50103. File Mirroring and Shifting
難度：4/5
在長度\(l\)的array中倒轉並left shift \(n\)格，相當於對前\(l-n\)項做reverse，再對最後\(n\)項做reverse。1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#define min(x,y) ((x)<(y)?(x):(y))
static inline uint8_t rev_bits(uint8_t num){
uint8_t ans = 0;
for(int i = 0; i < 8; i++){
if(((num >> i) & 1)) ans |= ((uint8_t)1 << (7 - i));
}
return ans;
}
static inline void reverse(uint8_t arr[], int n){
int l = 0, r = n - 1;
while(l < r){
uint8_t temp = arr[l];
arr[l] = arr[r];
arr[r] = temp;
l++, r--;
}
for(int i = 0; i < n; i++) arr[i] = rev_bits(arr[i]);
}
static inline void solve(FILE* in, FILE* out, int n){
fseek(in, 0, SEEK_END); int len = ftell(in);
int n_first = len - n;
uint8_t arr[4096];
fseek(in, n_first, SEEK_SET);
while(n_first){
int arr_size = min(n_first, 4096);
fseek(in, -arr_size, SEEK_CUR); fread(arr, sizeof(uint8_t), arr_size, in); fseek(in, -arr_size, SEEK_CUR);
reverse(arr, arr_size);
fwrite(arr, sizeof(uint8_t), arr_size, out);
n_first -= arr_size;
}
int n_second = n;
fseek(in, 0, SEEK_END);
while(n_second){
int arr_size = min(n_second, 4096);
fseek(in, -arr_size, SEEK_CUR); fread(arr, sizeof(uint8_t), arr_size, in); fseek(in, -arr_size, SEEK_CUR);
reverse(arr, arr_size);
fwrite(arr, sizeof(uint8_t), arr_size, out);
n_second -= arr_size;
}
}
int main(){
char in_name[64], out_name[64];
int n;
scanf("%s%s%d", in_name, out_name, &n);
FILE* in = fopen(in_name, "rb");
FILE* out = fopen(out_name, "wb");
solve(in, out, n);
fclose(in);
fclose(out);
return 0;
}
// l = 7, n = 3
// [1, 2, 3, 4, 5, 6, 7]
// [4, 3, 2, 1, 7, 6, 5]