const std = @import("std");

pub fn main() anyerror!void {
    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
    defer arena.deinit();
    const alloc = &arena.allocator;

    // Read our input
    var f = try std.fs.cwd().openFile("input", .{});
    defer f.close();
    var contents = try f.readToEndAlloc(alloc, std.math.maxInt(u32));
    defer alloc.free(contents);

    var values = std.ArrayList(u16).init(alloc);
    defer values.deinit();
    var it = std.mem.tokenize(contents, "\n");
    while (it.next()) |line| {
        var lit = std.mem.tokenize(line, ",");
        while (lit.next()) |value| {
            try values.append(try std.fmt.parseInt(u16, value, 10));
        }
    }
    //std.log.debug("{} items", .{values.items.len});
    std.sort.sort(u16, values.items, {}, comptime std.sort.asc(u16));
    //std.log.debug("{any}", .{values.items});

    // Try the median
    var median = values.items[values.items.len/2];
    std.log.debug("Median position {}, value: {}",
                  .{values.items.len/2, median});

    var fuel_required: u64 = 0;
    for (values.items) |v| {
        if (v < median) {
            fuel_required += @as(u64, median - v);
        }
        else {
            fuel_required += @as(u64, v - median);
        }
    }
    std.log.info("[Part 1] {} fuel required to get to median {}",
                 .{fuel_required, median});

    var i = values.items[0];
    const max = values.items[values.items.len-1];
    fuel_required = std.math.maxInt(u32);
    var best_position: u16 = 0;
    while (i <= max) : (i += 1) {
        var fu = calculate_fuel_to_pos(i, values.items[0..]);
        if (fu < fuel_required) {
            fuel_required = fu;
            best_position = 1;
        }
    }
    std.log.info("[Part 2] {} fuel required to get to best pos {}",
                 .{fuel_required, best_position});
}

// move 1: 1         = 1
// move 2: 1 + 2     = 3
// move 3: 1 + 2 + 3 = 6
// move 4: (1+2+3)+4 = 10
// move n: (1/2)n * (n+1)
fn calculate_fuel_to_pos(position: u16, crabs: []u16) u64 {
    var fuel_required: u64 = 0;
    for (crabs) |c| {
        var delta: u64 = 0;
        if (c < position) {
            delta = @as(u64, position - c);
        }
        else {
            delta = @as(u64, c - position);
        }
        fuel_required += (delta*(delta+1)) / 2;
    }
    return fuel_required;
}