AoC 2019 D5: Sunny with a Chance of Asteroids

| Problem statement | Source code | Tags: Intcode

The Intcode itself is relatively straightforward, but I spent some time ironing out the infrastructure for code sharing, especially since it's a bit tricky in C++.

At this point, my code is still functional. My intcode.hpp file contains two functions:

std::vector<int> parse_prog(const std::string &line);
std::vector<int> run_prog(std::vector<int> &codes, std::vector<int> inputs);

cpp

It modifies the codes vector in place, and returns a vector of outputs. The inputs are provided as a vector as well, and consumed in order.

Input and output instructions are straightforward to implement. One reads from the input vector, and the other appends to the output vector.
Parameter modes require changing codes[op1] to eval_param(codes, inst.params[0]) (ah yes, I also have a struct Inst that holds the opcode and parameters, each with mode/value, and a function to extract this information). eval_param just checks the mode and either returns the value at that position or the immediate value.
The main for loop doesn't increment by a fixed amount anymore; instead, each instruction in the switch statement specifies the new i value at the end.
The jump instructions just set i instead of incrementing it.

The new main loop looks like this:

int input_idx = 0;
std::vector<int> outputs;
for (int ip = 0; codes[ip] != 99;) {
    Inst inst = parse_inst(codes, ip);
    switch (inst.opcode) {
        case 1:
        case 2:
        case 7:
        case 8: {
            int val1 = eval_param(codes, inst.params[0]);
            int val2 = eval_param(codes, inst.params[1]);
            // int res = ...;
            write_to(codes, inst.params[2], res);
            ip += num_params_for_opcode(inst.opcode) + 1;
            break;
        }
        case 3: {
            write_to(codes, inst.params[0], inputs[input_idx]);
            input_idx++;
            ip += num_params_for_opcode(inst.opcode) + 1;
            break;
        }
        case 4: {
            int val = eval_param(codes, inst.params[0]);
            outputs.push_back(val);
            ip += num_params_for_opcode(inst.opcode) + 1;
            break;
        }
        case 5:
        case 6: {
            int val = eval_param(codes, inst.params[0]);
            bool should_jump = inst.opcode == 5 ? (val != 0) : (val == 0);
            if (should_jump) {
                ip = eval_param(codes, inst.params[1]);
            } else {
                ip += num_params_for_opcode(inst.opcode) + 1;
            }
            break;
        }
        default:
            throw std::invalid_argument("Unknown opcode");
    }
}

cpp

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