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// Copyright 2023 RISC Zero, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

extern crate alloc;

use alloc::{collections::VecDeque, vec::Vec};

use risc0_zkp::core::{
    digest::Digest,
    hash::sha::{cpu::Impl, Sha256},
};
use serde::{Deserialize, Serialize};

/// Represents the public state of a segment, needed for continuations and
/// receipt verification.
#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
pub struct SystemState {
    /// The program counter.
    pub pc: u32,

    /// The root hash of a merkle tree which confirms the
    /// integrity of the memory image.
    pub merkle_root: Digest,
}

impl SystemState {
    pub fn decode(flat: &mut VecDeque<u32>) -> Self {
        Self {
            pc: read_u32_bytes(flat),
            merkle_root: read_sha_halfs(flat),
        }
    }

    pub fn encode(&self, flat: &mut Vec<u32>) {
        write_u32_bytes(flat, self.pc);
        write_sha_halfs(flat, &self.merkle_root);
    }

    /// Hash the [crate::SystemState] to get a digest of the struct.
    pub fn digest(&self) -> Digest {
        tagged_struct("risc0.SystemState", &[self.merkle_root], &[self.pc])
    }
}

/// Implementation of the struct hash described in the recursion predicates RFC.
pub fn tagged_struct(tag: &str, down: &[Digest], data: &[u32]) -> Digest {
    let tag_digest: Digest = *Impl::hash_bytes(tag.as_bytes());
    let mut all = Vec::<u8>::new();
    all.extend_from_slice(tag_digest.as_bytes());
    for digest in down {
        all.extend_from_slice(digest.as_ref());
    }
    for word in data.iter().copied() {
        all.extend_from_slice(&word.to_le_bytes());
    }
    let down_count: u16 = down.len().try_into().unwrap();
    all.extend_from_slice(&down_count.to_le_bytes());
    *Impl::hash_bytes(&all)
}

pub fn read_sha_halfs(flat: &mut VecDeque<u32>) -> Digest {
    let mut bytes = Vec::<u8>::new();
    for half in flat.drain(0..16) {
        bytes.push((half & 0xff).try_into().unwrap());
        bytes.push((half >> 8).try_into().unwrap());
    }
    bytes.try_into().unwrap()
}

fn read_u32_bytes(flat: &mut VecDeque<u32>) -> u32 {
    u32::from_le_bytes(
        flat.drain(0..4)
            .map(|x| x as u8)
            .collect::<Vec<u8>>()
            .try_into()
            .unwrap(),
    )
}

pub fn write_sha_halfs(flat: &mut Vec<u32>, digest: &Digest) {
    for x in digest.as_words() {
        flat.push(*x & 0xffff);
        flat.push(*x >> 16);
    }
}

fn write_u32_bytes(flat: &mut Vec<u32>, word: u32) {
    for x in word.to_le_bytes() {
        flat.push(x as u32);
    }
}

#[cfg(test)]
mod tests {
    use super::tagged_struct;

    #[test]
    fn test_tagged_struct() {
        let digest1 = tagged_struct("foo", &[], &[1, 2013265920, 3]);
        let digest2 = tagged_struct("bar", &[digest1, digest1], &[2013265920, 5]);
        let digest3 = tagged_struct(
            "baz",
            &[digest1, digest2, digest1],
            &[6, 7, 2013265920, 9, 10],
        );

        println!("digest = {:?}", digest3);
        assert_eq!(
            digest3.to_string(),
            "9ff20cc6d365efa2af09181772f49013d05cdee6da896851614cae23aa5dd442"
        );
    }
}