// Copyright (c) 2022, 2024 blackshirt. All rights reserved.
// Use of this source code is governed by a MIT License
// that can be found in the LICENSE file.
module asn1

// ASN.1 Utility function

// read_bytes was safe version of bytes slicing, `src[pos..pos+size]`
fn read_bytes(src []u8, pos int, size int) ![]u8 {
    if src.len < 1 || pos > src.len || size > src.len {
        return error(' pos=${pos} or size=${size} bigger than len=${src.len}')
    }
    if pos + size > src.len {
        return error('pos + size maybe getting overflow')
    }

    ret := src[pos..pos + size]
    return ret
}

fn read_byte(src []u8, loc int) !(u8, int) {
    if src.len == 0 || loc + 1 > src.len {
        return error('invalid loc or len')
    }

    mut pos := loc
    result := src[pos]
    pos += 1

    return result, pos
}

fn read_digit(src []u8, loc int) !(u8, int) {
    val, pos := read_byte(src, loc)!
    // check its a digit, '0'-'9',
    // aka, 0x30 s/d 0x39 in hex, or 48-57 in dec
    if !val.is_digit() {
        return error('not digit byte')
    }

    digit := val - u8(0x30) // get the digit value
    return digit, pos
}

fn read_2_digits(src []u8, loc int) !(u8, int) {
    if loc >= src.len || src.len < 2 + loc {
        return error('not enough bytes')
    }
    mut val, mut pos := read_digit(src, loc)!

    first := val * 10

    if pos < src.len {
        val, pos = read_digit(src, pos)!
    }
    return first + val, pos
}

fn read_4_digits(src []u8, loc int) !(u16, int) {
    if loc >= src.len || src.len < 4 + loc {
        return error('not enough bytes')
    }
    mut val, mut pos := read_digit(src, loc)!
    first := u16(val) * 1000

    if pos < src.len {
        val, pos = read_digit(src, pos)!
    }
    second := u16(val) * 100

    if pos < src.len {
        val, pos = read_digit(src, pos)!
    }
    third := u16(val) * 10

    if pos < src.len {
        val, pos = read_digit(src, pos)!
    }
    fourth := u16(val)

    result := first + second + third + fourth
    return result, pos
}

fn validate_date(year u16, month u8, day u8) bool {
    if year < 0 {
        return false
    }
    if day < 1 {
        return false
    }
    mut dim := month
    match month {
        1, 3, 5, 7, 8, 10, 12 {
            dim = 31
        }
        4, 6, 9, 11 {
            dim = 30
        }
        2 {
            // kabisat
            if (year % 4 == 0 && year % 100 != 0) || year % 400 == 0 {
                dim = 29
            } else {
                dim = 28
            }
        }
        else {
            return false
        }
    }

    if day > dim {
        return false
    }

    return true
}