aoc2021-day5-sqlite.sql

// https://adventofcode.com/2021/day/5 in SQL with some algebra.
// I only checked this with the example, not the full data set :)
//
// Run this with `sqlite < aoc2021-day-5-sqlite.sql`.

WITH
  RECURSIVE ints(v) AS (
	  SELECT 0 UNION ALL
	  SELECT v + 1 FROM ints
	  WHERE v + 1<= 10
  ),

  points AS (SELECT a.v as x, b.v as y FROM ints a JOIN ints b),

  lines(x1, y1, x2, y2) AS (VALUES
	(0,9,5,9),
	(8,0,0,8),
	(9,4,3,4),
	(2,2,2,1),
	(7,0,7,4),
	(6,4,2,0),
	(0,9,2,9),
	(3,4,1,4),
	(0,0,8,8),
	(5,5,8,2)
  ),

  matching_points AS (
    SELECT x, y, x1, y1, x2, y2
    FROM points
    JOIN lines
    WHERE (y-y1)*(x2-x1) = (y2-y1)*(x-x1) AND (y BETWEEN y1 AND y2 OR y BETWEEN y2 AND y1) AND (x BETWEEN x1 and x2 OR x BETWEEN x2 and x1)
  ),

  points_with_counts AS (
        SELECT x, y, count(*) as cnt
        FROM matching_points
        group by 1, 2
  ),

  answer AS (select count(*) from points_with_counts where cnt > 1)

SELECT * FROM answer