day4.zig

const std = @import("std");

fn part(buffer: []const u8, init_rotations: f64, reverse: bool) !void {
    if (reverse) {
        var number_iter = std.mem.splitBackwardsScalar(u8, buffer, '\n');

        var previous_gear = try std.fmt.parseFloat(f64, number_iter.next().?);
        var total_rotations: f64 = init_rotations;

        while (number_iter.next()) |number| {
            const n = try std.fmt.parseFloat(f64, number);
            total_rotations = (previous_gear * total_rotations) / n;
            previous_gear = n;
        }

        std.debug.print("{d}\n", .{total_rotations});
    } else {
        var number_iter = std.mem.splitScalar(u8, buffer, '\n');

        var previous_gear = try std.fmt.parseFloat(f64, number_iter.next().?);
        var total_rotations: f64 = init_rotations;

        while (number_iter.next()) |number| {
            const n = try std.fmt.parseFloat(f64, number);
            total_rotations = (previous_gear * total_rotations) / n;
            previous_gear = n;
        }

        std.debug.print("{d}\n", .{total_rotations});
    }
}
fn part3(buffer: []const u8, init_r: f64) !void {
    var start_pointer: usize = 0;
    var end_pointer: usize = 0;
    var prev: f64 = undefined;
    var init_rotations = init_r;
    while (end_pointer < buffer.len) : (end_pointer += 1) {
        if (buffer[end_pointer] == '\n') {
            prev = try std.fmt.parseFloat(f64, buffer[start_pointer..end_pointer]);
            start_pointer = end_pointer + 1;
        } else if (buffer[end_pointer] == '|') {
            const current = try std.fmt.parseFloat(f64, buffer[start_pointer..end_pointer]);
            init_rotations = (init_rotations * prev) / current;
            start_pointer = end_pointer + 1;
        } else if (end_pointer == buffer.len - 1) {
            const current = try std.fmt.parseFloat(f64, buffer[start_pointer..end_pointer]);
            init_rotations = (init_rotations * prev) / current;
            std.debug.print("{d}", .{init_rotations});
        }
    }
}

pub fn main() !void {
    const allocator = std.heap.page_allocator;
    var file = try std.fs.cwd().openFile("src/day4/input.txt", .{});
    var buffer = try file.readToEndAlloc(allocator, std.math.maxInt(usize));
    try part(buffer, 2025, false);

    file = try std.fs.cwd().openFile("src/day4/input2.txt", .{});
    buffer = try file.readToEndAlloc(allocator, std.math.maxInt(usize));
    try part(buffer, 10000000000000, true);

    file = try std.fs.cwd().openFile("src/day4/input3.txt", .{});
    buffer = try file.readToEndAlloc(allocator, std.math.maxInt(usize));
    try part3(
        buffer,
        100,
    );
}