Burrows–Wheeler transform: Difference between revisions
→{{header|Sidef}}: added a more efficient/practical implementation
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BWT("SIX.MIXED.PIXIES.SIFT.SIXTY.PIXIE.DUST.BOXES") = "STEXYDST.E.IXXIIXXSSMPPS.B..EE.$.USFXDIIOIIIT"
</pre>
More efficient implementation, running in '''O(n*log(n))''' time, using '''O(n)''' space:
<syntaxhighlight lang="ruby">define LOOKAHEAD_LEN = 128
func bwt_sort (String s) { # O(n * LOOKAHEAD_LEN) space (fast)
^s.len -> map {|i|
var t = s.slice(i, LOOKAHEAD_LEN)
if (t.len < LOOKAHEAD_LEN) {
t += s.slice(0, min(i, LOOKAHEAD_LEN - t.len))
}
[t, i]
}.sort {|a,b|
(a[0] <=> b[0]) || (s.rotate(a[1]) <=> s.rotate(b[1]))
}.map { .[1] }
}
func bwt_encode(String s) {
var bwt = bwt_sort(s)
var ret = bwt.map {|i| s.slice(i-1, 1) }.join
var prefix = s.slice(0, LOOKAHEAD_LEN)
var len = prefix.len
var idx = 0;
for i in (bwt) {
var lookahead = s.slice(i, len)
if (lookahead.len < len) {
lookahead += s.slice(0, len - lookahead.len)
}
if (lookahead == prefix) {
var row = s.rotate(i)
if (row == s) {
break
}
}
++idx
}
return (ret, idx)
}
func bwt_decode(String bwt, Number idx) { # fast inversion
var tail = bwt.chars
var head = tail.sort
var indices = Hash()
tail.each_kv {|i,v|
indices{v} := [] << i
}
var table = []
head.each_kv {|i,v|
table[i] = indices{v}.shift
}
var dec = ''
var i = idx
head.len.times {
dec += head[i]
i = table[i]
}
return dec
}
var tests = [
"banana", "appellee", "dogwood", "TOBEORNOTTOBEORTOBEORNOT"
"SIX.MIXED.PIXIES.SIFT.SIXTY.PIXIE.DUST.BOXES",
]
tests.each { |str|
var (enc, idx) = bwt_encode(str)
var dec = bwt_decode(enc, idx)
say "BWT(#{dec.dump}) = (#{enc.dump}, #{idx})"
assert_eq(str, dec)
}</syntaxhighlight>
{{out}}
<pre>
BWT("banana") = ("nnbaaa", 3)
BWT("appellee") = ("eelplepa", 0)
BWT("dogwood") = ("odoodwg", 1)
BWT("TOBEORNOTTOBEORTOBEORNOT") = ("OOOBBBRRTTTEEENNOOORTTOO", 20)
BWT("SIX.MIXED.PIXIES.SIFT.SIXTY.PIXIE.DUST.BOXES") = ("TEXYDST.E.IXIXIXXSSMPPS.B..E.S.EUSFXDIIOIIIT", 29)
</pre>
=={{header|Swift}}==
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