Migrates the Wandbox compilation API from /api/compile.json (batch response) to /api/compile.ndjson (streaming response). The function interface remains unchanged—results are still collected and returned after completion.

Changes

• Added CompileNdjsonResult interface to represent individual ndjson response lines with type and data fields
• Updated compileAndRun() to consume ndjson stream using response.body.getReader() for incremental processing with proper buffering for incomplete lines
• Unified output structure by changing CompileResultWithOutput to use a single output: ReplOutput[] array instead of separate arrays, storing responses in the order they are received
• Updated cpp.ts to work with the new unified output structure
• Added HTTP error handling for non-OK responses

Implementation

The stream reader processes chunks incrementally, buffering incomplete lines and parsing complete JSON objects as they arrive:

const reader = response.body?.getReader();
const decoder = new TextDecoder();
let buffer = "";

while (true) {
  const { done, value } = await reader.read();
  if (done) break;
  
  buffer += decoder.decode(value, { stream: true });
  const lines = buffer.split("\n");
  buffer = lines.pop() || "";
  
  for (const line of lines) {
    if (line.trim().length > 0) {
      ndjsonResults.push(JSON.parse(line));
    }
  }
}

The merge logic accumulates streaming messages into both a CompileResult and a unified output array that preserves chronological order:

for (const r of ndjsonResults) {
  switch (r.type) {
    case "CompilerMessageS":
      result.compiler_output += r.data;
      result.compiler_message += r.data;
      // Add to output array in order
      for (const line of r.data.trim().split("\n")) {
        output.push({ type: "stdout", message: line });
      }
      break;
    // ... other cases
  }
}

This matches the Rust merge_compile_result implementation in the wandbox repository (feline/src/util.rs) while providing better flexibility for future interface changes.

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