// Good: Check bounds first
if data.len() < size_of::<u64>() {
    return Err(ProgramError::InvalidInstructionData);
}
let value = u64::from_le_bytes(data[0..8].try_into().unwrap());

// Bad: No bounds checking - could panic or cause UB
let value = u64::from_le_bytes(data[0..8].try_into().unwrap());

#[repr(C)]
struct BadOrder {
    small: u8,    // 1 byte
    // padding: [u8; 7] since `big` needs to be aligned at 8 bytes.
    big: u64,     // 8 bytes  
    medium: u16,  // 2 bytes
    // padding: [u8; 6] since the struct size needs to be aligned to 8 bytes.
}

#[repr(C)]
struct GoodOrder {
    big: u64,     // 8 bytes
    medium: u16,  // 2 bytes  
    small: u8,    // 1 byte
    // padding: [u8; 5] since the struct size needs to be aligned to 8 bytes.
}

#[repr(C)]
struct ByteArrayStruct {
    big: [u8; 8],    // represents u64
    medium: [u8; 2], // represents u16  
    small: u8,
}

pub struct DepositInstructionData {
    pub amount: u64,
    pub recipient: Pubkey,
}

impl<'a> TryFrom<&'a [u8]> for DepositInstructionData {
    type Error = ProgramError;

    fn try_from(data: &'a [u8]) -> Result<Self, Self::Error> {
        if data.len() < (size_of::<u64>() + size_of::<Pubkey>()) {
            return Err(ProgramError::InvalidInstructionData);
        }

        // No alignment issues: we're reading bytes and converting
        let amount = u64::from_le_bytes(
            data[0..8].try_into()
                .map_err(|_| ProgramError::InvalidInstructionData)?
        );
        
        let recipient = Pubkey::try_from(&data[8..40])
            .map_err(|_| ProgramError::InvalidInstructionData)?;

        Ok(Self { amount, recipient })
    }
}

#[repr(C)]
pub struct Config {
    pub authority: Pubkey,
    pub mint_x: Pubkey, 
    pub mint_y: Pubkey,
    pub seed: u64,        // This field requires 8-byte alignment
    pub fee: u16,         // This field requires 2-byte alignment  
    pub state: u8,
    pub config_bump: u8,
}

impl Config {
    pub const LEN: usize = size_of::<Self>();
    
    pub fn from_bytes(data: &[u8]) -> Result<&Self, ProgramError> {
        if data.len() != Self::LEN {
            return Err(ProgramError::InvalidAccountData);
        }

        // Critical: Check alignment for the most restrictive field (u64 in this case)
        if (data.as_ptr() as usize) % core::mem::align_of::<Self>() != 0 {
            return Err(ProgramError::InvalidAccountData);
        }

        // SAFETY: We've verified length and alignment
        Ok(unsafe { &*(data.as_ptr() as *const Self) })
    }
}

// Alternative: Avoid alignment issues entirely by using byte arrays for types with
// alignment requirement greater than 1 and provide accessor methods
#[repr(C)]
pub struct ConfigSafe {
    pub authority: Pubkey,
    pub mint_x: Pubkey, 
    pub mint_y: Pubkey,
    seed: [u8; 8],      // Convert with u64::from_le_bytes when needed
    fee: [u8; 2],       // Convert with u16::from_le_bytes when needed
    pub state: u8,
    pub config_bump: u8,
}

impl ConfigSafe {
    pub fn from_bytes(data: &[u8]) -> Result<&Self, ProgramError> {
        if data.len() != size_of::<Self>() {
            return Err(ProgramError::InvalidAccountData);
        }

        // SAFETY: No alignment check needed - everything is u8 aligned
        Ok(unsafe { &*(data.as_ptr() as *const Self) })
    }
    
    pub fn seed(&self) -> u64 {
        u64::from_le_bytes(self.seed)
    }
    
    pub fn fee(&self) -> u16 {
        u16::from_le_bytes(self.fee)
    }
}

#[repr(C, packed)]  // This can cause unaligned fields!
pub struct PackedConfig {
    pub state: u8,
    pub seed: u64,    // This u64 might not be 8-byte aligned due to packing
}

impl PackedConfig {
    pub fn seed(&self) -> u64 {
        self.seed  // Safe: Direct value copy, no reference created
    }
}

// Usage:
let config = PackedConfig::load(account)?;

// ❌ UNDEFINED BEHAVIOR: Creates a reference to potentially unaligned field
let seed_ref = &config.seed; // Compiler must create a reference here!

// ❌ UNDEFINED BEHAVIOR: Even this can be problematic
let seed_value = config.seed; // May create temporary reference internally

// ✅ SAFE: Accessor method reads value without creating reference
let seed_value = config.seed(); // No intermediate reference

pub struct StateAccount {
    pub is_active: bool,
    pub state_type: StateType,
    pub data: [u8; 32],
}

#[repr(u8)]
pub enum StateType {
    Inactive = 0,
    Active = 1,
    Paused = 2,
}

impl StateAccount {
    pub fn from_bytes(data: &[u8]) -> Result<Self, ProgramError> {
        if data.len() < size_of::<Self>() {
            return Err(ProgramError::InvalidAccountData);
        }

        // Safely handle bool (only 0 or 1 are valid)
        let is_active = match data[0] {
            0 => false,
            1 => true,
            _ => return Err(ProgramError::InvalidAccountData),
        };

        // Safely handle enum
        let state_type = match data[1] {
            0 => StateType::Inactive,
            1 => StateType::Active, 
            2 => StateType::Paused,
            _ => return Err(ProgramError::InvalidAccountData),
        };

        let mut data_array = [0u8; 32];
        data_array.copy_from_slice(&data[2..34]);

        Ok(Self {
            is_active,
            state_type,
            data: data_array,
        })
    }
}

// ❌ UNDEFINED BEHAVIOR: transmute requires proper alignment
let value: u64 = unsafe { core::mem::transmute(bytes_slice) };

#[repr(C, packed)]
pub struct PackedConfig {
    pub state: u8,
    pub seed: u64,     // This u64 is only 1-byte aligned!
    pub authority: Pubkey,
}

// ❌ UNDEFINED BEHAVIOR: Creates references to unaligned fields
let config = unsafe { &*(data.as_ptr() as *const PackedConfig) };
let seed_value = config.seed; // UB: May create reference to unaligned u64

#[repr(C, packed)]
pub struct PackedStruct {
    pub a: u8,
    pub b: u64,
}

let packed = /* ... */;
// ❌ UNDEFINED BEHAVIOR: Creates reference to unaligned field
let b_ref = &packed.b;
// ❌ UNDEFINED BEHAVIOR: May create temporary reference
let b_value = packed.b;

// ❌ UNDEFINED BEHAVIOR: No alignment check
let config = unsafe { &*(data.as_ptr() as *const Config) };

// ❌ WRONG: read_unaligned needs proper layout, not just size
#[repr(Rust)]  // Default layout - not guaranteed!
pub struct BadStruct {
    pub field: u64,
}

let value = unsafe { (data.as_ptr() as *const BadStruct).read_unaligned() };

impl Config {
    pub fn write_to_buffer(&self, data: &mut [u8]) -> Result<(), ProgramError> {
        if data.len() != Self::LEN {
            return Err(ProgramError::InvalidAccountData);
        }

        let mut offset = 0;
        
        // Write authority
        data[offset..offset + 32].copy_from_slice(self.authority.as_ref());
        offset += 32;
        
        // Write mint_x  
        data[offset..offset + 32].copy_from_slice(self.mint_x.as_ref());
        offset += 32;
        
        // Write mint_y
        data[offset..offset + 32].copy_from_slice(self.mint_y.as_ref());
        offset += 32;
        
        // Write seed
        data[offset..offset + 8].copy_from_slice(&self.seed.to_le_bytes());
        offset += 8;
        
        // Write fee
        data[offset..offset + 2].copy_from_slice(&self.fee.to_le_bytes());
        offset += 2;
        
        // Write state
        data[offset] = self.state;
        offset += 1;
        
        // Write config_bump
        data[offset] = self.config_bump;

        Ok(())
    }
}

impl Config {
    pub fn from_bytes_mut(data: &mut [u8]) -> Result<&mut Self, ProgramError> {
        if data.len() != Self::LEN {
            return Err(ProgramError::InvalidAccountData);
        }

        // Check alignment
        if (data.as_ptr() as usize) % core::mem::align_of::<Self>() != 0 {
            return Err(ProgramError::InvalidAccountData);
        }

        // SAFETY: We've verified length and alignment
        Ok(unsafe { &mut *(data.as_mut_ptr() as *mut Self) })
    }
}

#[repr(C)]
pub struct ConfigSafe {
    pub authority: Pubkey,
    pub mint_x: Pubkey, 
    pub mint_y: Pubkey,
    seed: [u8; 8],
    fee: [u8; 2],
    pub state: u8,
    pub config_bump: u8,
}

impl ConfigSafe {
    pub fn from_bytes_mut(data: &mut [u8]) -> Result<&mut Self, ProgramError> {
        if data.len() != size_of::<Self>() {
            return Err(ProgramError::InvalidAccountData);
        }

        // No alignment check needed - everything is u8 aligned
        Ok(unsafe { &mut *(data.as_mut_ptr() as *mut Self) })
    }
    
    pub fn seed(&self) -> u64 {
        u64::from_le_bytes(self.seed)
    }
    
    pub fn fee(&self) -> u16 {
        u16::from_le_bytes(self.fee)
    }

    // Setters that handle endianness correctly

    pub fn set_seed(&mut self, seed: u64) {
        self.seed = seed.to_le_bytes();
    }
    
    pub fn set_fee(&mut self, fee: u16) {
        self.fee = fee.to_le_bytes();
    }
}

#[repr(C)]
pub struct DynamicAccount {
    pub fixed_data: [u8; 32],
    pub counter: u64,
    // Dynamic data follows after the struct in memory
    // Layout: [fixed_data][counter][dynamic_data...]
}

impl DynamicAccount {
    pub const FIXED_SIZE: usize = size_of::<Self>();
    
    /// Safely parse account with dynamic data
    pub fn from_bytes_with_dynamic(data: &[u8]) -> Result<(&Self, &[u8]), ProgramError> {
        if data.len() < Self::FIXED_SIZE {
            return Err(ProgramError::InvalidAccountData);
        }

        // SAFETY: We've verified the buffer is large enough for the fixed part
        // The fixed part only contains [u8; 32] and u64, which have predictable layout
        let fixed_part = unsafe { &*(data.as_ptr() as *const Self) };
        
        // Everything after the fixed part is dynamic data
        let dynamic_part = &data[Self::FIXED_SIZE..];
        
        Ok((fixed_part, dynamic_part))
    }
    
    /// Get mutable references to both parts
    pub fn from_bytes_mut_with_dynamic(data: &mut [u8]) -> Result<(&mut Self, &mut [u8]), ProgramError> {
        if data.len() < Self::FIXED_SIZE {
            return Err(ProgramError::InvalidAccountData);
        }

        // Split the buffer to avoid borrowing issues
        let (fixed_bytes, dynamic_bytes) = data.split_at_mut(Self::FIXED_SIZE);
        
        // SAFETY: We've verified the size and split safely
        let fixed_part = unsafe { &mut *(fixed_bytes.as_mut_ptr() as *mut Self) };
        
        Ok((fixed_part, dynamic_bytes))
    }
}

/// Writing single dynamic field
impl DynamicAccount {
    pub fn write_with_dynamic(
        data: &mut [u8], 
        fixed_data: &[u8; 32], 
        counter: u64,
        dynamic_data: &[u8]
    ) -> Result<(), ProgramError> {
        let total_size = Self::FIXED_SIZE + dynamic_data.len();
        
        if data.len() != total_size {
            return Err(ProgramError::InvalidAccountData);
        }
        
        // Write fixed part field by field (safest approach)
        data[0..32].copy_from_slice(fixed_data);
        data[32..40].copy_from_slice(&counter.to_le_bytes());
        
        // Write dynamic part
        data[Self::FIXED_SIZE..].copy_from_slice(dynamic_data);
        
        Ok(())
    }
    
    /// Update just the dynamic portion
    pub fn update_dynamic_data(&mut self, account_data: &mut [u8], new_data: &[u8]) -> Result<(), ProgramError> {
        if account_data.len() < Self::FIXED_SIZE + new_data.len() {
            return Err(ProgramError::InvalidAccountData);
        }
        
        // Write new dynamic data
        account_data[Self::FIXED_SIZE..Self::FIXED_SIZE + new_data.len()].copy_from_slice(new_data);
        
        Ok(())
    }
}

const DYNAMIC_DATA_START_OFFSET: usize = size_of::<[u8; 32]>();

#[repr(C)]
pub struct DynamicallySizedAccount {
    pub sized_data: [u8; 32],
    // pub dynamic_data: &'info [u8], // Not part of the struct, but follows in the buffer
}

impl DynamicallySizedAccount {
    /// Returns the length of the dynamic data section.
    #[inline(always)]
    pub fn get_dynamic_data_len(data: &[u8]) -> Result<usize, ProgramError> {
        if data.len().le(&DYNAMIC_DATA_START_OFFSET) {
            return Err(ProgramError::InvalidAccountData);
        }

        Ok(data.len() - DYNAMIC_DATA_START_OFFSET)
    }

    /// Returns a slice of the dynamic data.
    #[inline(always)]
    pub fn read_dynamic_data(data: &[u8]) -> Result<&[u8], ProgramError> {
        if data.len().le(&DYNAMIC_DATA_START_OFFSET) {
            return Err(ProgramError::InvalidAccountData);
        }

        Ok(&data[DYNAMIC_DATA_START_OFFSET..])
    }
}

#[repr(C)]  
pub struct MultiDynamicAccount {
    pub fixed_data: [u8; 32],
    pub timestamp: u64,
    // Layout: [fixed_data][timestamp][len1: u8][data1][data2: remainder]
}

impl MultiDynamicAccount {
    pub const FIXED_SIZE: usize = size_of::<Self>();
    pub const LEN_PREFIX_SIZE: usize = size_of::<u8>();
    pub const MIN_SIZE: usize = Self::FIXED_SIZE + Self::LEN_PREFIX_SIZE;
    
    /// Parse account with two dynamic sections
    pub fn parse_dynamic_fields(data: &[u8]) -> Result<(&[u8; 32], u64, &[u8], &[u8]), ProgramError> {
        if data.len() < Self::MIN_SIZE {
            return Err(ProgramError::InvalidAccountData);
        }
        
        // Extract fixed data safely
        let fixed_data = data[..32].try_into()
            .map_err(|_| ProgramError::InvalidAccountData)?;
            
        let timestamp = u64::from_le_bytes(
            data[32..40].try_into()
                .map_err(|_| ProgramError::InvalidAccountData)?
        );
            
        // Read length of first dynamic field (single byte)
        let len = data[Self::FIXED_SIZE] as usize;
        
        // Validate we have enough data
        if data.len() <  Self::MIN_SIZE + len {
            return Err(ProgramError::InvalidAccountData);
        }
        
        let data_1 = &data[Self::MIN_SIZE..Self::MIN_SIZE + len];
        let data_2 = &data[Self::MIN_SIZE + len..]; // Remainder
        
        Ok((fixed_data, timestamp, data_1, data_2))
    }
    
    /// Write account with two dynamic sections
    pub fn write_with_multiple_dynamic(
        buffer: &mut [u8],
        fixed_data: &[u8; 32],
        timestamp: u64,
        data_1: &[u8],
        data_2: &[u8]
    ) -> Result<(), ProgramError> {
        let total_size = Self::MIN_SIZE + data_1.len() + data_2.len();
        
        if buffer.len() != total_size {
            return Err(ProgramError::InvalidAccountData);
        }
        
        // Validate data_1 length fits in u8
        if data_1.len() > u8::MAX as usize {
            return Err(ProgramError::InvalidInstructionData);
        }
        
        let mut offset = 0;
        
        // Write fixed data
        buffer[offset..offset + 32].copy_from_slice(fixed_data);
        offset += 32;
        
        buffer[offset..offset + 8].copy_from_slice(&timestamp.to_le_bytes());
        offset += 8;
        
        // Write length prefix for data1 (single byte)
        buffer[offset] = data_1.len() as u8;
        offset += 1;
        
        // Write data1
        buffer[offset..offset + data_1.len()].copy_from_slice(data_1);
        offset += data_1.len();
        
        // Write data2 (remainder - no length prefix needed)
        buffer[offset..].copy_from_slice(data_2);
        
        Ok(())
    }
}

pub fn resize_account(
    account: &AccountInfo,
    payer: &AccountInfo,
    new_size: usize,
    zero_out: bool,
) -> ProgramResult {
    // If the account is already the correct size, return early
    if new_size == account.data_len() {
        return Ok(());
    }

    // Calculate rent requirements
    let rent = Rent::get()?;
    let new_minimum_balance = rent.minimum_balance(new_size);

    // Adjust lamports to meet rent-exemption requirements
    match new_minimum_balance.cmp(&account.lamports()) {
        core::cmp::Ordering::Greater => {
            // Need more lamports for rent exemption
            let lamports_diff = new_minimum_balance.saturating_sub(account.lamports());
            **payer.try_borrow_mut_lamports()? -= lamports_diff;
            **account.try_borrow_mut_lamports()? += lamports_diff;
        }
        core::cmp::Ordering::Less => {
            // Return excess lamports to payer
            let lamports_diff = account.lamports().saturating_sub(new_minimum_balance);
            **account.try_borrow_mut_lamports()? -= lamports_diff;
            **payer.try_borrow_mut_lamports()? += lamports_diff;
        }
        core::cmp::Ordering::Equal => {
            // No lamport transfer needed
        }
    }

    // Reallocate the account
    account.resize(new_size)?;

    Ok(())
}