//! Mutation fuzzing for the filter decoders (tests only).
//!
//! A decoder fed a random or mutated frame may fail, but must not panic —
//! tests build with overflow checks and debug assertions, so an unchecked
//! subtraction, multiplication or shift on a header field, or an
//! out-of-range slice, fails the test — and must not return more than its
//! output limit.

#[cfg(not(feature = "std"))]
extern crate alloc;
#[cfg(not(feature = "std"))]
use alloc::vec::Vec;

use crate::error::FormatError;

/// xorshift64*: deterministic, so a failure reproduces.
pub(crate) struct Rng(u64);

impl Rng {
    pub(crate) fn new(seed: u64) -> Rng {
        Rng(seed.max(1))
    }

    pub(crate) fn next_u64(&mut self) -> u64 {
        let mut x = self.0;
        x ^= x >> 12;
        x ^= x << 25;
        x ^= x >> 27;
        self.0 = x;
        x.wrapping_mul(0x2545_F491_4F6C_DD1D)
    }

    /// Uniform in `0..n` (`n` > 0).
    pub(crate) fn below(&mut self, n: usize) -> usize {
        (self.next_u64() % n as u64) as usize
    }

    pub(crate) fn bytes(&mut self, n: usize) -> Vec<u8> {
        (0..n).map(|_| self.next_u64() as u8).collect()
    }

    /// A u32 that tends to hit edge cases in size and offset fields.
    fn interesting_u32(&mut self, len: usize) -> u32 {
        match self.below(10) {
            0 => 0,
            1 => 1,
            2 => self.below(20) as u32,
            3 => 15 + self.below(3) as u32,
            4 => u32::MAX - self.below(16) as u32,
            5 => 1 << self.below(32),
            6 => (len as u32)
                .wrapping_add(self.below(9) as u32)
                .wrapping_sub(4),
            7 => i32::MAX as u32,
            _ => self.next_u64() as u32,
        }
    }
}

/// One to four random edits of `seed`.
pub(crate) fn mutate(rng: &mut Rng, seed: &[u8]) -> Vec<u8> {
    let mut v = seed.to_vec();
    for _ in 0..1 + rng.below(4) {
        let len = v.len();
        match rng.below(9) {
            0 if len > 0 => {
                let i = rng.below(len);
                v[i] ^= 1 << rng.below(8);
            }
            1 if len > 0 => {
                let i = rng.below(len);
                v[i] = rng.next_u64() as u8;
            }
            2 if len > 0 => {
                let i = rng.below(len);
                v[i] = [0, 0xff, 0x7f, 0x80, 0x20, 0x1f][rng.below(6)];
            }
            // A size or offset field: little- or big-endian, anywhere, but
            // most often in the first 32 bytes where headers live.
            3 | 4 if len >= 4 => {
                let span = if rng.below(2) == 0 { len.min(32) } else { len };
                let i = rng.below(span - 3);
                let x = rng.interesting_u32(len);
                let b = if rng.below(2) == 0 {
                    x.to_le_bytes()
                } else {
                    x.to_be_bytes()
                };
                v[i..i + 4].copy_from_slice(&b);
            }
            5 if len > 0 => v.truncate(rng.below(len)),
            6 => {
                let n = 1 + rng.below(64);
                let extra = rng.bytes(n);
                v.extend_from_slice(&extra);
            }
            7 if len > 1 => {
                let a = rng.below(len);
                let b = a + rng.below(len - a);
                let copy = v[a..b].to_vec();
                let at = rng.below(len);
                v.splice(at..at, copy);
            }
            _ if len > 0 => {
                let i = rng.below(len);
                v[i] = v[i].wrapping_add(1 + rng.below(3) as u8);
            }
            _ => v.push(rng.next_u64() as u8),
        }
    }
    v
}

/// Feed `iters` inputs to `decode`: mostly mutations of `seeds`, some pure
/// noise and some truncated seeds. Asserts only "no panic, output within
/// `limit`".
pub(crate) fn fuzz_decoder(
    seed: u64,
    seeds: &[Vec<u8>],
    iters: usize,
    limit: usize,
    mut decode: impl FnMut(&[u8]) -> Result<Vec<u8>, FormatError>,
) {
    assert!(!seeds.is_empty());
    let mut rng = Rng::new(seed);
    for s in seeds {
        // The seeds themselves must be valid, or the fuzz explores nothing.
        decode(s).expect("seed frame must decode");
    }
    for _ in 0..iters {
        let input = match rng.below(16) {
            0 => {
                let n = rng.below(96);
                rng.bytes(n)
            }
            1 => {
                let s = &seeds[rng.below(seeds.len())];
                s[..rng.below(s.len() + 1)].to_vec()
            }
            _ => {
                let s = &seeds[rng.below(seeds.len())];
                mutate(&mut rng, s)
            }
        };
        if let Ok(out) = decode(&input) {
            assert!(out.len() <= limit, "decoded {} > limit {limit}", out.len());
        }
    }
}