{"record":{"id":"a73a443cce15fcec","repo":"rohitg00/ai-engineering-from-scratch","slug":"bleu4-requires-at-least-one-reference","errorCode":null,"errorMessage":"bleu4 requires at least one reference","messagePattern":"bleu4 requires at least one reference","errorType":"validation","errorClass":"ValueError","httpStatus":null,"severity":"error","filePath":"phases/19-capstone-projects/63-multimodal-eval/code/main.py","lineNumber":130,"sourceCode":"    return [tuple(seq[i:i + n]) for i in range(len(seq) - n + 1)]\n\n\ndef _count(ngrams: list[tuple[int, ...]]) -> dict[tuple[int, ...], int]:\n    out: dict[tuple[int, ...], int] = {}\n    for g in ngrams:\n        out[g] = out.get(g, 0) + 1\n    return out\n\n\ndef bleu4(generated: list[int], references: list[list[int]],\n          smoothing: bool = True) -> float:\n    \"\"\"BLEU-4 against multiple reference captions.\n\n    Uses Chen and Cherry \"method 1\" smoothing when any n-gram precision is 0\n    and `smoothing` is True.\n    \"\"\"\n    if not references:\n        raise ValueError(\"bleu4 requires at least one reference\")\n    if not generated:\n        return 0.0\n\n    weights = [0.25, 0.25, 0.25, 0.25]\n    log_p_sum = 0.0\n    for n in range(1, 5):\n        gen_ngrams = _ngrams(generated, n)\n        gen_counts = _count(gen_ngrams)\n        ref_max_counts: dict[tuple[int, ...], int] = {}\n        for ref in references:\n            ref_counts = _count(_ngrams(ref, n))\n            for g, c in ref_counts.items():\n                if c > ref_max_counts.get(g, 0):\n                    ref_max_counts[g] = c\n\n        clipped = 0\n        for g, c in gen_counts.items():\n            clipped += min(c, ref_max_counts.get(g, 0))","sourceCodeStart":112,"sourceCodeEnd":148,"githubUrl":"https://github.com/rohitg00/ai-engineering-from-scratch/blob/39ea8a1c6d0b61f071226eff7ede4d4105fed820/phases/19-capstone-projects/63-multimodal-eval/code/main.py#L112-L148","documentation":"Error \"bleu4 requires at least one reference\" thrown in rohitg00/ai-engineering-from-scratch.","triggerScenarios":"Thrown at phases/19-capstone-projects/63-multimodal-eval/code/main.py:130 when the library encounters an invalid state.","commonSituations":"See trigger scenarios.","solutions":[],"exampleFix":null,"handlingStrategy":null,"validationCode":null,"typeGuard":null,"tryCatchPattern":null,"preventionTips":[],"tags":[],"backgroundTag":null,"analyzedSha":"39ea8a1c6d0b61f071226eff7ede4d4105fed820","analyzedAt":"2026-08-26T03:13:46.626Z","schemaVersion":2},"datasetVersion":"2026-08-26T07:17:17.940Z"}