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author | S. Gilles <sgilles@math.umd.edu> | 2017-12-31 02:49:07 -0500 |
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committer | Ori Bernstein <ori@eigenstate.org> | 2018-01-02 10:01:45 -0800 |
commit | 6c561486a8359a7c6858be5b47170e9341633da5 (patch) | |
tree | e4fb21e3b0dd204aaa739a2bd2adb9583be9bcae | |
parent | 5106fe5edb4664688fe87b6340b32c0c1dbd6157 (diff) | |
download | mc-6c561486a8359a7c6858be5b47170e9341633da5.tar.gz |
add permutation iterator
A good exposition of this algorithm is at
https://www.nayuki.io/page/next-lexicographical-permutation-algorithm
-rw-r--r-- | lib/iter/bld.sub | 1 | ||||
-rw-r--r-- | lib/iter/perm.myr | 77 | ||||
-rw-r--r-- | lib/iter/test/perm.myr | 54 |
3 files changed, 132 insertions, 0 deletions
diff --git a/lib/iter/bld.sub b/lib/iter/bld.sub index 805676a..2b8f5e3 100644 --- a/lib/iter/bld.sub +++ b/lib/iter/bld.sub @@ -1,6 +1,7 @@ lib iter = chunk.myr enum.myr + perm.myr ref.myr reverse.myr zip.myr diff --git a/lib/iter/perm.myr b/lib/iter/perm.myr new file mode 100644 index 0000000..c520919 --- /dev/null +++ b/lib/iter/perm.myr @@ -0,0 +1,77 @@ +use std + +pkg iter = + type permiter(@a) = struct + cmp : (a : @a, b : @a -> std.order) + seq : @a[:] + first : bool + ;; + + impl iterable permiter(@a) -> @a[:] + generic byperm : (a : @a[:], cmp : (a : @a, b : @a -> std.order) -> permiter(@a)) +;; + +generic byperm = {a, cmp + /* start off by backwards-sorting a */ + std.sort(a, cmp) + + -> [.cmp = cmp, .seq = std.sort(a, cmp), .first = true] +} + +impl iterable permiter(@a) -> @a[:] = + __iternext__ = {itp, valp + var j : std.size = seq.len - 1 + var i : std.size = seq.len - 1 + var seq : @a[:] = itp.seq + + /* We're permuting seq in place */ + valp# = itp.seq + + if itp.first + itp.first = false + -> true + ;; + + /* Find the longest decreasing sequence at end */ + j = seq.len - 1 + while true + if j == 0 + -> false + ;; + match itp.cmp(seq[j - 1], seq[j]) + | `std.After: j-- + | _: break + ;; + ;; + j-- + + /* Now seq[j+1:] is all decreasing */ + + /* Find highest i s.t. seq[j] < seq[i] */ + i = seq.len - 1 + while true + if i <= j + -> false + ;; + match itp.cmp(seq[j], seq[i]) + | `std.Before: break + | _: i-- + ;; + ;; + + /* First, swap seq[i] and seq[j] */ + std.swap(&seq[i], &seq[j]) + + /* Now reverse seq[j+1:] */ + i = 1 + while j + i < seq.len - i + std.swap(&seq[j + i], &seq[seq.len - i]) + i++ + ;; + + -> true + } + + __iterfin__ = {itp, valp + } +;; diff --git a/lib/iter/test/perm.myr b/lib/iter/test/perm.myr new file mode 100644 index 0000000..cfebb03 --- /dev/null +++ b/lib/iter/test/perm.myr @@ -0,0 +1,54 @@ +use std +use testr + +use iter + +const main = { + testr.run([ + [.name = "byperm-01", .fn = byperm01], + [.name = "byperm-02", .fn = byperm02], + [.name = "byperm-03", .fn = byperm03], + ][:]) +} + +const byperm01 = {c + var v : int[:] = [ 0, 2, 1 ][:] + var sb : std.strbuf# = std.mksb() + std.sbfmt(sb, " ") + + for w : iter.byperm(v, std.numcmp) + std.sbfmt(sb, "{} ", w) + ;; + + var expected : byte[:] = " [0, 1, 2] [0, 2, 1] [1, 0, 2] [1, 2, 0] [2, 0, 1] [2, 1, 0] " + var actual : byte[:] = std.sbfin(sb) + testr.check(c, std.eq(expected, actual), "expected “{}”, got “{}”", expected, actual) +} + +const byperm02 = {c + var v : int[:] = [ 1, 1 ][:] + var sb : std.strbuf# = std.mksb() + std.sbfmt(sb, " ") + + for w : iter.byperm(v, std.numcmp) + std.sbfmt(sb, "{} ", w) + ;; + + var expected : byte[:] = " [1, 1] " + var actual : byte[:] = std.sbfin(sb) + testr.check(c, std.eq(expected, actual), "expected “{}”, got “{}”", expected, actual) +} + +const byperm03 = {c + var v : int[:] = [ 3, 1, 1 ][:] + var sb : std.strbuf# = std.mksb() + std.sbfmt(sb, " ") + + for w : iter.byperm(v, std.numcmp) + std.sbfmt(sb, "{} ", w) + ;; + + var expected : byte[:] = " [1, 1, 3] [1, 3, 1] [3, 1, 1] " + var actual : byte[:] = std.sbfin(sb) + testr.check(c, std.eq(expected, actual), "expected “{}”, got “{}”", expected, actual) +} |