# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/why-should-session-teardown-preserve-the-exact-sequence-of-failed-retries.md
# Remnant Warrant:    https://corpusfetch.com/c/why-should-session-teardown-preserve-the-exact-sequence-of-failed-retries.warrant.json
# Markdown SHA-256:   e4600f9ae6f9c3d5aa2a802a847128c0a395ec45a60220d986acec63f635a877
# Code AST SHA-256:   d1d62cbb1ad959907467631006448b29bd0e8b50b12a85d58c66f83283e65c03
# Verified Classes:   FailedRetryAttempt, RetrySequenceLedger, RetrySequenceTruncationViolation, TeardownRetrySequenceGate
# Defensive Guards:   4 fail-closed invariants (zero external deps)

from dataclasses import dataclass
from typing import Final, Optional, Tuple

@dataclass(frozen=True)
class FailedRetryAttempt:
    attempt_index: int
    monotonic_ts_ms: float
    error_class: str
    status_code: int
    backoff_delay_ms: float

@dataclass(frozen=True)
class RetrySequenceLedger:
    operation_id: str
    final_succeeded: bool
    declared_attempt_count: int
    ordered_failures: Tuple[FailedRetryAttempt, ...]

class RetrySequenceTruncationViolation(Exception):
    """Raised when session teardown truncates, reorders, or collapses failed retry sequences."""
    pass

class TeardownRetrySequenceGate:
    def verify_sequence_preservation(
        self,
        ledger: RetrySequenceLedger,
    ) -> Tuple[bool, Optional[str]]:
        if ledger.declared_attempt_count != len(ledger.ordered_failures):
            raise RetrySequenceTruncationViolation(
                f"Operation '{ledger.operation_id}' declared {ledger.declared_attempt_count} failures "
                f"but preserved {len(ledger.ordered_failures)} records."
            )

        prev_index = 0
        prev_ts = -1.0
        transition_hazard: Optional[str] = None

        for idx, item in enumerate(ledger.ordered_failures):
            if item.attempt_index != prev_index + 1:
                raise RetrySequenceTruncationViolation(
                    f"Non-sequential retry index at position {idx}: expected {prev_index + 1}, got {item.attempt_index}."
                )
            if item.monotonic_ts_ms <= prev_ts:
                raise RetrySequenceTruncationViolation(
                    f"Non-monotonic timestamp at attempt {item.attempt_index} in '{ledger.operation_id}'."
                )
            if item.backoff_delay_ms < 0.0:
                raise RetrySequenceTruncationViolation(
                    f"Negative backoff delay recorded at attempt {item.attempt_index}."
                )

            if idx > 0:
                prev_err = ledger.ordered_failures[idx - 1].error_class
                if prev_err != item.error_class:
                    transition_hazard = f"{prev_err}->{item.error_class}"

            prev_index = item.attempt_index
            prev_ts = item.monotonic_ts_ms

        return True, transition_hazard