AdventOfCode-2023/day09-hard/src/main.zig

const std = @import("std");

fn StackList(comptime T: type, comptime capacity_type: type, comptime capacity: capacity_type) type {
    return struct {
        const Self = @This();
        mem: [capacity]T,
        length: capacity_type,

        fn add(self: *Self, value: T) void {
            self.mem[self.length] = value;
            self.length += 1;
        }

        fn has(self: *Self, needle: T) bool {
            for (0..self.length) |i| {
                if (self.mem[i] == needle) {
                    return true;
                }
            }

            return false;
        }

        fn getMutableSlice(self: *Self) []T {
            return (&self.mem)[0..self.length];
        }

        fn getSlice(self: *const Self) []const T {
            return self.mem[0..self.length];
        }

        fn init() Self {
            return Self{
                .mem = undefined,
                .length = 0,
            };
        }
    };
}

fn addDigit(result: anytype, digit: u8) void {
    result.* = (result.* * 10) + (digit - '0');
}

fn canStartWithGroup(springs: []const u8, group_size: usize) bool {
    if (springs.len < group_size) {
        return false;
    }

    if (springs.len > group_size and springs[group_size] == '#') {
        return false;
    }

    var i: usize = 0;
    while (i < group_size) : (i += 1) {
        if (springs[i] == '.') {
            return false;
        }
    }

    return true;
}

fn solveBackward(readings: []i64) i64 {
    var i: usize = 1;
    while (i < readings.len) : (i += 1) {
        var j = readings.len - 1;
        while (j >= i) : (j -= 1) {
            readings[j] = readings[j] - readings[j - 1];
        }
    }

    var result: i64 = 0;
    var sign: i64 = 1;
    for (readings) |value| {
        result += sign * value;
        sign *= -1;
    }
    //std.debug.print("backwards: derivatives: {any}, result: {d}\n", .{ readings, result });
    return result;
}

fn solveForward(readings: []i64) i64 {
    var i: usize = 1;
    while (i < readings.len) : (i += 1) {
        var j: usize = 0;
        while (j < readings.len - i) : (j += 1) {
            readings[j] = readings[j + 1] - readings[j];
        }
    }

    var result: i64 = 0;
    for (readings) |value| {
        result += value;
    }
    //std.debug.print("forwards: derivatives: {any}, result: {d}\n", .{ readings, result });
    return result;
}

fn solveLine(line: []const u8) i64 {
    var i: usize = 0;
    var readings = StackList(i64, usize, 31).init();
    while (i < line.len) {
        var number: i64 = 0;
        var is_negative = false;
        if (line[i] == '-') {
            is_negative = true;
            i += 1;
        }
        while (i < line.len and line[i] != ' ') : (i += 1) {
            addDigit(&number, line[i]);
        }

        if (is_negative) {
            number = -number;
        }

        readings.add(number);
        i += 1;
    }

    return solveBackward(readings.getMutableSlice());
}

pub fn main() !void {
    const stdout = std.io.getStdOut().writer();

    const raw_in = std.io.getStdIn();
    var buffered_reader = std.io.bufferedReader(raw_in.reader());
    var reader = buffered_reader.reader();

    var result: i64 = 0;
    var line_buffer: [1000]u8 = undefined;
    while (try reader.readUntilDelimiterOrEof(&line_buffer, '\n')) |line| {
        result += solveLine(line);
    }
    try stdout.print("{d}\n", .{result});
}