Coding — Bootloader Instruction Simulator (Fix the Loop)

SWE coding prompt seen in both phone and virtual-onsite loops. A list of `plus` / `next` / `jump` instructions mutates an accumulator and program counter; first detect the loop, then identify or repair the single bad instruction that makes execution cycle. Requirements You are given a file where each line is an instruction with an operand: plus <n> — add <n> to a global accumulated value, then advance to the next line. next <n> — skip directly to the next line (the operand is ignored for control flow). jump <n> — jump to line current_index + n. The program halts when it either runs past the last line or revisits a line it has already executed (an infinite loop). One line has its jump and next instruction swapped, which is what forces the program into a loop. The task: Run the program and detect the loop — the offending line is the first line that would be visited twice. Repair the run by flipping the behavior on a revisited line: a jump that lands on an already-visited line is treated as a next, and a next that would revisit is treated as a jump. Return the accumulated value reached once the program halts without looping. A reported clarification: the expected return is the accumulated value at the point the (repaired) program terminates — not the sum of every plus operand in the file. Partial progress is acceptable; the interviewer wanted whatever value is reached without looping. Notes The hard part is reading comprehension, not algorithms: the base mechanic is a visited-set loop detector over a program counter, but the prompt's framing (which instruction is "wrong", what exactly to return) is reported as confusing and worth clarifying explicitly before coding. The wording around "an instruction is reversed" trips candidates into thinking every jump/next is mislabeled; in practice exactly one line revisits, and that revisit point is the line to fix. A virtual-onsite variant phrases the first part as "find the loop" and the second part as "one instruction is wrong; find that instruction." Treat the prompt wording as part of the problem and restate your interpretation before coding. The full follow-up set has not surfaced — candidates ran out of time on the base problem. Preparation Implement a program-counter simulator with a visited set that halts on the first repeat — the same skeleton as the classic "find where the boot sequence loops" puzzle. Practice parsing <op> <signed-int> lines and dispatching on the op. Be ready to clarify the return contract up front (terminal accumulator vs. total) and to handle the repair rule (treat a revisiting jump as next and vice versa) as a small state tweak inside the loop.