exercism/zig/luhn/luhn.zig

const std = @import("std");
const ascii = std.ascii;

pub fn isValid(s: []const u8) bool {
    var idx: usize = s.len;
    var luhn_idx: usize = 1;
    var total: u64 = 0;

    while (idx > 0) {
        idx -= 1;
        if (s[idx] == 32) continue;
        if (!ascii.isDigit(s[idx])) {
            return false;
        }

        var curr: u8 = s[idx] - 48;
        if (luhn_idx % 2 == 0) {
            curr *= 2;
            if (curr > 9) {
                curr -= 9;
            }
        }
        total += curr;
        luhn_idx += 1;
    }

    if (luhn_idx <= 2) return false;
    return total % 10 == 0;
}