symjit 2.16.3

a lightweight just-in-time (JIT) optimizer compiler
Documentation 
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use std::collections::HashMap;

pub type Jumper = fn(x: i32, code: u32) -> u32;

#[derive(Debug)]
pub struct Assembler {
    pub buf: Vec<u8>,
    labels: HashMap<String, usize>,
    jumps: Vec<(String, usize, u32, Jumper, bool)>,
}

impl Assembler {
    pub fn new() -> Assembler {
        Assembler {
            buf: Vec::new(),
            labels: HashMap::new(),
            jumps: Vec::new(),
        }
    }

    pub fn bytes(&self) -> Vec<u8> {
        self.buf.clone()
    }

    pub fn append_byte(&mut self, b: u8) {
        self.buf.push(b)
    }

    pub fn append_bytes(&mut self, bs: &[u8]) {
        for b in bs {
            self.append_byte(*b);
        }
    }

    pub fn append_word(&mut self, mut u: u32) {
        // appends u (uint32) as little-endian
        for _ in 0..4 {
            self.append_byte((u & 0xff) as u8);
            u >>= 8;
        }
    }

    pub fn append_quad(&mut self, mut u: u64) {
        // appends u (uint32) as little-endian
        for _ in 0..8 {
            self.append_byte((u & 0xff) as u8);
            u >>= 8;
        }
    }

    pub fn ip(&self) -> usize {
        self.buf.len()
    }

    pub fn set_label(&mut self, label: &str) {
        self.labels.insert(label.to_string(), self.ip());
    }

    pub fn jump(&mut self, label: &str, code: u32, f: Jumper) {
        self.jumps
            .push((label.to_string(), self.ip(), code, f, true));
        self.append_word(0);
    }

    pub fn jump_abs(&mut self, label: &str, code: u32, f: Jumper) {
        self.jumps
            .push((label.to_string(), self.ip(), code, f, false));
        self.append_word(0);
    }

    pub fn apply_jumps(&mut self) {
        for (label, ip, code, f, rel) in self.jumps.iter() {
            let target = self
                .labels
                .get(label)
                .unwrap_or_else(|| panic!("label {} not found", label));
            let offset = (*target as i32) - if *rel { *ip as i32 } else { 0 };

            let x = f(offset, *code);

            self.buf[*ip] |= (x & 0xff) as u8;
            self.buf[*ip + 1] |= ((x >> 8) & 0xff) as u8;
            self.buf[*ip + 2] |= ((x >> 16) & 0xff) as u8;
            self.buf[*ip + 3] |= ((x >> 24) & 0xff) as u8;
        }
    }
}