AoC-2022 Day 15b -- no allocs + heavy steroids

// AocZz.cpp : This file contains the 'main' function. Program execution begins and ends there.
//

#include <stdio.h>
#include <stdlib.h>

struct Sensor
{
    Sensor* prev{ 0 };
    int sx{ 0 };
    int sy{ 0 };
    int dist{ 0 };
};

bool isFound(Sensor* last, int x, int y)
{
    if (x > 4000000)
        return false;
    if (y > 4000000)
        return false;
    if (x < 0)
        return false;
    if (y < 0)
        return false;

    for (Sensor* p = last; p; p = p->prev)
    {
        int len = abs(p->sx - x) + abs(p->sy - y);
        if (len <= p->dist)
            return false;
    }

    printf("%lld", 4000000ull * x + y);
    return true; // exit of recursion
}



bool readValue(FILE * f, int& val)
{
    do
    {
        int c = fgetc(f);
        if (c < 0)
            return false;
        if ('=' == c)
            break;
    } while (true);

    val = 0;
    int sign = 1;
    do
    {
        int c = fgetc(f);
        if (c < 0)
            return false;
        if ('-' == c)
        {
            sign = -1;
        }
        else if (('9' >= c) && ('0' <= c))
            val = val * 10 + sign * (c - '0');
        else
            return val;
    } while (true);
}

void traverse(Sensor * last)
{
    for (Sensor* p = last; p; p = p->prev)
    {   // for each sensor, walk around it's perimeter and check points
        int len = p->dist + 1;

        int dirX[] = { -1, +1, -1, +1 };
        int dirY[] = { -1, -1, +1, +1 };

        for (int i = 0; i <= len; i++)
        {
            for (int k = 0; k < 4; k++)
            {   // diamond with four sides
                if (isFound(last, p->sx + dirX[k] * (len - i), p->sy + dirY[k] * i))
                    return;
            }
        }
    }
}

void recursiveReadValues(FILE* f, Sensor* pPrev)
{
    Sensor s; // list of sensors gets stored in process stack
    s.prev = pPrev;
    int bx;
    int by;
    if (readValue(f, s.sx) && readValue(f, s.sy) && readValue(f, bx) && readValue(f, by))
    {
        s.dist = abs(bx - s.sx) + abs(by - s.sy);
        recursiveReadValues(f, &s);
    }
    else
    {
        fclose(f);
        traverse(pPrev); // exit of recursion
    }
}

int main()
{
    FILE* f = NULL;
    fopen_s(&f, "input.txt", "r");

    recursiveReadValues(f, nullptr);

    return 0;
}