40 lines
1.2 KiB
Factor
40 lines
1.2 KiB
Factor
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! Copyright (c) 2012 Anonymous
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! See http://factorcode.org/license.txt for BSD license.
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USING: kernel math sequences ;
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IN: rosetta-code.equilibrium-index
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! http://rosettacode.org/wiki/Equilibrium_index
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! An equilibrium index of a sequence is an index into the sequence such that the sum of elements at lower indices is equal to the sum of elements at higher indices. For example, in a sequence A:
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! A0 = − 7
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! A1 = 1
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! A2 = 5
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! A3 = 2
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! A4 = − 4
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! A5 = 3
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! A6 = 0
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! 3 is an equilibrium index, because:
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! A0 + A1 + A2 = A4 + A5 + A6
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! 6 is also an equilibrium index, because:
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! A0 + A1 + A2 + A3 + A4 + A5 = 0
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! (sum of zero elements is zero)
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! 7 is not an equilibrium index, because it is not a valid index
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! of sequence A.
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! Write a function that, given a sequence, returns its
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! equilibrium indices (if any). Assume that the sequence may be
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! very long.
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: accum-left ( seq id quot -- seq )
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accumulate nip ; inline
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: accum-right ( seq id quot -- seq )
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[ <reversed> ] 2dip accum-left <reversed> ; inline
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: equilibrium-indices ( seq -- inds )
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0 [ + ] [ accum-left ] [ accum-right ] 3bi [ = ] 2map
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V{ } swap dup length iota [ [ suffix ] curry [ ] if ] 2each ;
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