aoc2024/day07.zig

const std = @import("std");

const input = @embedFile("day07");
const Equation = struct {
    result: u64,
    values: std.ArrayList(u32)
};
var puzzle: std.ArrayList(Equation) = undefined;

pub fn main () !void {

    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
    defer {
        switch (gpa.deinit()) {
            .leak => @panic("leaked memory"),
            else => {},
        }
    }
    const gpa_alloc = gpa.allocator();
    var arena = std.heap.ArenaAllocator.init(gpa_alloc);
    defer arena.deinit();

    const alloc = arena.allocator();

    try parse_puzzle(alloc);

    var result1: u64 = 0;
    var result2: u64 = 0;
    for(0..puzzle.items.len) |eqi| {
        const eq = puzzle.items[eqi];
        const val = eq.values.items[0];
        const val_cnt = eq.values.items.len;
        if(part1(eqi, 1, val_cnt, val)) {
            result1 += eq.result;
        }
        if(part2(eqi, 1, val_cnt, val)) {
            result2 += eq.result;
        }
    }

    const out = std.io.getStdOut().writer();
    try out.print("Result 1: {d}\n", .{result1});
    try out.print("Result 2: {d}\n", .{result2});
}

fn part1(eqi: usize, vali: usize, endi: usize, cur: u64) bool {
    const eq = puzzle.items[eqi];

    if(cur>eq.result) return false;
    if(vali==endi and cur==eq.result) return true;
    if(vali==endi) return false;


    const val = eq.values.items[vali];
    return part1(eqi, vali+1, endi, cur+val)
        or part1(eqi, vali+1, endi, cur*val);
}

inline fn concat(a: u64, b: u32) u64 {
    var buf: [20]u8 = undefined;
    const str = std.fmt.bufPrint(&buf, "{d}{d}", .{a,b}) catch @panic("lol");
    return std.fmt.parseInt(u64, str, 10) catch @panic("lol");
}

fn part2(eqi: usize, vali: usize, endi: usize, cur: u64) bool {
    const eq = puzzle.items[eqi];

    if(cur>eq.result) return false;
    if(vali==endi and cur==eq.result) return true;
    if(vali==endi) return false;

    const val = eq.values.items[vali];
    return part2(eqi, vali+1, endi, cur+val)
        or part2(eqi, vali+1, endi, cur*val)
        or part2(eqi, vali+1, endi, concat(cur,val));
}

fn parse_puzzle(alloc: std.mem.Allocator) !void {
    puzzle = std.ArrayList(Equation).init(alloc);
    errdefer puzzle.deinit();

    var line_it = std.mem.tokenizeScalar(u8, input, '\n');
    while(line_it.next()) |line| {
        var res_it = std.mem.tokenizeScalar(u8, line, ':');

        const result: u64 = try std.fmt.parseInt(u64, res_it.next().?, 10);

        var values = std.ArrayList(u32).init(alloc);
        errdefer values.deinit();

        var val_it = std.mem.tokenizeScalar(u8, res_it.next().?, ' ');
        while(val_it.next()) |val| {
            try values.append(try std.fmt.parseInt(u32, val, 10));
        }

        try puzzle.append(.{.result=result, .values=values});
    }
}