AoC-2024 Day 7b -- single pass, no allocs

const std = @import("std");
const Allocator = std.mem.Allocator;

const data = @embedFile("input0.txt");

// arena for ints
var numsArr: [20]i64 = undefined;
var numsCnt: u8 = 0;
var testVal: i64 = 0;
var cnt: i64 = 0;

fn recursiveOp(op: u8, len: u8, accum: i64) bool {
    if (len == numsCnt) {
        if (accum == testVal)
            return true;
        return false;
    }

    var newval: i64 = undefined;
    if (op == '+') {
        newval = accum + numsArr[len];
    }
    if (op == '*') {
        newval = accum * numsArr[len];
    }

    if (op == '|') {
        newval = accum;
        var b = numsArr[len];
        while (b > 0) {
            newval *= 10;
            b = @divTrunc(b, 10);
        }
        newval += numsArr[len];
    }

    if (recursiveOp('+', len + 1, newval))
        return true;

    if (recursiveOp('*', len + 1, newval))
        return true;

    if (recursiveOp('|', len + 1, newval))
        return true;

    return false;
}

fn doProcessing() void {
    const accum = numsArr[0];
    if (recursiveOp('+', 1, accum)) {
        cnt += testVal;
        return;
    }

    if (recursiveOp('*', 1, accum)) {
        cnt += testVal;
        return;
    }

    if (recursiveOp('|', 1, accum)) {
        cnt += testVal;
        return;
    }
}

pub fn main() !void {
    var currNum: i64 = 0;
    for (data) |c| {
        if (c == '\r')
            continue;

        if (c == ':') {
            testVal = currNum;
            currNum = 0;
            continue;
        }

        if ((c == ' ') or (c == '\n')) {
            if (0 == currNum)
                continue;

            numsArr[numsCnt] = currNum;
            numsCnt += 1;
            currNum = 0;
            if (c == ' ')
                continue;
        }
        if (c == '\n') {
            doProcessing();
            currNum = 0;
            numsCnt = 0;
            continue;
        }

        currNum *= 10;
        currNum += c - '0';
    }

    std.debug.print("Result: {d}.\n", .{cnt});
}