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Neil Gall
Neil Gall

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Advent of Code 2021 Day 3

I really wanted to switch away from C today but it was a bit twiddling problem. The challenge was discarding entries from the report structure. In higher level languages you'd build new lists of the remaining items but doing all that copying by hand is tough in C, so I opted for efficiency (both programmer and CPU) and added a "keep" flag to each entry, ignoring the entries whose flag had been cleared on subsequent passes.

Is it more efficient to be passing over all those discarded entries? Probably, for this size of data set which fits in the CPU cache. Checking the keep flag is very cheap. But if you increased the problem to millions or billions of entries, copying to a new smaller array would make sense. You'd only need about 50% more memory as on average you're halving the data set each time. Just double-buffer, copying the retained items back and forth between two buffers on each iteration.

Reusing the string/file readers from day1 and day2...

#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include "reader.h"

// ----------------------------------------------
// Report structure

typedef struct entry {
    uint16_t value;
    uint8_t keep;
} entry_t;

struct Report {
    size_t capacity;
    size_t count;
    size_t entry_length;
    entry_t entries[];
};

size_t report_size(size_t count) {
    return sizeof(struct Report) + sizeof(entry_t) * count;
}

struct Report *empty_report(size_t capacity) {
    struct Report *report = (struct Report *)malloc(report_size(capacity));
    report->capacity = capacity;
    report->count = 0;
    report->entry_length = 0;
    return report;
}

void append_entry(struct Report **report_p, entry_t entry, size_t length) {
    struct Report *report = *report_p;
    if (report->count == report->capacity) {
        report->capacity *= 2;
        *report_p = report = (struct Report *)realloc(report, report_size(report->capacity));
    }
    if (length > report->entry_length)
        report->entry_length = length;
    report->entries[report->count++] = entry;
}

struct Report *read_report(struct Reader *reader) {
    struct Report *report = empty_report(100);
    struct entry entry = { .value = 0, .keep = 1 };
    size_t length = 0;
    char c;
    while ((c = reader->next(reader)) != 0) {
        if (isdigit(c)) {
            entry.value = (entry.value << 1) + (c - '0');
            length += 1;
        } else {
            if (length > 0) {
                append_entry(&report, entry, length);
            }
            entry.value = 0;
            length = 0;
        }
    }
    if (length > 0) {
        append_entry(&report, entry, length);
    }
    reader->delete(reader);
    return report;
}


// ----------------------------------------------
// Problems

size_t ones_in_bit_position(struct Report *report, unsigned bit) {
    size_t ones = 0;
    for (size_t e = 0; e < report->count; ++e) {
        if (report->entries[e].keep && report->entries[e].value & bit)
            ones++;
    }
    return ones;    
}


int part1(struct Report *report) {
    unsigned gamma = 0;
    unsigned epsilon = 0;
    for (size_t i = 0; i < report->entry_length; ++i) {
        unsigned bit = 1 << i;
        size_t ones = ones_in_bit_position(report, bit);
        if (ones * 2 > report->count) {
            gamma |= bit;
        } else {
            epsilon |= bit;
        }
    }
    return gamma * epsilon;
}


void keep_all_entries(struct Report *report) {
    for (size_t i = 0; i < report->count; ++i) {
        report->entries[i].keep = 1;
    }
}


size_t discard_entries(struct Report *report, unsigned bit, unsigned discard) {
    size_t discarded = 0;
    for (size_t i = 0; i < report->count; ++i) {
        if (report->entries[i].keep &&
                !(report->entries[i].value & bit) == discard) {
            report->entries[i].keep = 0;
            discarded++;
        }
    }
    return discarded;
}


unsigned first_kept_entry(struct Report *report) {
    for (size_t i = 0; i < report->count; ++i) {
        if (report->entries[i].keep)
            return report->entries[i].value;
    }
    abort();
}


unsigned oxygen_generator_rating(struct Report *report) {
    size_t remaining = report->count;
    size_t bit_pos = report->entry_length;

    keep_all_entries(report);
    while (bit_pos-- > 0 && remaining > 1) {
        printf("bit %d remaining %u\n", bit_pos, remaining);
        unsigned bit = 1 << bit_pos;
        size_t ones = ones_in_bit_position(report, bit);
        unsigned keep = (ones * 2 >= remaining) ? 1 : 0;
        remaining -= discard_entries(report, bit, !keep);
    }

    return first_kept_entry(report);
}


unsigned co2_scrubber_rating(struct Report *report) {
    size_t remaining = report->count;
    size_t bit_pos = report->entry_length;

    keep_all_entries(report);
    while (bit_pos-- > 0 && remaining > 1) {
        unsigned bit = 1 << bit_pos;
        size_t ones = ones_in_bit_position(report, bit);
        unsigned keep = (ones * 2 < remaining) ? 1 : 0;
        remaining -= discard_entries(report, bit, !keep);
    }

    return first_kept_entry(report);
}


int part2(struct Report *report) {
    return oxygen_generator_rating(report) * co2_scrubber_rating(report);
}


// ----------------------------------------------
// tests

void test(const char *input, int (*f)(struct Report *), int expect) {
    struct Report *report = read_report(string_reader(input));
    int actual = f(report);
    free(report);
    if (actual != expect) {
        fprintf(stderr, "test failed input='%s' expect=%d actual=%d\n",
            input, expect, actual);
        abort();
    }
}

static const char test_data[] = 
    "00100 11110 10110 10111 10101 01111 00111 11100 10000 11001 00010 01010";

void tests() {
    test(test_data, part1, 198);
    test(test_data, part2, 230);
}

// ----------------------------------------------
// Main

int main() {
    tests();

    struct Report *report = read_report(file_reader("day3/input.txt"));

    printf("Part 1: %d\n", part1(report));
    printf("Part 2: %d\n", part2(report));

    return 0;
}
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Discussion (1)

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Neil Gall Author

Ok interesting! I reworked it to copy the valid report entries rather than just flag them discarded. And it runs about 33% faster, even without trying to be clever about double-buffering. I wonder how far I can take this...

Newer code at github.com/neilgall/advent-of-code...