Not FAST, but is this correct?
H =. 1e6
L =. _1
D =: H, H, 1, H, 5, H, 8, H, 11, L, L, H, H, 20,3
P =: 1, 1, 1, 2, 4, 2, 1, 1, 1, 1, 1, 1, 2, 1, 2
proc=: dyad define
P=. x
D=. y
while. +./mask=.(L~:H,}:D)*.D>D<.P+H,}:D do.
D=. mask{"0 1|: D,:P+H,}:D
end.
D
)
D,:P proc D
1000000 1000000 1 1000000 5 1000000 8 1000000 11 _1 _1 1000000 1000000 20 3
1000000 1000000 1 3 5 7 8 9 10 _1 _1 1000000 1000000 20 3
D~:P proc D
0 0 0 1 0 1 0 1 1 0 0 0 0 0 0
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