expry 0.4.0

use crate::raw_utils::EncodingError;

pub use crate::rewind;
pub use crate::clear;
pub use crate::pool;

use core::mem::*;
use core::ops::*;

// See also https://manishearth.github.io/blog/2021/03/15/arenas-in-rust/
// See also https://doc.rust-lang.org/std/ptr/fn.write.html
// See also https://github.com/rust-lang/rust/issues/27779 (about absence of placement new)
// mem::align_of::<T>() ???
// https://doc.rust-lang.org/nomicon/vec/vec-alloc.html

pub fn from_hex(c: char) -> Result<u8,EncodingError> {
    if c.is_ascii_digit() {
        return Ok(c as u8 - b'0');
    } else if matches!(c, 'a'..='f') {
        return Ok(c as u8 - b'a' + 10);
    } else if matches!(c, 'A'..='F') {
        return Ok(c as u8 - b'A' + 10);
    }
    Err(EncodingError{line_nr: line!() })
}
pub fn from_hex_u8(c: u8) -> Result<u8,EncodingError> {
    if c.is_ascii_digit() {
        return Ok(c - b'0');
    } else if (b'a'..=b'f').contains(&c) {
        return Ok(c - b'a' + 10);
    } else if (b'A'..=b'F').contains(&c) {
        return Ok(c - b'A' + 10);
    }
    Err(EncodingError{line_nr: line!() })
}

pub fn to_hex(s: &[u8]) -> String {
    let mut item = String::with_capacity(s.len()*2);
    for b in s {
        item.push(b"0123456789abcdef"[(b >> 4) as usize] as char);
        item.push(b"0123456789abcdef"[(b & 0xF) as usize] as char);
    }
    item
}

/// Escaping for inside the `script` HTML tag.
pub fn html_escape_outside_attribute_u8(c: u8) -> Option<&'static [u8]> {
    match c {
        b'&' => Some(b"&amp;"),
        b'<' => Some(b"&lt;"),
        b'>' => Some(b"&gt;"),
        _ => None,
    }
}
pub fn html_escape_inside_attribute_u8(c: u8) -> Option<&'static [u8]> {
    match c {
        b'&' => Some(b"&amp;"),
        b'<' => Some(b"&lt;"),
        b'>' => Some(b"&gt;"),
        b'"' => Some(b"&quot;"),
        b'\'' => Some(b"&#39;"),
        _ => None,
    }
}
/// Escapes a whole URL so only valid URL chars are inside the result. Not to be confused if you
/// want to encode a random string as part of a URL (e.g. in a form), use [`url_encode_u8`] for
/// that.
pub fn url_escape_u8(c: u8, buffer: &mut Vec<u8>) -> Option<&[u8]> {
    match c {
        b'-' |
        b'~' |
        b'.' |
        b'/' |
        b',' |
        b'=' |
        b'&' |
        b':' |
        b'?' |
        b'#' | // for anchors links in html: #name
        b'_' => return None,
        b' ' => return Some(b"+"),
        c if c.is_ascii_lowercase() ||
             c.is_ascii_uppercase() ||
             c.is_ascii_digit() => return None,
        _ => {},
    };
    buffer.clear();
    buffer.push(b'%');
    const CHARS : &[u8; 16] = b"0123456789ABCDEF";
    buffer.push(CHARS[(c >> 4) as usize]);
    buffer.push(CHARS[(c & 0xF) as usize]);
    Some(buffer)
}
/// Encodes a part of a URL so it can be used in any part of a URL. Not to be confused if you
/// want to escape a whole URL (e.g. the string `http://example.org`), use [`url_escape_u8`] for
/// that.
pub fn url_encode_u8(c: u8, buffer: &mut Vec<u8>) -> Option<&[u8]> {
    match c {
        // RFC 3986 section 2.2 Reserved Characters
        b'-' |
        b'~' |
        b'.' |
        b',' |
        b':' |
        b'_' => return None,
        b' ' => return Some(b"+"),
        c if c.is_ascii_lowercase() ||
             c.is_ascii_uppercase() ||
             c.is_ascii_digit() => return None,
        _ => {},
    };
    buffer.clear();
    buffer.push(b'%');
    const CHARS : &[u8; 16] = b"0123456789ABCDEF";
    buffer.push(CHARS[(c >> 4) as usize]);
    buffer.push(CHARS[(c & 0xF) as usize]);
    Some(buffer)
}

fn from_digit(v: u8) -> u8 {
    if (b'A'..=b'F').contains(&v) {
        10 + v - b'A'
    } else if (b'a'..=b'f').contains(&v) {
        10 + v - b'a'
    } else if v.is_ascii_digit() {
        v - b'0'
    } else {
        0
    }
}

pub fn url_unescape_u8(input: &[u8]) -> alloc::borrow::Cow<'_, [u8]> {
    for (mut i,b) in input.iter().enumerate() {
        if *b == b'+' || *b == b'%' {
            let mut output = Vec::new();
            output.extend_from_slice(&input[0..i]);
            while i < input.len() {
                // convert '+' to a space
                if input[i] == b'+' {
                    output.push(b' ');
                // covert something with % to their real value
                } else if input[i] == b'%' && i + 2 < input.len() {
                    output.push(from_digit(input[i+1]) << 4 | from_digit(input[i+2]));
                    i += 2;
                } else {
                    output.push(input[i]);
                }
                i += 1;
            }
            return std::borrow::Cow::Owned(output);
        }
    }
    std::borrow::Cow::Borrowed(input)
}

pub fn url_unescape_u8_to_scope<'c>(input: &[u8], scope: &mut MemoryScope<'c>) -> &'c [u8] {
    let mut output = ScopedArrayBuilder::new(scope);
    let mut i = 0;
    while i < input.len() {
        // convert '+' to a space
        if input[i] == b'+' {
            output.push(b' ');
        // covert something with % to their real value
        } else if input[i] == b'%' && i + 2 < input.len() {
            output.push(from_digit(input[i+1]) << 4 | from_digit(input[i+2]));
            i += 2;
        } else {
            output.push(input[i]);
        }
        i += 1;
    }
    output.build()
}

pub type ReplaceFn = fn (u8, &mut Vec<u8>) -> Option<&[u8]>;

struct MemoryPage {
    available: usize,
    size: usize,
    ptr: *mut u8,
}

fn align_offset<T>(ptr: * mut T, align: usize) -> usize {
    let extra = (ptr as usize) & (align-1);
    if extra > 0 {
        align - extra
    } else {
        0
    }
}

impl MemoryPage {
    pub fn new(bytes: usize) -> Self {
        unsafe {
            let layout = std::alloc::Layout::from_size_align_unchecked(bytes, 16);
            let ptr = std::alloc::alloc(layout);
            Self {
                available: bytes,
                size: bytes,
                ptr,
            }
        }
    }

    fn alloc<T>(&mut self, count: usize) -> *mut T {
        let length = count*core::mem::size_of::<T>();
        debug_assert!(length <= self.available);
        unsafe {
            let ptr = self.ptr.add(self.size - self.available) as * mut T;
            let offset = align_offset(ptr, core::mem::align_of::<T>());
            let retval = (ptr as *mut u8).add(offset) as *mut T;
            self.available -= length + offset;
            retval
        }
    }
}

impl Drop for MemoryPage {
    fn drop(&mut self) {
        unsafe {
            let layout = std::alloc::Layout::from_size_align_unchecked(self.size, 16);
            std::alloc::dealloc(self.ptr, layout);
        }
    }
}

type Delayed = (unsafe fn(*mut ()), *mut ());

pub struct MemoryPool {
    content: Vec<MemoryPage>, // can have different size then DEFAULT_MEMORY_PAGE_SIZE
    next: Vec<MemoryPage>,    // have all the same DEFAULT_MEMORY_PAGE_SIZE
    // FIXME: store in the memory page, as a linked list to avoid extra allocations
    delay_execution: Vec<Delayed>,
}

impl MemoryPool {
    #[must_use]
    pub fn _rewind<'c,'d>(self: &'d mut &'c mut Self) -> MemoryScope<'d> where 'c: 'd {
        //println!("MemoryPool::scope");
        let len_content = self.content.len();
        let len_last_available = if let Some(last) = self.content.last() { last.available } else { 0 };
        let len_delay_execution = self.delay_execution.len();
        MemoryScope {
            pool: self,
            len_content,
            len_last_available,
            len_delay_execution,
            clear: false,
        }
    }
    #[must_use]
    pub fn _clear<'c,'d>(self: &'d mut &'c mut Self) -> MemoryScope<'d> where 'c: 'd {
        //println!("MemoryPool::scope");
        let len_content = self.content.len();
        let len_last_available = if let Some(last) = self.content.last() { last.available } else { 0 };
        let len_delay_execution = self.delay_execution.len();
        MemoryScope {
            pool: self,
            len_content,
            len_last_available,
            len_delay_execution,
            clear: true,
        }
    }
    pub fn shrink(&mut self) {
        self.next.clear();
        self.next.shrink_to_fit();
        self.content.shrink_to_fit();
        self.delay_execution.shrink_to_fit();
    }
    pub fn new() -> Self {
        Self {
            content: Vec::with_capacity(0),
            next: Vec::with_capacity(0),
            delay_execution: Vec::new(),
        }
    }
}

impl Default for MemoryPool {
    fn default() -> Self {
        Self::new()
    }
}

pub struct MemoryScope<'c> {
    pool: &'c mut MemoryPool,
    len_content: usize,
    len_last_available: usize,
    len_delay_execution: usize,
    clear: bool,
}

impl<'c> MemoryScope<'c> {
    const DEFAULT_MEMORY_PAGE_SIZE: usize = 16384;

    // make a new scope with a new lifetime
    #[must_use]
    pub fn _rewind<'d>(self: &'d mut &mut Self) -> MemoryScope<'d> where 'c: 'd {
        //println!("scope::scope");
        let len_content = self.pool.content.len();
        let len_last_available = if let Some(last) = self.pool.content.last() { last.available } else { 0 };
        let len_delay_execution = self.pool.delay_execution.len();
        MemoryScope {
            pool: self.pool,
            len_content,
            len_last_available,
            len_delay_execution,
            clear: false,
        }
    }

    // make a new scope with a new lifetime
    #[must_use]
    pub fn _clear<'d>(self: &'d mut &mut Self) -> MemoryScope<'d> where 'c: 'd {
        //println!("scope::scope");
        let len_content = self.pool.content.len();
        let len_last_available = if let Some(last) = self.pool.content.last() { last.available } else { 0 };
        let len_delay_execution = self.pool.delay_execution.len();
        MemoryScope {
            pool: self.pool,
            len_content,
            len_last_available,
            len_delay_execution,
            clear: true,
        }
    }

    fn available<T>(&self) -> usize {
        if let Some(last) = self.pool.content.last() {
            let ptr = unsafe { last.ptr.add(last.size - last.available) as * mut T };
            let offset = align_offset(ptr, core::mem::align_of::<T>());
            (last.available-offset)/core::mem::size_of::<T>()
        } else {
            0
        }
    }

    fn add_page(&mut self, length: usize) {
        let length = ceil_to_power_of_two(length);
        if length <= Self::DEFAULT_MEMORY_PAGE_SIZE {
            if let Some(page) = self.pool.next.pop() {
                debug_assert!(page.size == Self::DEFAULT_MEMORY_PAGE_SIZE);
                self.pool.content.push(page);
            } else {
                let page = MemoryPage::new(Self::DEFAULT_MEMORY_PAGE_SIZE);
                self.pool.content.push(page);
            }
        } else {
            let page = MemoryPage::new(length);
            self.pool.content.push(page);
        }
    }

    fn alloc_ptr<T>(&mut self, count: usize) -> * mut T {
        self._alloc_ptr(count, core::mem::size_of::<T>(), core::mem::align_of::<T>()) as * mut T
    }
    fn _alloc_ptr(&mut self, count: usize, size: usize, align: usize) -> * mut u8 {
        let length = std::cmp::max(1,count)*size;
        let at = self.pool.content.len();
        if at > 0 {
            let page = &mut self.pool.content[at-1];
            let ptr = unsafe { page.ptr.add(page.size - page.available) };
            let offset = align_offset(ptr, align);
            if page.available >= offset+length {
                page.available -= offset+length;
                let retval = unsafe { ptr.add(offset) };
                unsafe {
                    debug_assert!(retval.add(length) <= page.ptr.add(page.size));
                }
                return retval;
            }
        }
        self.add_page(length);
        debug_assert!(at < self.pool.content.len());
        let page = &mut self.pool.content[at];
        debug_assert!(page.available >= length);
        let ptr = page.ptr;
        let offset = align_offset(ptr, align);
        page.available -= offset+length;
        let retval = unsafe { ptr.add(offset) };
        unsafe {
            debug_assert!(retval.add(length) <= page.ptr.add(page.size));
        }
        retval
    }
    // Allocates the remaining memory in the last page. If there is not enough memory, a new page is added.
    // Make excess memory (unused) data available again with unused()
    // Returns a pointer and the number of T elements that can be stored there (which is >= `count`).
    fn alloc_at_least<T>(&mut self, count: usize) -> (*mut T, usize) {
        let length = count*core::mem::size_of::<T>();
        if let Some(page) = self.pool.content.last_mut() {
            let ptr = unsafe { page.ptr.add(page.size - page.available) as * mut T };
            let offset = align_offset(ptr, core::mem::align_of::<T>());
            if page.available >= offset+length {
                page.available -= offset;
                let extra = page.available/core::mem::size_of::<T>();
                page.available -= extra*core::mem::size_of::<T>();
                return unsafe { ((ptr as * mut u8).add(offset) as * mut T, extra) };
            }
        }
        // should never return 0, because slice::from_raw_parts requires non-null (even for zero length)
        self.add_page(length);
        self.alloc_at_least(count)
    }

    // Internal function to return some previous reserved memory back to the memory pool.
    unsafe fn unused<T>(&mut self, count: usize) {
        if let Some(last) = self.pool.content.last_mut() {
            last.available += count * core::mem::size_of::<T>();
        }
    }

    pub fn alloc<'b, T>(&mut self, count: usize) -> &'b mut [MaybeUninit<T>] where 'c: 'b {
        unsafe {
            let retval : *mut MaybeUninit<T> = self.alloc_ptr::<MaybeUninit<T>>(count);
            core::slice::from_raw_parts_mut(retval, count)
        }
    }
    // copied from MaybeUninit::slice_assume_init_mut (which is unstable as for now), but without const
    /// # Safety
    /// Same safety as [`MaybeUninit::slice_assume_init_mut`].
    pub(crate) unsafe fn slice_assume_init_mut<T>(slice: &mut [MaybeUninit<T>]) -> &mut [T] {
        // SAFETY: similar to safety notes for `slice_get_ref`, but we have a
        // mutable reference which is also guaranteed to be valid for writes.
        unsafe { &mut *(slice as *mut [MaybeUninit<T>] as *mut [T]) }
    }

    pub fn copy_u8<'b>(&mut self, bytes: &'_ [u8]) -> &'b mut [u8] where 'c: 'b {
        let length = bytes.len();
        unsafe {
            let retval : *mut u8 = self.alloc_ptr(length);
            std::ptr::copy(bytes.as_ptr(), retval, length);
            core::slice::from_raw_parts_mut(retval, length)
        }
    }

    pub fn concat_u8<'b>(&mut self, chunks: &[&[u8]]) -> &'b mut [u8] where 'c: 'b {
        // FIXME: maybe add optimisation for if chunks[0] is last entry in memory pool, to reuse that
        let length = chunks.iter().fold(0, |x,y| x + y.len());
        unsafe {
            let retval : *mut u8 = self.alloc_ptr(length);
            let mut current = retval;
            for c in chunks {
                std::ptr::copy(c.as_ptr(), current, c.len());
                current = current.add(c.len());
            }
            core::slice::from_raw_parts_mut(retval, length)
        }
    }

    pub fn join_str<'b>(&mut self, chunks: &[&str], glue: &str) -> &'b mut str where 'c: 'b {
        // FIXME: maybe add optimisation for if chunks[0] is last entry in memory pool, to reuse that
        let mut length = chunks.iter().fold(0, |x,y| x + y.len());
        length += (chunks.len()-1) * glue.len();
        unsafe {
            let retval : *mut u8 = self.alloc_ptr(length);
            let mut current = retval;
            for (i,c) in chunks.iter().enumerate() {
                if i != 0 {
                    std::ptr::copy(glue.as_ptr(), current, glue.len());
                    current = current.add(glue.len());
                }
                std::ptr::copy(c.as_ptr(), current, c.len());
                current = current.add(c.len());
            }
            core::str::from_utf8_unchecked_mut(core::slice::from_raw_parts_mut(retval, length))
        }
    }

    pub fn concat_str<'b>(&mut self, chunks: &[&str]) -> &'b mut str where 'c: 'b {
        // FIXME: maybe add optimisation for if chunks[0] is last entry in memory pool, to reuse that
        let length = chunks.iter().fold(0, |x,y| x + y.len());
        unsafe {
            let retval : *mut u8 = self.alloc_ptr(length);
            let mut current = retval;
            for c in chunks {
                std::ptr::copy(c.as_ptr(), current, c.len());
                current = current.add(c.len());
            }
            core::str::from_utf8_unchecked_mut(core::slice::from_raw_parts_mut(retval, length))
        }
    }

    pub fn copy_str<'b>(&mut self, str: &'_ str) -> &'b mut str where 'c: 'b {
        let retval = self.copy_u8(str.as_bytes());
        unsafe {
            core::str::from_utf8_unchecked_mut(retval)
        }
    }

    pub fn copy_hex<'b>(&mut self, s: &'_ [u8]) -> &'b mut str where 'c: 'b {
        let item = self.alloc(s.len()*2);
        let mut i = 0;
        for b in s {
            item[i].write(b"0123456789abcdef"[(b >> 4) as usize]);
            item[i+1].write(b"0123456789abcdef"[(b & 0xF) as usize]);
            i += 2;
        }
        unsafe {
            core::str::from_utf8_unchecked_mut(Self::slice_assume_init_mut(item))
        }
    }

    pub fn copy_unhex<'b>(&mut self, s: &'_ [u8]) -> Result<&'b mut [u8],EncodingError> where 'c: 'b {
        if s.len() % 2 != 0 {
            return Err(EncodingError{line_nr: line!() });
        }
        let item = self.alloc(s.len()/2);
        for i in 0..item.len() {
            let l = from_hex_u8(s[i*2])?;
            let r = from_hex_u8(s[i*2+1])?;
            item[i].write((l << 4) | r);
        }
        unsafe {
            Ok(Self::slice_assume_init_mut(item))
        }
    }

    // 1-on-1 or 1-on-0 replacements
    pub fn copy_with_replacement<'b>(&mut self, str: &'b [u8], replace: fn (u8) -> Option<&'static [u8]>) -> &'b [u8] where 'c: 'b {
        for (i, c) in str.iter().enumerate() {
            if let Some(replacement) = replace(*c) {
                let mut escaped_string = ScopedArrayBuilder::new(self);
                escaped_string.extend_from_slice(&str[..i]);
                escaped_string.extend_from_slice(replacement);
                for c in &str[i+1..] {
                    match replace(*c) {
                        Some(escaped_char) => escaped_string.extend_from_slice(escaped_char),
                        None => escaped_string.push(*c),
                    };
                }
                return escaped_string.build();
            }
        }
        str
    }

    pub fn copy_with_dynamic_replacement<'b>(&mut self, str: &'b [u8], replace: ReplaceFn) -> &'b [u8] where 'c: 'b
    {
        let mut buffer : Vec<u8> = Vec::new();
        for (i, c) in str.iter().enumerate() {
            if let Some(replacement) = replace(*c, &mut buffer) {
                let mut escaped_string = ScopedArrayBuilder::new(self);
                escaped_string.extend_from_slice(&str[..i]);
                escaped_string.extend_from_slice(replacement);
                for c in &str[i+1..] {
                    buffer.clear();
                    match replace(*c, &mut buffer) {
                        Some(escaped_char) => escaped_string.extend_from_slice(escaped_char),
                        None => escaped_string.push(*c),
                    };
                }
                return escaped_string.build();
            }
        }
        str
    }

    /// Copies objects without a destructor in the memory pool.
    pub fn copy_object<'b,T>(&mut self, object: &'_ T) -> &'b mut T
    where
        'c: 'b,
        T: Copy,
    {
        unsafe {
            let retval : *mut T = self.alloc_ptr::<T>(1);
            *retval = *object;
            if std::mem::needs_drop::<T>() {
                let f = drop_wrapper::<T>;
                self.pool.delay_execution.push((f, retval as * mut ()));
            }
            &mut *retval
        }
    }

    /// Moves objects in the memory pool, the destructor is called when the MemoryScope gets out of
    /// scope.
    pub fn move_object<'b,T>(&mut self, object: T) -> &'b mut T
    where
        'c: 'b,
        T: 'b,
    {
        unsafe {
            let retval : *mut T = self.alloc_ptr::<T>(1);
            let object = ManuallyDrop::new(object);
            // in effect doing *retval = object, but with retval uninitialized
            // so we can not just move the object there
            std::ptr::copy_nonoverlapping(object.deref(), retval, 1);
            if std::mem::needs_drop::<T>() {
                let f = drop_wrapper::<T>;
                self.pool.delay_execution.push((f, retval as * mut ()));
            }
            &mut *retval
        }
    }

    pub fn write_fmt<'b>(&mut self, args: std::fmt::Arguments<'_>) -> &'b str where 'c: 'b {
        if let Some(s) = args.as_str() {
            return s;
        }

        let mut output = ScopedStringBuilder::new(self);
        // unwrap() similar to format!: see https://doc.rust-lang.org/src/alloc/fmt.rs.html#597
        core::fmt::write(&mut output, args).unwrap();
        output.build()
    }

    pub fn slice_from_iter<'b, I: Iterator<Item = T>, T: Unpin>(&mut self, iter: I) -> &'b mut [T] where 'c: 'b {
        let mut builder = ScopedArrayBuilder::new(self);
        for v in iter {
            builder.push(v);
        }
        builder.build()
    }

    pub fn slice_from_array<'b, T: Unpin+Default, const N: usize>(&mut self, mut array: [T; N], range: Range<usize>) -> &'b mut [T] where 'c: 'b {
        let mut builder = ScopedArrayBuilder::with_capacity(self, range.len());
        for i in range {
            builder.push(std::mem::take(&mut array[i]));
        }
        builder.build()
    }
}

unsafe fn drop_wrapper<T>(object: *mut ()) {
    let object = std::mem::transmute::<* mut (), *mut T>(object);
    std::ptr::drop_in_place(object);
}

/// Ceil to next power of two.
fn ceil_to_power_of_two(length: usize) -> usize {
    let next_power_of_two = 1 << (core::mem::size_of::<usize>()*8 - (length.leading_zeros() as usize));
    debug_assert!(next_power_of_two >= length);
    // check if it was already a power of two
    if length == (next_power_of_two>>1) {
        length
    } else {
        next_power_of_two
    }
}

pub struct ScopedArrayBuilder<'a, 'c, T> {
    scope: &'a mut MemoryScope<'c>,
    ptr: *mut T,
    len: usize,
    capacity: usize,
}

impl<'a, 'c, T> Drop for ScopedArrayBuilder<'a, 'c, T> {
    fn drop(&mut self) {
        for i in 0..self.len {
            unsafe { 
                std::ptr::drop_in_place(self.ptr.add(i));
            }
        }
        if self.capacity > 0 {
            // this won't release earlier capacity (that was available when the builder grew in
            // size), but we don't have the ability to track that.
            // We can of course incorporate len_content, len_last_available 
            // like MemoryScope in this struct, but the size grows with 16
            // bytes, so not sure if that is the correct path
            unsafe { self.scope.unused::<T>(self.capacity); }
        }
    }
}

impl<'a, 'c, T> ScopedArrayBuilder<'a, 'c, T> where T: Unpin {
    pub fn new(scope: &'a mut MemoryScope<'c>) -> Self {
        let (ptr, capacity) = scope.alloc_at_least::<T>(1);
        Self { scope, ptr, len: 0, capacity }
    }

    pub fn with_capacity(scope: &'a mut MemoryScope<'c>, count: usize) -> Self {
        let (ptr, capacity) = scope.alloc_at_least::<T>(count);
        Self { scope, ptr, len: 0, capacity }
    }

    pub fn with_size(scope: &'a mut MemoryScope<'c>, count: usize, value: T) -> Self where T: Clone {
        let (ptr, capacity) = scope.alloc_at_least::<T>(count);
        let mut retval = Self { scope, ptr, len: 0, capacity };
        for _ in 0..count {
            retval.push(value.clone());
        }
        retval
    }

    // returns temporary str, useful to evaluate of this candidate is any good (if not, don't call build())
    pub fn as_slice(&self) -> &[T] {
        unsafe {
            core::slice::from_raw_parts(self.ptr, self.len)
        }
    }

    pub fn as_mut_slice(&mut self) -> &mut [T] {
        unsafe {
            core::slice::from_raw_parts_mut(self.ptr, self.len)
        }
    }

    pub fn extend(&mut self, extra: usize, value: T) where T: Clone {
        if self.len+extra >= self.capacity {
            self.alloc(self.len+extra);
        }
        unsafe {
            for i in self.len..self.len+extra {
                std::ptr::copy(&value.clone(), self.ptr.add(i), 1);
            }
        }
        self.len += extra;
    }

    // returns a more permanent str
    // resulting lifetime is min('c, T)
    #[must_use]
    pub fn build<'b>(mut self) -> &'b mut [T] where T: 'b, 'c: 'b {
        unsafe { self.scope.unused::<T>(self.capacity - self.len); }
        let len = self.len;
        self.capacity = 0;
        self.len = 0;
        if std::mem::needs_drop::<T>() {
            let f = drop_wrapper::<T>;
            for i in 0..len {
                self.scope.pool.delay_execution.push((f, unsafe { self.ptr.add(i) } as * mut ()));
            }
        }
        unsafe {
            core::slice::from_raw_parts_mut(self.ptr, len)
        }
    }

    pub fn clear(&mut self) {
        if std::mem::needs_drop::<T>() {
            for i in 0..self.len {
                unsafe { 
                    std::ptr::drop_in_place(self.ptr.add(i));
                }
            }
        }
        self.len = 0;
    }

    // new elements are left in uninitialized state, so don't make this method publically available.
    fn alloc(&mut self, count: usize) {
        let (ptr,capacity) = self.scope.alloc_at_least::<T>(count + self.len);
        unsafe {
            if self.len > 0 {
                std::ptr::copy(self.ptr, ptr, self.len);
            }
            self.ptr = ptr;
            self.capacity = capacity;
        }
    }

    pub fn insert(&mut self, pos: usize, v: T) {
        assert!(pos <= self.len, "index is out of bounds");
        if self.len == self.capacity {
            self.alloc(self.len+1);
        }
        unsafe {
            std::ptr::copy(self.ptr.add(pos), self.ptr.add(pos+1), self.len-pos);
            std::ptr::copy(&v, self.ptr.add(pos), 1);
        }
        std::mem::forget(v);
        self.len += 1;
    }

    pub fn push(&mut self, v: T) {
        if self.len == self.capacity {
            self.alloc(self.len+1);
        }
        unsafe {
            std::ptr::copy(&v, self.ptr.add(self.len), 1);
        }
        std::mem::forget(v);
        self.len += 1;
    }

    pub fn extend_from_slice(&mut self, v: &[T]) where T: Copy {
        if self.len+v.len() > self.capacity {
            self.alloc(self.len+v.len());
        }
        unsafe {
            std::ptr::copy(v.as_ptr(), self.ptr.add(self.len), v.len());
        }
        self.len += v.len();
    }

    #[must_use]
    pub fn len(&self) -> usize {
        self.len
    }

    #[must_use]
    pub fn is_empty(&self) -> bool {
        self.len == 0
    }
}

pub struct ScopedStringBuilder<'a, 'c> {
    scope: &'a mut MemoryScope<'c>,
    ptr: *mut u8,
    len: usize,
    capacity: usize,
}

impl<'a, 'c> Drop for ScopedStringBuilder<'a, 'c> {
    fn drop(&mut self) {
        if self.capacity > 0 {
            unsafe { self.scope.unused::<u8>(self.capacity); }
        }
    }
}

impl<'a, 'c> ScopedStringBuilder<'a, 'c> {
    pub fn new(scope: &'a mut MemoryScope<'c>) -> Self {
        let (ptr, capacity) = scope.alloc_at_least::<u8>(1);
        Self { scope, ptr, len: 0, capacity }
    }

    pub fn with_capacity(scope: &'a mut MemoryScope<'c>, size_in_bytes: usize) -> Self {
        let (ptr, capacity) = scope.alloc_at_least::<u8>(size_in_bytes);
        Self { scope, ptr, len: 0, capacity }
    }


    // returns temporary str, useful to evaluate of this candidate is any good (if not, don't call build())
    pub fn as_str(&mut self) -> &str {
        unsafe {
            core::str::from_utf8_unchecked_mut(core::slice::from_raw_parts_mut(self.ptr, self.len))
        }
    }

    // returns a more permanent str
    #[must_use]
    pub fn build<'b>(mut self) -> &'b mut str where 'c: 'b {
        unsafe { self.scope.unused::<u8>(self.capacity - self.len); }
        self.capacity = 0;
        // SAFETY: all things appended to this buffer are valid utf8, so no need to check
        unsafe {
            core::str::from_utf8_unchecked_mut(core::slice::from_raw_parts_mut(self.ptr, self.len))
        }
    }

    fn alloc(&mut self, count: usize) {
        let (ptr,capacity) = self.scope.alloc_at_least::<u8>(count + self.len);
        unsafe {
            if self.len > 0 {
                std::ptr::copy(self.ptr, ptr, self.len);
            }
            self.ptr = ptr;
            self.capacity = capacity;
        }
    }

    /// # Safety
    /// `c` should only contain UTF-8 chars.
    pub(crate) unsafe fn write_raw(&mut self, c: &[u8]) {
        let len = c.len();
        if self.len+len > self.capacity {
            self.alloc(self.len+len);
        }
        std::ptr::copy(c.as_ptr(), self.ptr.add(self.len), len);
        self.len += len;

    }
    pub fn push_char(&mut self, c: char) {
        let len = c.len_utf8();
        if self.len+len > self.capacity {
            self.alloc(self.len+len);
        }
        let val = unsafe { core::slice::from_raw_parts_mut(self.ptr.add(self.len), len) };
        c.encode_utf8(val);
        self.len += len;
    }

    #[must_use]
    pub fn len(&self) -> usize {
        self.len
    }

    #[must_use]
    pub fn is_empty(&self) -> bool {
        self.len == 0
    }
}

impl<'a, 'c> core::fmt::Write for ScopedStringBuilder<'a, 'c> {
    fn write_str(&mut self, s: &str) -> core::fmt::Result {
        unsafe { self.write_raw(s.as_bytes()); }
        Ok(())
    }
}

impl<'c> Drop for MemoryScope<'c> {
    fn drop(&mut self) {
        let content: &mut _ = &mut self.pool.content;

        // execute delayed functions
        let delay_execution: &mut _ = &mut self.pool.delay_execution;
        //eprintln!("drop scope: {} and {}/{}", self.len_content, self.len_delay_execution, delay_execution.len());
        debug_assert!(self.len_delay_execution <= delay_execution.len());
        if self.len_delay_execution < delay_execution.len() {
            for i in (self.len_delay_execution..delay_execution.len()).rev() {
                //println!("execute delay {}", i);
                let (f,arg) = delay_execution.remove(i);
                unsafe {
                    f(arg);
                }
            }
        }

        //println!("truncating memorypool from {} to {}, freeing {} bytes", content.len(), self.len_content, content.iter().map(|x| x.len()).sum::<usize>());
        //eprintln!("dropping from {} to {}", content.len(), self.len_content);
        if self.clear {
            content.truncate(self.len_content);
        } else {
            for mut page in content.drain(self.len_content..) {
                // to avoid certain worst case behaviour, we need to always free non-regular
                // size MemoryPages: when in a loop files are read, that are of increasingly size,
                // many MemoryPages are used that can not be used and are directly skipped. This
                // results in large memory consumption.
                if page.size == Self::DEFAULT_MEMORY_PAGE_SIZE {
                    page.available = page.size;
                    self.pool.next.push(page);
                }
            }
        }
        if let Some(last) = content.last_mut() {
            //eprintln!("set last from {} to {}", last.available, self.len_last_available);
            last.available = self.len_last_available;
        }
    }
}

#[cfg(test)]
mod tests {
    use crate::*;

    #[test]
    fn format() {
        pool!(scope);
        let s : &str = write!(scope, "foo {}", 42);
        assert_eq!("foo 42", s);
    }

    #[test]
    fn scoped_string_builder() {
        pool!(scope);
        let mut builder = ScopedStringBuilder::new(&mut scope);
        for i in 0..3 {
            write!(builder, "{i}, ").unwrap();
        }
        let output = builder.build();
        assert_eq!("0, 1, 2, ", output);
    }

    #[test]
    fn scoped_array_builder() {
        pool!(scope);
        let mut builder = ScopedArrayBuilder::new(&mut scope);
        for i in 0..3 {
            builder.push(format!("-{i}-"));
        }
        let output : &mut [String] = builder.build();
        assert_eq!(3, output.len());
        for i in 0..3 {
            let expected = format!("-{i}-");
            assert_eq!(expected, output[i]);
        }
    }

    fn test(mut scope: &mut MemoryScope, available: usize) {
        rewind!(scope in scope);
        scope.alloc::<u8>(1024);
        eprintln!("{} < {}", scope.available::<u8>(), available);
        assert!(scope.available::<u8>() < available);
    }

    #[test]
    fn scoped_allocation() {
        pool!(scope);
        scope.alloc::<u8>(1);
        let available = scope.available::<u8>();
        test(&mut scope, available);
        assert_eq!(available, scope.available::<u8>());
    }

    #[test]
    fn power_of_two() {
        let mut length = 2048usize;
        let next_power_of_two = 1 << (core::mem::size_of::<usize>()*8 - (length.leading_zeros() as usize));
        eprintln!("{} -> {} -> {}", length, next_power_of_two, length & ((next_power_of_two>>1)-1));
        if length & ((next_power_of_two>>1)-1) != 0 {
            length = next_power_of_two;
        }
        assert!(length == 2048);
    }
    pub fn expry<'b,'c>(_bytecode: &'b [u8], _scope: &mut MemoryScope<'c>) -> &'b [u8] where 'c: 'b {
        todo!();
    }
    pub fn scoping_compile_test(flag: bool, expr: &[u8]) -> bool {
        let mut scope = MemoryPool::new();
        let mut value : &[u8] = b"";
        clear!(scope);
        if flag {
            let retval = expry(expr, &mut scope);
            value = retval;
        }
        !value.is_empty()
    }

    #[test]
    fn ceil_power_of_two() {
        for i in 0..16 {
            let result = super::ceil_to_power_of_two(i);
            eprintln!("{} -> {}", i, result);
        }
    }
}