From 5936eb16d8d6224b0b3605abb77e90ddf1ccc0db Mon Sep 17 00:00:00 2001 From: Roberto Ierusalimschy Date: Wed, 25 Nov 2015 10:48:57 -0200 Subject: randomness in 'table.sort' used only when needed (big imbalance in partition result) + small refactoring --- ltablib.c | 111 +++++++++++++++++++++++++++++++++++++++++--------------------- 1 file changed, 73 insertions(+), 38 deletions(-) diff --git a/ltablib.c b/ltablib.c index ef39d5df..a03d0db2 100644 --- a/ltablib.c +++ b/ltablib.c @@ -1,5 +1,5 @@ /* -** $Id: ltablib.c,v 1.86 2015/11/20 12:30:20 roberto Exp roberto $ +** $Id: ltablib.c,v 1.89 2015/11/24 16:54:32 roberto Exp roberto $ ** Library for Table Manipulation ** See Copyright Notice in lua.h */ @@ -231,24 +231,68 @@ static int unpack (lua_State *L) { ** ======================================================= */ + +/* +** Produce a "random" 'unsigned int' to randomize pivot choice. This +** macro is used only when 'sort' detects a big imbalance in the result +** of a partition. (If you don't want/need this "randomness", ~0 is a +** good choice.) +*/ +#if !defined(l_randomizePivot) /* { */ + +#include + +/* size of 'e' measured in number of 'unsigned int's */ +#define sof(e) (sizeof(e) / sizeof(unsigned int)) + +/* +** Use 'time' and 'clock' as sources of "randomness". Because we don't +** know the types 'clock_t' and 'time_t', we cannot cast them to +** anything without risking overflows. A safe way to use their values +** is to copy them to an array of a known type and use the array values. +*/ +static unsigned int l_randomizePivot (void) { + clock_t c = clock(); + time_t t = time(NULL); + unsigned int buff[sof(c) + sof(t)]; + unsigned int i, rnd = 0; + memcpy(buff, &c, sof(c) * sizeof(unsigned int)); + memcpy(buff + sof(c), &t, sof(t) * sizeof(unsigned int)); + for (i = 0; i < sof(buff); i++) + rnd += buff[i]; + return rnd; +} + +#endif /* } */ + + +/* arrays larger than 'RANLIMIT' may use randomized pivots */ +#define RANLIMIT 100u + + static void set2 (lua_State *L, unsigned int i, unsigned int j) { lua_seti(L, 1, i); lua_seti(L, 1, j); } + +/* +** Return true iff value at stack index 'a' is less than the value at +** index 'b' (according to the order of the sort). +*/ static int sort_comp (lua_State *L, int a, int b) { - if (!lua_isnil(L, 2)) { /* function? */ + if (lua_isnil(L, 2)) /* no function? */ + return lua_compare(L, a, b, LUA_OPLT); /* a < b */ + else { /* function */ int res; - lua_pushvalue(L, 2); + lua_pushvalue(L, 2); /* push function */ lua_pushvalue(L, a-1); /* -1 to compensate function */ lua_pushvalue(L, b-2); /* -2 to compensate function and 'a' */ - lua_call(L, 2, 1); - res = lua_toboolean(L, -1); - lua_pop(L, 1); + lua_call(L, 2, 1); /* call function */ + res = lua_toboolean(L, -1); /* get result */ + lua_pop(L, 1); /* pop result */ return res; } - else /* a < b? */ - return lua_compare(L, a, b, LUA_OPLT); } @@ -293,39 +337,26 @@ static unsigned int partition (lua_State *L, unsigned int lo, /* -** Choose a "random" pivot in the middle part of the interval [lo, up]. -** (If you don't want/need this "randomness", (lo+up)/2 is an excellent -** choice.) +** Choose an element in the middle (2nd-3th quarters) of [lo,up] +** "randomized" by 'rnd' */ -#if !defined(l_sortpivot) -/* Use 'time' and 'clock' as sources of "randomness" */ -#include - -#define szi (sizeof(unsigned int)) -#define sof(e) (sizeof(e)/szi) - -static unsigned int choosePivot (unsigned int lo, unsigned int up) { - clock_t c = clock(); - time_t t = time(NULL); - unsigned int buff[sof(c) + sof(t)]; +static unsigned int choosePivot (unsigned int lo, unsigned int up, + unsigned int rnd) { unsigned int r4 = (unsigned int)(up - lo) / 4u; /* range/4 */ - unsigned int p, i, h = 0; - memcpy(buff, &c, sof(c) * szi); - memcpy(buff + sof(c), &t, sof(t) * szi); - for (i = 0; i < sof(buff); i++) - h += buff[i]; - p = h % (r4 * 2) + (lo + r4); + unsigned int p = rnd % (r4 * 2) + (lo + r4); lua_assert(lo + r4 <= p && p <= up - r4); return p; } -#define l_sortpivot(lo,up) choosePivot(lo,up) -#endif - -static void auxsort (lua_State *L, unsigned int lo, unsigned int up) { +/* +** QuickSort algorithm (recursive function) +*/ +static void auxsort (lua_State *L, unsigned int lo, unsigned int up, + unsigned int rnd) { while (lo < up) { /* loop for tail recursion */ unsigned int p; /* Pivot index */ + unsigned int n; /* to be used later */ /* sort elements 'lo', 'p', and 'up' */ lua_geti(L, 1, lo); lua_geti(L, 1, up); @@ -335,10 +366,10 @@ static void auxsort (lua_State *L, unsigned int lo, unsigned int up) { lua_pop(L, 2); /* remove both values */ if (up - lo == 1) /* only 2 elements? */ return; /* already sorted */ - if (up - lo < 100u) /* small interval? */ + if (up - lo < RANLIMIT || rnd == 0) /* small interval or no randomize? */ p = (lo + up)/2; /* middle element is a good pivot */ else /* for larger intervals, it is worth a random pivot */ - p = l_sortpivot(lo, up); + p = choosePivot(lo, up, rnd); lua_geti(L, 1, p); lua_geti(L, 1, lo); if (sort_comp(L, -2, -1)) /* a[p] < a[lo]? */ @@ -360,14 +391,18 @@ static void auxsort (lua_State *L, unsigned int lo, unsigned int up) { p = partition(L, lo, up); /* a[lo .. p - 1] <= a[p] == P <= a[p + 1 .. up] */ if (p - lo < up - p) { /* lower interval is smaller? */ - auxsort(L, lo, p - 1); /* call recursively for lower interval */ + auxsort(L, lo, p - 1, rnd); /* call recursively for lower interval */ + n = p - lo; /* size of smaller interval */ lo = p + 1; /* tail call for [p + 1 .. up] (upper interval) */ } else { - auxsort(L, p + 1, up); /* call recursively for upper interval */ + auxsort(L, p + 1, up, rnd); /* call recursively for upper interval */ + n = up - p; /* size of smaller interval */ up = p - 1; /* tail call for [lo .. p - 1] (lower interval) */ } - } /* tail call auxsort(L, lo, up) */ + if ((up - lo) / 128u > n) /* partition too imbalanced? */ + rnd = l_randomizePivot(); /* try a new randomization */ + } /* tail call auxsort(L, lo, up, rnd) */ } @@ -379,7 +414,7 @@ static int sort (lua_State *L) { if (!lua_isnoneornil(L, 2)) /* is there a 2nd argument? */ luaL_checktype(L, 2, LUA_TFUNCTION); /* must be a function */ lua_settop(L, 2); /* make sure there are two arguments */ - auxsort(L, 1, (unsigned int)n); + auxsort(L, 1, (unsigned int)n, 0u); } return 0; } -- cgit v1.2.3-55-g6feb