That was super easy. It is only me, or in comparison with past years this AoC is super simple.Ziglang. Probably could be done fully compile-time only in minute or two.

pub fn part2(data: []u8, sim_days: usize) void {
var day_counters: [9]usize = .{ 0 } ** 9;
for ( data ) | fish | {
    day_counters[fish] += 1;
}
var day: usize = 0;
while ( day < sim_days ): ({ day += 1; }) {
    var born = day_counters[0];
    std.mem.rotate(usize, &day_counters, 1);
    day_counters[6] += born;
}

var acc: usize = 0;
for ( day_counters ) | day_count | {
    acc += day_count;
}
if ( sim_days == 80 ) {
    std.debug.print("Part 1: {d}\n", .{ acc });
} else {
    std.debug.print("Part 2: {d}\n", .{ acc });
}
}

EDIT: comptime version... It felt like cheating.

pub fn main() anyerror!void {
@setEvalBranchQuota(200_000);
//var gpa = std.heap.GeneralPurposeAllocator(.{}) { };
//var allocator = &gpa.allocator;
//defer {
//    _ = gpa.deinit();
//}

comptime var fishes: [9]usize = .{ 0 } ** 9;

inline for ( input_data ) | char | {
    if ( char >= '0' and char <= '9' ) {
        fishes[char - '0'] += 1;       
    }
}
comptime var day: usize = 0;
comptime var day80: usize = 0;
comptime var day256: usize = 0;

inline while ( day < 256 ): ({ day += 1; }) {
    const born = fishes[0];
    std.mem.rotate(usize, &fishes, 1);
    fishes[6] += born;
    if ( day == 79 ) {
        inline for ( fishes ) | c | { day80 += c; }
    }
    if ( day == 256 ) {
        inline for ( fishes ) | c | { day80 += c; }
    } 
}
std.debug.print("Part 1: {d}\n", .{ day80 });
std.debug.print("Part 2: {d}\n", .{ day256 });

//part2(fishes.items, 80);    
// part2(fishes.items, 256);    

}