WITH MUTUALLY RECURSIVE
lines(r INT, line TEXT) AS (
SELECT r, regexp_split_to_array(input, '\n')[r] as block
FROM input, generate_series(1, array_length(regexp_split_to_array(input, '\n'), 1)) r
),
cells(r INT, c INT, symbol TEXT) AS (
SELECT r, c, substring(line, c, 1)
FROM lines, generate_series(1, length(line)) c
),
steps(r INT, c INT) AS (
SELECT r, c FROM cells WHERE symbol = 'S'
EXCEPT ALL
SELECT * FROM s_delay
UNION
SELECT cells.r, cells.c
FROM cells, (
SELECT r + 1, c FROM steps
UNION SELECT r - 1, c FROM steps
UNION SELECT r, c + 1 FROM steps
UNION SELECT r, c - 1 FROM steps
) as potato(r,c)
WHERE cells.r = potato.r
AND cells.c = potato.c
AND cells.symbol != '#'
),
s_delay(r INT, c INT) AS (
SELECT r, c FROM cells WHERE symbol = 'S'
),
part1(part1 BIGINT) AS (
SELECT COUNT(*) FROM (SELECT DISTINCT * FROM steps)
),
-- PART 2 wants a much larger step count on an infinite repeating grid.
-- We know it will be quadratic based on the clear paths if nothing else.
-- Map out enough points to reverse out polynomial coefficients.
-- For me they were `ax^2 + bx + c` with a = 60724, b = 30602, c = 3849.
dists(r INT, c INT, d INT) AS (
SELECT r, c, MIN(d)
FROM (
SELECT r, c, 0 d
FROM cells
WHERE symbol = 'S'
UNION ALL
SELECT potato.r, potato.c, d + 1
FROM cells, (
SELECT r + 1, c, d FROM dists
UNION SELECT r - 1, c, d FROM dists
UNION SELECT r, c + 1, d FROM dists
UNION SELECT r, c - 1, d FROM dists
) as potato(r,c,d)
WHERE cells.r = 1 + (((potato.r - 1) % 131) + 131) % 131
AND cells.c = 1 + (((potato.c - 1) % 131) + 131) % 131
AND cells.symbol != '#'
AND potato.d < 1000
)
GROUP BY r, c
),
part2(x0 BIGINT, x2 BIGINT, x4 BIGINT, x6 BIGINT) AS (
SELECT
(SELECT COUNT(*) FROM dists WHERE d <= 0 * 131 + 65 AND d % 2 = 1),
(SELECT COUNT(*) FROM dists WHERE d <= 2 * 131 + 65 AND d % 2 = 1),
(SELECT COUNT(*) FROM dists WHERE d <= 4 * 131 + 65 AND d % 2 = 1),
(SELECT COUNT(*) FROM dists WHERE d <= 6 * 131 + 65 AND d % 2 = 1)
),
potato (x INT) AS ( SELECT 1 )
SELECT 'idk';Day 21 was brought to you by: @frankmcsherry