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// Copyright 2024 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::BTreeMap;

use anyhow::{anyhow, bail, Context, Result};
use elf::{endian::LittleEndian, file::Class, ElfBytes};
use risc0_zkvm_platform::WORD_SIZE;

/// A RISC Zero program
pub struct Program {
    /// The entrypoint of the program
    pub entry: u32,

    /// The initial memory image
    pub image: BTreeMap<u32, u32>,
}

impl Program {
    /// Initialize a RISC Zero Program from an appropriate ELF file
    pub fn load_elf(input: &[u8], max_mem: u32) -> Result<Program> {
        let mut image: BTreeMap<u32, u32> = BTreeMap::new();
        let elf = ElfBytes::<LittleEndian>::minimal_parse(input)
            .map_err(|err| anyhow!("Elf parse error: {err}"))?;
        if elf.ehdr.class != Class::ELF32 {
            bail!("Not a 32-bit ELF");
        }
        if elf.ehdr.e_machine != elf::abi::EM_RISCV {
            bail!("Invalid machine type, must be RISC-V");
        }
        if elf.ehdr.e_type != elf::abi::ET_EXEC {
            bail!("Invalid ELF type, must be executable");
        }
        let entry: u32 = elf
            .ehdr
            .e_entry
            .try_into()
            .map_err(|err| anyhow!("e_entry was larger than 32 bits. {err}"))?;
        if entry >= max_mem || entry % WORD_SIZE as u32 != 0 {
            bail!("Invalid entrypoint");
        }
        let segments = elf.segments().ok_or(anyhow!("Missing segment table"))?;
        if segments.len() > 256 {
            bail!("Too many program headers");
        }
        for segment in segments.iter().filter(|x| x.p_type == elf::abi::PT_LOAD) {
            let file_size: u32 = segment
                .p_filesz
                .try_into()
                .map_err(|err| anyhow!("filesize was larger than 32 bits. {err}"))?;
            if file_size >= max_mem {
                bail!("Invalid segment file_size");
            }
            let mem_size: u32 = segment
                .p_memsz
                .try_into()
                .map_err(|err| anyhow!("mem_size was larger than 32 bits {err}"))?;
            if mem_size >= max_mem {
                bail!("Invalid segment mem_size");
            }
            let vaddr: u32 = segment
                .p_vaddr
                .try_into()
                .map_err(|err| anyhow!("vaddr is larger than 32 bits. {err}"))?;
            if vaddr % WORD_SIZE as u32 != 0 {
                bail!("vaddr {vaddr:08x} is unaligned");
            }
            let offset: u32 = segment
                .p_offset
                .try_into()
                .map_err(|err| anyhow!("offset is larger than 32 bits. {err}"))?;
            for i in (0..mem_size).step_by(WORD_SIZE) {
                let addr = vaddr.checked_add(i).context("Invalid segment vaddr")?;
                if addr >= max_mem {
                    bail!("Address [0x{addr:08x}] exceeds maximum address for guest programs [0x{max_mem:08x}]");
                }
                if i >= file_size {
                    // Past the file size, all zeros.
                    image.insert(addr, 0);
                } else {
                    let mut word = 0;
                    // Don't read past the end of the file.
                    let len = core::cmp::min(file_size - i, WORD_SIZE as u32);
                    for j in 0..len {
                        let offset = (offset + i + j) as usize;
                        let byte = input.get(offset).context("Invalid segment offset")?;
                        word |= (*byte as u32) << (j * 8);
                    }
                    image.insert(addr, word);
                }
            }
        }
        Ok(Program { entry, image })
    }
}