Having solution for Part 1 for Y=2_000_000 I made Part 2 by trivial modifications, mainly running for Y in 0 .. 4_000_000 loop Do_Part_1 (Y); end loop;. And this worked for the example but not for my input file. This made me create another solution (that doesn’t work for Part 1, however). It produced a correct answer. With the answer I was able to debug the first solution and found a bug (in a slice intersection test function). Now I have 2 solutions for Part 2 
.
My second solution takes 0.3s to find the first result, but if I remove return; to check all possible results it takes about 8 seconds:
$ time ./bin/day_15 < /tmp/input
3316868 2686239 13267474686239
real 0m0,348s
user 0m0,335s
sys 0m0,013s
The idea is to walk along the contour of each sensor checking that each position of the contour isn’t covered by any sensor. Here is the source code:
with Ada.Text_IO;
with Ada.Integer_Text_IO;
with Ada.Long_Long_Integer_Text_IO;
with Ada.Strings.Fixed;
with Ada.Containers.Doubly_Linked_Lists;
procedure Day_15 is
type Position is record
X, Y : Integer;
end record;
function Distance (A, B : Position) return Natural is
(abs (A.X - B.X) + abs (A.Y - B.Y));
type Pair is record
Sensor : Position;
Beacon : Position;
end record;
procedure Read_Pair (V : out Pair) is
Line : String (1 … 80);
Last : Natural;
Pos : Natural;
Colon : Natural;
begin
Ada.Text_IO.Get_Line (Line, Last);
pragma Assert (Last < Line’Last);
Colon := Ada.Strings.Fixed.Index (Line, “:”);
Ada.Integer_Text_IO.Get (Line (13 … Last), V.Sensor.X, Pos);
Ada.Integer_Text_IO.Get (Line (Pos + 5 … Colon - 1), V.Sensor.Y, Pos);
Ada.Integer_Text_IO.Get (Line (Colon + 25 … Last), V.Beacon.X, Pos);
Ada.Integer_Text_IO.Get (Line (Pos + 5 … Last), V.Beacon.Y, Pos);
end Read_Pair;
package Pair_Lists is new Ada.Containers.Doubly_Linked_Lists (Pair);
Pairs : Pair_Lists.List;
subtype Target is Natural range 0 … 4000000;
type Position_Array is array (Positive range <>) of Position;
function Contour (S : Pair) return Position_Array is
Dist : constant Positive := Distance (S.Sensor, S.Beacon);
Result : Position_Array (1 … 4 * (Dist + 1));
begin
for J in 0 … Dist loop
Result (4 * J + 1) := (S.Sensor.X + J, S.Sensor.Y - Dist - 1 + J);
Result (4 * J + 2) := (S.Sensor.X + Dist + 1 - J, S.Sensor.Y + J);
Result (4 * J + 3) := (S.Sensor.X - J, S.Sensor.Y + Dist + 1 - J);
Result (4 * J + 4) := (S.Sensor.X - Dist - 1 + J, S.Sensor.Y - J);
end loop;
return Result;
end Contour;
begin
while not Ada.Text_IO.End_Of_File loop
declare
Item : Pair;
begin
Read_Pair (Item);
Pairs.Append (Item);
end;
end loop;
for S of Pairs loop
for Pos of Contour (S) loop
if Pos.X in Target and then Pos.Y in Target then
if (for all X of Pairs =>
Distance (X.Sensor, Pos) > Distance (X.Sensor, X.Beacon))
then
Ada.Integer_Text_IO.Put (Pos.X);
Ada.Integer_Text_IO.Put (Pos.Y);
Ada.Long_Long_Integer_Text_IO.Put
(Long_Long_Integer (Pos.Y) +
Long_Long_Integer (Pos.X) * 4000000);
Ada.Text_IO.New_Line;
return;
end if;
end if;
end loop;
end loop;
end Day_15;
and also, the Code completion in Gnat Studio doesn’t work for