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comments added to 'lua_rotate'

This commit is contained in:
Roberto Ierusalimschy 2014-05-14 11:20:17 -03:00
parent 5a5a834975
commit 965993da07
1 changed files with 11 additions and 7 deletions
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18
lapi.c
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@ -1,5 +1,5 @@
/* /*
** $Id: lapi.c,v 2.209 2014/05/01 18:21:32 roberto Exp roberto $ ** $Id: lapi.c,v 2.210 2014/05/13 19:40:28 roberto Exp roberto $
** Lua API ** Lua API
** See Copyright Notice in lua.h ** See Copyright Notice in lua.h
*/ */
@ -195,17 +195,21 @@ static void reverse (lua_State *L, StkId from, StkId to) {
} }
/*
** Let x = AB, where A is a prefix of length 'n'. Then,
** rotate x n == BA. But BA == (A^r . B^r)^r.
*/
LUA_API void lua_rotate (lua_State *L, int idx, int n) { LUA_API void lua_rotate (lua_State *L, int idx, int n) {
StkId p, t, m; StkId p, t, m;
lua_lock(L); lua_lock(L);
t = L->top - 1; t = L->top - 1; /* end of stack segment being rotated */
p = index2addr(L, idx); p = index2addr(L, idx); /* start of segment */
m = (n >= 0 ? t - n : p - n - 1); m = (n >= 0 ? t - n : p - n - 1); /* end of prefix */
api_checkstackindex(L, idx, p); api_checkstackindex(L, idx, p);
api_check(L, p <= m + 1 && m <= t, "invalid 'n'"); api_check(L, p <= m + 1 && m <= t, "invalid 'n'");
reverse(L, p, m); reverse(L, p, m); /* reverse the prefix with length 'n' */
reverse(L, m + 1, t); reverse(L, m + 1, t); /* reverse the suffix */
reverse(L, p, t); reverse(L, p, t); /* reverse the entire segment */
lua_unlock(L); lua_unlock(L);
} }