use std::fs;
use std::path::{Path, PathBuf};

pub struct RomInfo {
    path: PathBuf,
    file_size: usize,
    has_copier_header: bool,
    header: Option<SnesHeader>,
}

struct SnesHeader {
    title: String,
    mapping: &'static str,
    region: &'static str,
    version: u8,
}

#[derive(Clone, Copy)]
enum Mapping {
    Low,
    High,
    ExtendedHigh,
}

impl RomInfo {
    pub fn read(path: &Path) -> Result<Self, String> {
        let bytes = fs::read(path).map_err(|error| format!("could not read the ROM ({error})"))?;
        let has_copier_header = bytes.len() >= 512 && bytes.len() % 0x8000 == 512;
        let rom = if has_copier_header {
            &bytes[512..]
        } else {
            &bytes[..]
        };

        Ok(Self {
            path: path.to_path_buf(),
            file_size: bytes.len(),
            has_copier_header,
            header: find_header(rom),
        })
    }

    pub fn summary(&self) -> String {
        let file_name = self
            .path
            .file_name()
            .and_then(|name| name.to_str())
            .unwrap_or("Selected file");
        let location = self
            .path
            .parent()
            .map(|parent| parent.display().to_string())
            .filter(|parent| !parent.is_empty())
            .unwrap_or_else(|| "Unknown".to_string());
        let copier_header = if self.has_copier_header { "Yes" } else { "No" };

        let mut details = vec![
            format!("File: {file_name}"),
            format!("Location: {location}"),
            format!("Size: {}", format_file_size(self.file_size)),
            format!("Copier header: {copier_header}"),
        ];

        if let Some(header) = &self.header {
            details.splice(
                0..0,
                [
                    format!("Game title: {}", header.title),
                    format!("Mapping: {}", header.mapping),
                    format!("Region: {}", header.region),
                    format!("Revision: 1.{}", header.version),
                    String::new(),
                ],
            );
        } else {
            details.splice(
                0..0,
                [
                    "Game title: Not found".to_string(),
                    "SNES header: Not recognized".to_string(),
                    String::new(),
                ],
            );
        }

        details.join("\n")
    }
}

fn find_header(rom: &[u8]) -> Option<SnesHeader> {
    let candidates = [
        (0x7fc0, Mapping::Low),
        (0xffc0, Mapping::High),
        (0x40ffc0, Mapping::ExtendedHigh),
    ];

    candidates
        .iter()
        .filter_map(|(offset, mapping)| parse_header(rom, *offset, *mapping))
        .max_by_key(|(score, _)| *score)
        .filter(|(score, _)| *score >= 5)
        .map(|(_, header)| header)
}

fn parse_header(rom: &[u8], offset: usize, mapping: Mapping) -> Option<(i32, SnesHeader)> {
    let bytes = rom.get(offset..offset + 0x40)?;
    let title = parse_title(&bytes[..21])?;
    let map_mode = bytes[0x15];
    let checksum_complement = u16::from_le_bytes([bytes[0x1c], bytes[0x1d]]);
    let checksum = u16::from_le_bytes([bytes[0x1e], bytes[0x1f]]);
    let reset_vector = u16::from_le_bytes([bytes[0x3c], bytes[0x3d]]);

    let mut score = 0;
    if mapping_matches(map_mode, mapping) {
        score += 3;
    }
    if checksum != 0 && checksum != u16::MAX && checksum ^ checksum_complement == u16::MAX {
        score += 5;
    }
    if reset_vector >= 0x8000 {
        score += 3;
    }

    Some((
        score,
        SnesHeader {
            title,
            mapping: mapping_name(mapping),
            region: region_name(bytes[0x19]),
            version: bytes[0x1b],
        },
    ))
}

fn parse_title(bytes: &[u8]) -> Option<String> {
    if bytes.iter().any(|byte| !matches!(*byte, 0 | 0x20..=0x7e)) {
        return None;
    }

    let end = bytes
        .iter()
        .rposition(|byte| *byte != 0 && *byte != b' ')
        .map(|position| position + 1)?;
    Some(String::from_utf8_lossy(&bytes[..end]).to_string())
}

fn mapping_matches(map_mode: u8, mapping: Mapping) -> bool {
    match mapping {
        Mapping::Low => matches!(map_mode & 0x0f, 0 | 2 | 3),
        Mapping::High => map_mode & 0x0f == 1,
        Mapping::ExtendedHigh => map_mode & 0x0f == 5,
    }
}

fn mapping_name(mapping: Mapping) -> &'static str {
    match mapping {
        Mapping::Low => "LoROM",
        Mapping::High => "HiROM",
        Mapping::ExtendedHigh => "ExHiROM",
    }
}

fn region_name(region: u8) -> &'static str {
    match region {
        0 => "Japan",
        1 => "USA / Canada",
        2 => "Europe / Oceania / Asia",
        3 => "Sweden",
        4 => "Finland",
        5 => "Denmark",
        6 => "France",
        7 => "Netherlands",
        8 => "Spain",
        9 => "Germany / Austria / Switzerland",
        10 => "Italy",
        11 => "Hong Kong / China",
        12 => "Indonesia",
        13 => "South Korea",
        _ => "Unknown",
    }
}

fn format_file_size(bytes: usize) -> String {
    let exact = format_integer(bytes);
    if bytes < 1024 {
        return format!("{exact} bytes");
    }

    const UNITS: [&str; 4] = ["KiB", "MiB", "GiB", "TiB"];
    let mut size = bytes as f64 / 1024.0;
    let mut unit = UNITS[0];
    for candidate in &UNITS[1..] {
        if size < 1024.0 {
            break;
        }
        size /= 1024.0;
        unit = candidate;
    }
    format!("{size:.2} {unit} ({exact} bytes)")
}

fn format_integer(value: usize) -> String {
    let digits = value.to_string();
    let mut result = String::with_capacity(digits.len() + digits.len() / 3);
    for (index, character) in digits.chars().enumerate() {
        if index > 0 && (digits.len() - index).is_multiple_of(3) {
            result.push(',');
        }
        result.push(character);
    }
    result
}

#[cfg(test)]
mod tests {
    use super::{find_header, format_file_size};

    #[test]
    fn reads_a_lorom_header() {
        let mut rom = vec![0; 0x8000];
        let header = &mut rom[0x7fc0..0x8000];
        header[..21].copy_from_slice(b"SUPER MARIOWORLD     ");
        header[0x15] = 0x20;
        header[0x19] = 1;
        header[0x1b] = 2;
        header[0x1c..0x1e].copy_from_slice(&0x4321_u16.to_le_bytes());
        header[0x1e..0x20].copy_from_slice(&0xbcde_u16.to_le_bytes());
        header[0x3c..0x3e].copy_from_slice(&0x8000_u16.to_le_bytes());

        let header = find_header(&rom).expect("the header should be recognized");
        assert_eq!(header.title, "SUPER MARIOWORLD");
        assert_eq!(header.mapping, "LoROM");
        assert_eq!(header.region, "USA / Canada");
        assert_eq!(header.version, 2);
    }

    #[test]
    fn rejects_data_without_a_snes_header() {
        assert!(find_header(&vec![0; 0x10000]).is_none());
    }

    #[test]
    fn formats_file_sizes_for_people() {
        assert_eq!(format_file_size(512), "512 bytes");
        assert_eq!(format_file_size(1_048_576), "1.00 MiB (1,048,576 bytes)");
    }
}