76 lines
2.8 KiB
C
76 lines
2.8 KiB
C
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/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
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│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8 :vi│
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╞══════════════════════════════════════════════════════════════════════════════╡
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│ Copyright 2020 Justine Alexandra Roberts Tunney │
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│ │
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│ This program is free software; you can redistribute it and/or modify │
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│ it under the terms of the GNU General Public License as published by │
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│ the Free Software Foundation; version 2 of the License. │
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│ │
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│ This program is distributed in the hope that it will be useful, but │
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│ WITHOUT ANY WARRANTY; without even the implied warranty of │
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│ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU │
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│ General Public License for more details. │
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│ │
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│ You should have received a copy of the GNU General Public License │
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│ along with this program; if not, write to the Free Software │
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│ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA │
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│ 02110-1301 USA │
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╚─────────────────────────────────────────────────────────────────────────────*/
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#include "libc/bits/hilbert.h"
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static axdx_t RotateQuadrant(long n, long y, long x, long ry, long rx) {
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long t;
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if (ry == 0) {
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if (rx == 1) {
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y = n - 1 - y;
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x = n - 1 - x;
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}
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t = x;
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x = y;
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y = t;
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}
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return (axdx_t){y, x};
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}
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/**
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* Generates Hilbert space-filling curve.
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*
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* @see morton()
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*/
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long hilbert(long n, long y, long x) {
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axdx_t m;
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long d, s, ry, rx;
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d = 0;
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for (s = n / 2; s > 0; s /= 2) {
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rx = (x & s) > 0;
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ry = (y & s) > 0;
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d += s * s * ((3 * rx) ^ ry);
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m = RotateQuadrant(n, y, x, ry, rx);
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x = m.dx;
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y = m.ax;
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}
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return d;
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}
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/**
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* Decodes Hilbert space-filling curve.
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*
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* @see unmorton()
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*/
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axdx_t unhilbert(long n, long i) {
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axdx_t m;
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long s, t, y, x, ry, rx;
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t = i;
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x = y = 0;
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for (s = 1; s < n; s *= 2) {
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rx = (t / 2) & 1;
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ry = (t ^ rx) & 1;
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m = RotateQuadrant(s, y, x, ry, rx);
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x = m.dx + s * rx;
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y = m.ax + s * ry;
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t /= 4;
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}
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return (axdx_t){y, x};
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}
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