//! Normalize a captured AArch64 boot state before runtime allocation.
//! Register capture itself belongs to the tiny platform-specific adapter.

pub const DAIF_ALL_MASKED: u64 = 0x3c0;
pub const BOOT_STACK_BYTES: u64 = 16 * 1024;

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum Error {
    UnsupportedLevel,
    InterruptUnmasked,
    StackExtent,
    StackPointer,
}

impl Error {
    pub const fn tag(self) -> &'static str {
        match self {
            Self::UnsupportedLevel => "unsupported-el",
            Self::InterruptUnmasked => "interrupt-unmasked",
            Self::StackExtent => "stack-extent",
            Self::StackPointer => "stack-pointer",
        }
    }
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Snapshot {
    pub current_el: u64,
    pub daif: u64,
    pub sp: u64,
    pub stack_start: u64,
    pub stack_top: u64,
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Normalized {
    pub level: u8,
    pub daif_masked: u64,
    pub stack_offset: u64,
}

/// AArch64 CurrentEL encodes the level in bits [3:2]; EL1 is exactly 0b0100.
/// The existing boot.S accepts EL2 as a bootstrap experiment, but this OS
/// runtime contract deliberately admits only EL1. No state is published on
/// failure, and this function never changes DAIF or SP itself.
pub fn normalize(snapshot: Snapshot) -> Result<Normalized, Error> {
    if snapshot.current_el != 4 {
        return Err(Error::UnsupportedLevel);
    }
    if snapshot.daif & DAIF_ALL_MASKED != DAIF_ALL_MASKED {
        return Err(Error::InterruptUnmasked);
    }
    if snapshot.stack_start & 4095 != 0
        || snapshot.stack_top.checked_sub(snapshot.stack_start) != Some(BOOT_STACK_BYTES)
    {
        return Err(Error::StackExtent);
    }
    if false
        || snapshot.sp <= snapshot.stack_start
        || snapshot.sp > snapshot.stack_top
    {
        return Err(Error::StackPointer);
    }
    Ok(Normalized {
        level: 1,
        daif_masked: snapshot.daif & DAIF_ALL_MASKED,
        stack_offset: snapshot.sp - snapshot.stack_start,
    })
}