exercism/zig/isbn-verifier/isbn_verifier.zig

const std = @import("std");

pub fn isValidIsbn10(s: []const u8) bool {
    if (s.len < 10) return false;

    var isbn: [10]u8 = undefined;
    var pos: usize = 0;

    for (s) |c| {
        if (pos == 10) return false;

        isbn[pos] = switch (c) {
            '-' => continue,
            '0'...'9' => blk: {
                pos += 1;
                break :blk std.fmt.parseInt(u8, &[_]u8{c}, 10) catch unreachable;
            },
            else => blk: {
                if (c == 'X' and pos == 9) {
                    pos += 1;
                    break :blk 10;
                } else {
                    return false;
                }
            },
        };
    }

    var sum: u32 = 0;
    var factor: usize = 10;
    var i: usize = 0;
    while (factor > 0) : ({
        factor -= 1;
        i += 1;
    }) {
        sum += @intCast(isbn[i] * factor);
    }

    return sum % 11 == 0;
}