"""Cheap content-sanity checks for the truncated-response continuation path.

A model in a degenerate repetition loop can spend its ENTIRE output budget
echoing one fragment; the ``finish_reason=length`` continuation path would
then stitch that fragment into the final response with a "continue" nudge
(one incident: a 60k-char turn delivered as 31 Discord messages). These
helpers detect repetition-dominated fragments BEFORE the nudge so the turn
aborts with a clear error (like the ``_thinking_exhausted`` guard).

Deliberately conservative: only LONG verbatim repeats (60+ chars) covering a
majority of the fragment trip the guard, so ordinary truncations (a sentence
cut mid-word, a repeated heading, similar code lines) are never blocked.
"""

from __future__ import annotations

import math

# Below this length the check doesn't run: short truncations trivially
# contain repeated tokens and are legitimately continued.
MIN_FRAGMENT_LENGTH = 400

# Exact-repeat window; a verbatim repeat this long is far beyond ordinary
# phrasing reuse (citations, headings, similar code).
_REPEAT_WINDOW = 60

# A window repeating at least this often is a signal even for short fragments.
_MIN_REPEAT_COUNT = 5

# "Repetition-dominated" = repeated windows cover at least this fraction.
_DOMINANCE_RATIO = 0.5


def is_repetition_dominated(text: str) -> bool:
    """True when a single 60+ char substring recurs often enough to cover at
    least half of ``text`` — the signature of a repetition loop, where
    continuing would only stitch in more repeats.

    Fail-open: False for non-string / empty / short inputs.
    """
    if not isinstance(text, str):
        return False
    n = len(text)
    if n < MIN_FRAGMENT_LENGTH:
        return False

    # Fast path: one normalized line duplicated enough to cover half the
    # fragment (the most common echo shape). Cheap, no big allocations.
    if _line_repetition_dominated(text, n):
        return True

    # General path: fixed-size windows sliding one char at a time, catching
    # loops that don't align to line boundaries. A window must appear
    # ``needed`` times to cover >= _DOMINANCE_RATIO (and >= _MIN_REPEAT_COUNT).
    window = _REPEAT_WINDOW
    needed = max(_MIN_REPEAT_COUNT, math.ceil(n * _DOMINANCE_RATIO / window))
    counts: dict[str, int] = {}
    for i in range(n - window + 1):
        key = text[i : i + window]
        c = counts.get(key, 0) + 1
        if c >= needed:
            return True
        counts[key] = c
    return False


def _line_repetition_dominated(text: str, n: int) -> bool:
    """True when a single normalized line covers half the fragment via repeats."""
    counts: dict[str, int] = {}
    for line in text.splitlines():
        norm = line.strip()
        if norm:
            counts[norm] = counts.get(norm, 0) + 1
    return any(
        c >= _MIN_REPEAT_COUNT and c * len(line) >= n * _DOMINANCE_RATIO
        for line, c in counts.items()
    )