AoC64/2021/day16/main.cpp at main · hmatejx/AoC64 · GitHub

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
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "AoC64.h"
#if __has_include("input.h")
  #include "input.h"          // user’s private input (gitignored)
#else
  #include "input.example.h"  // checked-in sample input
#endif

inline uint8_t to_nibble(const char c) {
    if (c >= '0' && c <= '9')
        return c - '0';
    return c - 'A' + 10;
}

uint16_t get_bits_increment_offset(uint8_t nbits, const char* packet, uint16_t& bit_offset) {
    uint16_t res = 0;
    for (int8_t bit = 0; bit < nbits; ++bit) {
        const uint16_t offset = bit_offset + bit;
        if ((to_nibble(packet[offset >> 2]) >> (3 - (offset & 3))) & 1)
            res |= 1 << (nbits - 1 - bit);
    }
    bit_offset += nbits;
    return res;
}
#define get_bits(nbits) get_bits_increment_offset((nbits), packet, bit_offset)


// global variable hack for part 1
uint16_t g_sum_versions = 0;

// returns the number of bits in the packet
uint64_t decode(const char* packet, uint16_t& bit_offset, uint16_t& bits_read) {

    uint16_t start_offset = bit_offset;
    uint64_t value;

    // get version and type
    uint8_t version = get_bits(3);
    uint8_t type = get_bits(3);
    g_sum_versions += version;

    // consider type
    switch (type) {
        // literal value packet
        case 4:
            {
                value = 0;
                //uint8_t n_nibbles = 0, more;
                for (;;) {
                    uint8_t more = get_bits(1);
                    value |= get_bits(4);
                    // nibble padded with bit 0, last nibble
                    if (!more)
                        break;
                    // make place for next nibble
                    value <<= 4;
                    //++n_nibbles;
                }
            }
            break;

        // operator packet
        case 0:
        case 1:
        case 2:
        case 3:
            {
                // get the total total bit length or total number of sub-packets
                uint16_t subpackets_len, num_subpackets;
                if (get_bits(1) == 0) {
                    subpackets_len = get_bits(15);
                    num_subpackets = 0xFFFF;
                } else {
                    subpackets_len = 0xFFFF;
                    num_subpackets = get_bits(11);
                }
                // process the subpackets
                uint16_t tot_len = 0, len;
                uint8_t num = 0;
                if (type == 0 || type == 3)
                    value = 0;
                else if (type == 2)
                    value = 0xFFFFFFFFFFFFFFFF;
                else
                    value = 1;
                while (tot_len < subpackets_len && num < num_subpackets) {
                    uint64_t v = decode(packet, bit_offset, len);
                    switch (type) {
                        case 0: value += v; break;
                        case 1: value *= v; break;
                        case 2: if (v < value) value = v; break;
                        case 3: if (v > value) value = v; break;
                    }
                    tot_len += len;
                    num++;
                }
            }
            break;
        default:
            uint16_t len;
            get_bits(get_bits(1) == 0 ? 15 : 11);
            uint64_t v1 = decode(packet, bit_offset, len);
            uint64_t v2 = decode(packet, bit_offset, len);
            if (type == 5)
                value = v1 > v2 ? 1 : 0;
            else if (type == 6)
                value = v1 < v2 ? 1 : 0;
            else
                value = v1 == v2 ? 1 : 0;
    }
    // update the number of bits read (bit_offset is updated automatically in get_bits)
    bits_read = bit_offset - start_offset;
    tick((bit_offset >> 8) & (uint8_t)7);

    return value;
}


int main(void) {
    init(16);

    uint16_t bit_offset = 0, len;
    uint64_t value = decode(input[0], bit_offset, len);

    printf("part 1: %u\n", g_sum_versions);
    printf("part 2: %llu\n", value);

    finish();
}