Solved day 8 part 1.

Signed-off-by: jmug <u.g.a.mariano@gmail.com>

inputs/day08.example.txt

@@ -0,0 +1,12 @@
+............
+........0...
+.....0......
+.......0....
+....0.......
+......A.....
+............
+............
+........A...
+.........A..
+............
+............

inputs/day08.txt

@@ -0,0 +1,50 @@
+....K..........................8.................z
+.....n..............r...........z.................
+.......................w....8.............3...E...
+.....Q.....U..4.........8.........................
+...............rc...w........i....................
+...........K.........i..2.........................
+..................4.....i.........................
+K.....n....................w...........z..........
+..U......Q........................I...............
+..........i.....I.....Q....g....................5E
+..Q......................................5........
+..........c............8......w...g..........5....
+.............................I.O..................
+.Z.............4....b.....................k.......
+..n........4......r..g..6..c.............3........
+....Z............c................................
+...................................x..............
+.......................................O..........
+...............U...................E..........5...
+.....f..........................OI3......k........
+..m.......o......F.......R........x...............
+m...........o..v6..3...............X..............
+..............H6v.....F.g.....................W...
+...........o....Fb....v...............E...........
+...Z.............a................................
+......U6.............V............................
+.9.............b..............pTk.................
+.......m........V.........H1....x.................
+...m.................H....................MX......
+............t.a............H......................
+........Z...a............v.....1..T..p.W..X.......
+.............................9...x.......p........
+.....J.....................V..1................0..
+...........r..j..........a............pT..........
+.G..................J...N......f..................
+...........G......T....B........W.e...........M...
+..........j.............Rk.............M..........
+.........q.............MB......R.F..1..P....X...f.
+............................V....o...........h....
+...........................................W......
+......b......u............................e.......
+.............................................0....
+..CA....Gt..O........................7.....e....0.
+C.u......A..9J..N........................h.....e..
+uj....q..........N.2..................7...........
+G....N.....uJ...............................0.....
+.................B................P.......h.......
+...C....q...........R.........P...................
+.....q..tC....2.9.....B............P....f.........
+...............2.................................7

src/day08.zig

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+const std = @import("std");
+const print = std.debug.print;
+const args = @import("lib/args.zig");
+const files = @import("lib/files.zig");
+
+const Pos = struct {
+    x: isize,
+    y: isize,
+};
+
+fn normalizeCharacter(c: u8) u8 {
+    return switch (c) {
+        '0'...'9' => c - '0',
+        'a'...'z' => c - 'a' + 10,
+        'A'...'Z' => c - 'A' + 37,
+        else => unreachable,
+    };
+}
+
+fn getAntinodePos(a: Pos, b: Pos, size: usize) ?Pos {
+    var res_x = a.x;
+    var res_y = a.y;
+    res_x += a.x - b.x;
+    res_y += a.y - b.y;
+    if (res_x < 0 or
+        res_x >= size or
+        res_y < 0 or
+        res_y >= size)
+    {
+        return null;
+    }
+    return Pos{
+        .x = res_x,
+        .y = res_y,
+    };
+}
+
+// returns true if the position being set hadn't been set before.
+fn setAntinode(antinodes: *[50][50]bool, pos: Pos) bool {
+    const x: usize = @bitCast(pos.x);
+    const y: usize = @bitCast(pos.y);
+    if (antinodes[y][x]) {
+        return false;
+    }
+    antinodes[y][x] = true;
+    return true;
+}
+
+pub fn main() !void {
+    const input_path = args.getFirstArg();
+    const input = try files.openForReading(input_path);
+    defer input.close();
+
+    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+    defer arena.deinit();
+    const allocator = arena.allocator();
+
+    var antinodes: [50][50]bool = .{.{false} ** 50} ** 50;
+    var positions: [64]std.ArrayList(Pos) = undefined;
+    for (0..64) |i| {
+        positions[i] = std.ArrayList(Pos).init(allocator);
+    }
+
+    var line_buf: [51]u8 = undefined;
+    var input_size: usize = 0;
+    while (files.readLine(input, &line_buf)) |line| {
+        defer input_size += 1;
+        for (line, 0..) |c, x| {
+            if (c == '.') {
+                continue;
+            }
+            try positions[normalizeCharacter(c)].append(Pos{ .x = @bitCast(x), .y = @bitCast(input_size) });
+        }
+    } else |err| {
+        print("done reading input with error: {any}\n", .{err});
+    }
+    var unique_antinodes: usize = 0;
+    for (0..64) |c| {
+        for (0..positions[c].items.len) |pos_i| {
+            for (pos_i + 1..positions[c].items.len) |mirror_pos_i| {
+                const pos = positions[c].items[pos_i];
+                const pos_mirror = positions[c].items[mirror_pos_i];
+                if (getAntinodePos(pos, pos_mirror, input_size)) |anti_pos| {
+                    if (setAntinode(&antinodes, anti_pos)) {
+                        unique_antinodes += 1;
+                    }
+                }
+                if (getAntinodePos(pos_mirror, pos, input_size)) |anti_pos| {
+                    if (setAntinode(&antinodes, anti_pos)) {
+                        unique_antinodes += 1;
+                    }
+                }
+            }
+        }
+    }
+    print("Unique antinodes: {d}\n", .{unique_antinodes});
+}