#![no_std]

extern crate alloc;
extern crate log;

use core::{fmt, str, convert};
use alloc::string::String;
use log::{info, trace};
use elf::*;

pub mod elf;
mod file;
mod image;
use image::{DynamicSection, Image};
mod reloc;

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Arch {
    Arm,
    OpenRisc,
}


#[derive(Debug)]
pub enum Error {
    Parsing(&'static str),
    Lookup(String)
}

impl convert::From<&'static str> for Error {
    fn from(desc: &'static str) -> Error {
        Error::Parsing(desc)
    }
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            &Error::Parsing(desc) =>
                write!(f, "parse error: {}", desc),
            &Error::Lookup(ref sym) =>
                write!(f, "symbol lookup error: {}", sym),
        }
    }
}

fn elf_hash(name: &[u8]) -> u32 {
    let mut h: u32 = 0;
    for c in name {
        h = (h << 4) + *c as u32;
        let g = h & 0xf0000000;
        if g != 0 {
            h ^= g >> 24;
            h &= !g;
        }
    }
    h
}

pub struct Library {
    image: Image,
    dyn_section: DynamicSection,
}

impl Library {
    fn strtab(&self) -> &[u8] {
        self.image.get_ref_slice_unchecked(&self.dyn_section.strtab)
    }

    fn symtab(&self) -> &[Elf32_Sym] {
        self.image.get_ref_slice_unchecked(&self.dyn_section.symtab)
    }

    fn hash(&self) -> &[Elf32_Word] {
        self.image.get_ref_slice_unchecked(&self.dyn_section.hash)
    }

    fn hash_bucket(&self) -> &[Elf32_Word] {
        &self.hash()[self.dyn_section.hash_bucket.clone()]
    }

    fn hash_chain(&self) -> &[Elf32_Word] {
        &self.hash()[self.dyn_section.hash_chain.clone()]
    }

    fn rel(&self) -> &[Elf32_Rel] {
        self.image.get_ref_slice_unchecked(&self.dyn_section.rel)
    }

    fn rela(&self) -> &[Elf32_Rela] {
        self.image.get_ref_slice_unchecked(&self.dyn_section.rela)
    }

    fn pltrel(&self) -> &[Elf32_Rel] {
        self.image.get_ref_slice_unchecked(&self.dyn_section.pltrel)
    }

    pub fn lookup(&self, name: &[u8]) -> Option<Elf32_Word> {
        let hash = elf_hash(name);
        let mut index = self.hash_bucket()[hash as usize % self.hash_bucket().len()] as usize;

        loop {
            if index == STN_UNDEF { return None }

            let sym = &self.symtab()[index];
            let sym_name_off = sym.st_name as usize;
            match self.strtab().get(sym_name_off..sym_name_off + name.len()) {
                Some(sym_name) if sym_name == name => {
                    if ELF32_ST_BIND(sym.st_info) & STB_GLOBAL == 0 {
                        return None
                    }

                    match sym.st_shndx {
                        SHN_UNDEF => return None,
                        SHN_ABS => return Some(self.image.ptr() as u32 + sym.st_value),
                        _ => return Some(self.image.ptr() as u32 + sym.st_value)
                    }
                }
                _ => (),
            }

            index = self.hash_chain()[index] as usize;
        }
    }

    pub fn name_starting_at(&self, offset: usize) -> Result<&[u8], Error> {
        let size = self.strtab().iter().skip(offset).position(|&x| x == 0)
                              .ok_or("symbol in symbol table not null-terminated")?;
        Ok(self.strtab().get(offset..offset + size)
           .ok_or("cannot read symbol name")?)
    }
}

pub fn load(
    data: &[u8],
    resolve: &dyn Fn(&[u8]) -> Option<Elf32_Word>
) -> Result<Library, Error> {
    // validate ELF file
    let file = file::File::new(data)
        .ok_or("cannot read ELF header")?;
    if file.ehdr.e_type != ET_DYN {
        return Err("not a shared library")?
    }
    let arch = file.arch()
        .ok_or("not for a supported architecture")?;

    // prepare target memory
    let image_size = file.program_headers()
        .filter_map(|phdr| phdr.map(|phdr| phdr.p_vaddr + phdr.p_memsz))
        .max()
        .unwrap_or(0) as usize;
    let image_align = file.program_headers()
        .filter_map(|phdr| phdr.and_then(|phdr| {
            if phdr.p_type == PT_LOAD {
                Some(phdr.p_align)
            } else {
                None
            }
        }))
        .max()
        .unwrap_or(4) as usize;
    // 1 image for all segments
    let mut image = image::Image::new(image_size, image_align)
        .map_err(|_| "cannot allocate target image")?;
    info!("ELF target: {} bytes, align to {:X}, allocated at {:08X}", image_size, image_align, image.ptr() as usize);

    // LOAD
    for phdr in file.program_headers() {
        let phdr = phdr.ok_or("cannot read program header")?;
        if phdr.p_type != PT_LOAD { continue; }

        trace!("Program header: {:08X}+{:08X} to {:08X}",
              phdr.p_offset, phdr.p_filesz,
              image.ptr() as u32
        );
        let src = file.get(phdr.p_offset as usize..(phdr.p_offset + phdr.p_filesz) as usize)
            .ok_or("program header requests an out of bounds load (in file)")?;
        let dst = image.get_mut(phdr.p_vaddr as usize..
                                (phdr.p_vaddr + phdr.p_filesz) as usize)
            .ok_or("program header requests an out of bounds load (in target)")?;
        dst.copy_from_slice(src);
    }

    // relocate DYNAMIC
    let dyn_range = file.dyn_header_vaddr()
        .ok_or("cannot find a dynamic header")?;
    let dyn_section = image.dyn_section(dyn_range.clone())?;
    info!("Relocating {} rela, {} rel, {} pltrel",
          dyn_section.rela.len(), dyn_section.rel.len(), dyn_section.pltrel.len());
    let lib = Library {
        image,
        dyn_section,
    };

    for rela in lib.rela() {
        reloc::relocate(arch, &lib, rela, resolve)?;
    }
    for rel in lib.rel() {
        reloc::relocate(arch, &lib, rel, resolve)?;
    }
    for pltrel in lib.pltrel() {
        reloc::relocate(arch, &lib, pltrel, resolve)?;
    }

    Ok(lib)
}