3	4	12
10	5	3	15
11	6	2	12
Total			207

sum = 207
sum mod 11 = 207 mod 11 = 9
11 - 9 = 2
2 mod 10 = 2

Expected check digit: 2
Actual digit 13: 2 ✓

Rust Implementation

pub fn verify_id(id: &str) -> bool {
    // Check length and all digits
    if id.len() != 13 || !id.bytes().all(|b| b.is_ascii_digit()) {
        return false;
    }

    let digits: Vec<u32> = id.bytes().map(|b| (b - b'0') as u32).collect();

    // Calculate weighted checksum
    let total: u32 = digits[..12]
        .iter()
        .enumerate()
        .map(|(i, &d)| d * (13 - i as u32))
        .sum();

    let calculated_checksum = (11 - total % 11) % 10;

    calculated_checksum == digits[12]
}

Validation Rules

An ID is considered invalid if:

• Length is not exactly 13 characters
• Contains any non-digit characters
• The check digit does not match the calculated value

Note

The verify_id function only validates the checksum. It does not verify that the province or district codes are real. Use get_id_meaning to check if the embedded codes correspond to known locations.

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