AdventOfCode-2023/day03-hard/src/main.zig

const std = @import("std");

const Offset = enum(u2) {
    Before = 0,
    Same = 1,
    After = 2,

    fn applyTo(self: *const Offset, index: usize) usize {
        return (index + @intFromEnum(self.*)) - 1;
    }
};

const Char = enum(u4) {
    Zero = 0,
    One = 1,
    Two = 2,
    Three = 3,
    Four = 4,
    Five = 5,
    Six = 6,
    Seven = 7,
    Eight = 8,
    Nine = 9,
    Dot = 12,
    MaybeGear = 13,
    OtherSymbol = 14,

    fn isDigit(self: *const Char) bool {
        return @intFromEnum(self.*) < 10;
    }

    fn getDigit(self: *const Char) u4 {
        return @intFromEnum(self.*);
    }
};

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

const CharInfo = struct {
    adjacents: [8]Char,
    this: Char,

    fn isAdjacentToSymbol(self: *const CharInfo) bool {
        for (0..8) |i| {
            if (self.adjacents[i] == .MaybeGear or self.adjacents[i] == .OtherSymbol) {
                return true;
            }
        }

        return false;
    }

    fn getGearNumbersLocations(self: *const CharInfo) ?[2][2]Offset {
        if (self.this != .MaybeGear) {
            return null;
        }

        var locations = StackList(comptime [2]Offset, comptime u8, comptime 6).init();

        // previous line
        if (self.adjacents[0].isDigit()) {
            locations.add(.{ .Before, .Before });
            if (!self.adjacents[1].isDigit() and self.adjacents[2].isDigit()) {
                locations.add(.{ .Before, .After });
            }
        } else if (self.adjacents[1].isDigit()) {
            locations.add(.{ .Before, .Same });
        } else if (self.adjacents[2].isDigit()) {
            locations.add(.{ .Before, .After });
        }

        // this line
        if (self.adjacents[3].isDigit()) {
            locations.add(.{ .Same, .Before });
        }
        if (self.adjacents[4].isDigit()) {
            locations.add(.{ .Same, .After });
        }

        // next line
        if (self.adjacents[5].isDigit()) {
            locations.add(.{ .After, .Before });
            if (!self.adjacents[6].isDigit() and self.adjacents[7].isDigit()) {
                locations.add(.{ .After, .After });
            }
        } else if (self.adjacents[6].isDigit()) {
            locations.add(.{ .After, .Same });
        } else if (self.adjacents[7].isDigit()) {
            locations.add(.{ .After, .After });
        }

        if (locations.length == 2) {
            return .{ locations.mem[0], locations.mem[1] };
        }

        return null;
    }
};

const State = struct {
    number: u32 = 0,
    is_adjacent_to_symbol: bool = false,

    fn addDigit(self: *State, char_info: CharInfo) void {
        self.number = self.number * 10 + char_info.this.getDigit();
        self.is_adjacent_to_symbol = self.is_adjacent_to_symbol or char_info.isAdjacentToSymbol();
    }
};

fn getNumberAt(engine: [][]const CharInfo, x: usize, y: usize) u32 {
    const line = engine[x];
    var index: usize = y;
    while (index > 0) : (index -= 1) {
        if (!line[index].this.isDigit()) {
            break;
        }
    }

    if (!line[index].this.isDigit()) {
        index += 1;
    }

    var result: u32 = 0;
    while (index < line.len) : (index += 1) {
        if (!line[index].this.isDigit()) {
            break;
        }

        result = result * 10 + line[index].this.getDigit();
    }

    return result;
}

fn solve(engine: [][]const CharInfo) u32 {
    var result: u32 = 0;

    for (engine, 0..) |line, i| {
        //std.debug.print("handling new line\n", .{});
        for (line, 0..) |char_info, j| {
            //std.debug.print("handling: {s}({s})\n", .{ [_]u8{char_info.this}, char_info.adjacents });
            const gearNumberLocations = char_info.getGearNumbersLocations();
            if (gearNumberLocations) |*value| {
                //std.debug.print("found gear at {d},{d}; relative number positions are {d},{d} and {d},{d}\n", .{ i, j, value[0][0], value[0][1], value[1][0], value[1][1] });
                const a = getNumberAt(engine, value[0][0].applyTo(i), value[0][1].applyTo(j));
                const b = getNumberAt(engine, value[1][0].applyTo(i), value[1][1].applyTo(j));
                result += a * b;
            }
        }
    }

    return result;
}

fn charLinesToCharInfoLines(allocator: std.mem.Allocator, char_lines: [][]const Char) ![][]const CharInfo {
    var char_info_lines = try allocator.alloc([]CharInfo, char_lines.len);
    for (0..char_lines.len) |i| {
        const char_line = char_lines[i];
        var char_info_line = try allocator.alloc(CharInfo, char_line.len);
        for (0..char_line.len) |j| {
            char_info_line[j] = CharInfo{ .this = char_line[j], .adjacents = .{
                if (i > 0 and j > 0) char_lines[i - 1][j - 1] else .Dot,
                if (i > 0) char_lines[i - 1][j] else .Dot,
                if (i > 0 and j + 1 < char_line.len) char_lines[i - 1][j + 1] else .Dot,
                if (j > 0) char_line[j - 1] else .Dot,
                if (j + 1 < char_line.len) char_line[j + 1] else .Dot,
                if (i + 1 < char_lines.len and j > 0) char_lines[i + 1][j - 1] else .Dot,
                if (i + 1 < char_lines.len) char_lines[i + 1][j] else .Dot,
                if (i + 1 < char_lines.len and j + 1 < char_line.len) char_lines[i + 1][j + 1] else .Dot,
            } };
        }

        char_info_lines[i] = char_info_line;
    }

    return char_info_lines;
}

fn readInput(allocator: std.mem.Allocator, reader: anytype) ![][]const Char {
    var lines = std.ArrayList([]Char).init(allocator);
    var line_buffer: [1000]u8 = undefined;
    while (try reader.readUntilDelimiterOrEof(&line_buffer, '\n')) |line_ref| {
        var line = try allocator.alloc(Char, line_ref.len);
        for (line_ref, line) |raw_char, *char| {
            char.* = switch (raw_char) {
                '0'...'9' => @enumFromInt(raw_char - '0'),
                '.' => .Dot,
                '*' => .MaybeGear,
                else => .OtherSymbol,
            };
        }
        try lines.append(line);
    }

    return lines.items;
}

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 allocator_buffer: [1_000_000]u8 = undefined;
    var fba = std.heap.FixedBufferAllocator.init(&allocator_buffer);
    var allocator = fba.allocator();

    var lines_arena = std.heap.ArenaAllocator.init(allocator);
    const lines_allocator = lines_arena.allocator();
    const char_lines = try readInput(lines_allocator, reader);

    var info_lines_arena = std.heap.ArenaAllocator.init(allocator);
    const info_lines_allocator = info_lines_arena.allocator();
    const info_lines = try charLinesToCharInfoLines(info_lines_allocator, char_lines);
    lines_arena.deinit();

    const result = solve(info_lines);
    info_lines_arena.deinit();

    try stdout.print("{d}\n", .{result});
}