優化計數排序值域的後綴數組

#include <algorithm>
#include <cstdio>
#include <cstring>
#include <iostream>

using namespace std;

const int N = 1000010;

char s[N];
int n, sa[N], rk[N], oldrk[N << 1], id[N], px[N], cnt[N];
// px[i] = rk[id[i]]（用於排序的數組所以叫 px）

bool cmp(int x, int y, int w) {
  return oldrk[x] == oldrk[y] && oldrk[x + w] == oldrk[y + w];
}

int main() {
  int i, m = 300, p, w;

  scanf("%s", s + 1);
  n = strlen(s + 1);
  for (i = 1; i <= n; ++i) ++cnt[rk[i] = s[i]];
  for (i = 1; i <= m; ++i) cnt[i] += cnt[i - 1];
  for (i = n; i >= 1; --i) sa[cnt[rk[i]]--] = i;

  for (w = 1; w < n; w <<= 1, m = p) {  // m=p 就是優化計數排序值域
    for (p = 0, i = n; i > n - w; --i) id[++p] = i;
    for (i = 1; i <= n; ++i)
      if (sa[i] > w) id[++p] = sa[i] - w;
    memset(cnt, 0, sizeof(cnt));
    for (i = 1; i <= n; ++i) ++cnt[px[i] = rk[id[i]]];
    for (i = 1; i <= m; ++i) cnt[i] += cnt[i - 1];
    for (i = n; i >= 1; --i) sa[cnt[px[i]]--] = id[i];
    memcpy(oldrk, rk, sizeof(rk));
    for (p = 0, i = 1; i <= n; ++i)
      rk[sa[i]] = cmp(sa[i], sa[i - 1], w) ? p : ++p;
  }

  for (i = 1; i <= n; ++i) printf("%d ", sa[i]);

  return 0;
}