Solved day 8 part 2.

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

src/day08-2.zig

@@ -0,0 +1,98 @@
+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 getAntinodePositions(a: Pos, b: Pos, size: usize, buf: *[50]Pos) []Pos {
+    var res_x = a.x;
+    var res_y = a.y;
+    const delta_x = a.x - b.x;
+    const delta_y = a.y - b.y;
+    var pos_count: usize = 0;
+    while (res_x >= 0 and
+        res_x <= size - 1 and
+        res_y >= 0 and
+        res_y <= size - 1) : (pos_count += 1)
+    {
+        buf[pos_count] = Pos{ .x = res_x, .y = res_y };
+        res_x += delta_x;
+        res_y += delta_y;
+    }
+    return buf[0..pos_count];
+}
+
+// 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;
+    var positions_buf: [50]Pos = undefined;
+    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];
+                for (getAntinodePositions(pos, pos_mirror, input_size, &positions_buf)) |anti_pos| {
+                    if (setAntinode(&antinodes, anti_pos)) {
+                        unique_antinodes += 1;
+                    }
+                }
+                for (getAntinodePositions(pos_mirror, pos, input_size, &positions_buf)) |anti_pos| {
+                    if (setAntinode(&antinodes, anti_pos)) {
+                        unique_antinodes += 1;
+                    }
+                }
+            }
+        }
+    }
+    print("Unique antinodes: {d}\n", .{unique_antinodes});
+}