/// XEX2 main header parsing.
///
/// The main header is located at the very beginning of the XEX2 file (offset 0x00)
/// and contains the magic bytes, module flags, header size, security info offset,
/// and the count of optional header entries that follow.
use std::fmt;

use crate::error::{Result, Xex2Error};
use crate::util::read_u32_be;

/// Expected magic value at offset 0x00: ASCII "XEX2" = 0x58455832.
pub const XEX2_MAGIC: u32 = 0x58455832;

/// Size of the fixed portion of the main header (before optional header entries).
pub const HEADER_SIZE: usize = 0x18;

/// The parsed XEX2 main header.
#[derive(Debug, Clone)]
pub struct Xex2Header {
    /// Magic bytes — must be `XEX2_MAGIC` (0x58455832).
    pub magic: u32,
    /// Bitfield indicating the module type (title, DLL, patch, etc.).
    pub module_flags: ModuleFlags,
    /// Total size of all headers in bytes. The PE image data starts at this offset.
    pub header_size: u32,
    /// Reserved field (typically 0).
    pub reserved: u32,
    /// File offset to the `xex2_security_info` structure.
    pub security_offset: u32,
    /// Number of optional header entries following the main header.
    pub header_count: u32,
}

/// Parses the XEX2 main header from the beginning of `data`.
///
/// Validates that the magic bytes match `XEX2_MAGIC` and that the buffer is
/// large enough to contain the fixed header fields.
pub fn parse_header(data: &[u8]) -> Result<Xex2Header> {
    if data.len() < HEADER_SIZE {
        return Err(Xex2Error::FileTooSmall {
            expected: HEADER_SIZE,
            actual: data.len(),
        });
    }

    let magic = read_u32_be(data, 0x00)?;
    if magic != XEX2_MAGIC {
        return Err(Xex2Error::InvalidMagic(magic));
    }

    Ok(Xex2Header {
        magic,
        module_flags: ModuleFlags(read_u32_be(data, 0x04)?),
        header_size: read_u32_be(data, 0x08)?,
        reserved: read_u32_be(data, 0x0C)?,
        security_offset: read_u32_be(data, 0x10)?,
        header_count: read_u32_be(data, 0x14)?,
    })
}

/// Wrapper around the module flags bitmask from the XEX2 header.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct ModuleFlags(pub u32);

impl ModuleFlags {
    pub const TITLE: u32 = 0x00000001;
    pub const EXPORTS_TO_TITLE: u32 = 0x00000002;
    pub const SYSTEM_DEBUGGER: u32 = 0x00000004;
    pub const DLL_MODULE: u32 = 0x00000008;
    pub const MODULE_PATCH: u32 = 0x00000010;
    pub const PATCH_FULL: u32 = 0x00000020;
    pub const PATCH_DELTA: u32 = 0x00000040;
    pub const USER_MODE: u32 = 0x00000080;

    /// All known flags paired with their display names, in bit order.
    const FLAGS: &[(u32, &str)] = &[
        (Self::TITLE, "TITLE"),
        (Self::EXPORTS_TO_TITLE, "EXPORTS_TO_TITLE"),
        (Self::SYSTEM_DEBUGGER, "SYSTEM_DEBUGGER"),
        (Self::DLL_MODULE, "DLL_MODULE"),
        (Self::MODULE_PATCH, "MODULE_PATCH"),
        (Self::PATCH_FULL, "PATCH_FULL"),
        (Self::PATCH_DELTA, "PATCH_DELTA"),
        (Self::USER_MODE, "USER_MODE"),
    ];

    /// Returns the raw `u32` value.
    pub fn bits(self) -> u32 {
        self.0
    }

    /// Returns a list of human-readable flag names that are set.
    pub fn flag_names(self) -> Vec<&'static str> {
        Self::FLAGS
            .iter()
            .filter(|(bit, _)| self.0 & bit != 0)
            .map(|(_, name)| *name)
            .collect()
    }
}

impl fmt::Display for ModuleFlags {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let names = self.flag_names();
        if names.is_empty() {
            write!(f, "0x{:08X}", self.0)
        } else {
            write!(f, "0x{:08X} [{}]", self.0, names.join(", "))
        }
    }
}

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

    /// Builds a minimal valid XEX2 header buffer with the given field values.
    fn make_header(
        magic: u32,
        module_flags: u32,
        header_size: u32,
        reserved: u32,
        security_offset: u32,
        header_count: u32,
    ) -> Vec<u8> {
        let mut buf = Vec::with_capacity(HEADER_SIZE);
        buf.extend_from_slice(&magic.to_be_bytes());
        buf.extend_from_slice(&module_flags.to_be_bytes());
        buf.extend_from_slice(&header_size.to_be_bytes());
        buf.extend_from_slice(&reserved.to_be_bytes());
        buf.extend_from_slice(&security_offset.to_be_bytes());
        buf.extend_from_slice(&header_count.to_be_bytes());
        buf
    }

    #[test]
    fn test_parse_valid_header() {
        let data = make_header(XEX2_MAGIC, 0x01, 0x3000, 0, 0x90, 15);
        let header = parse_header(&data).unwrap();
        assert_eq!(header.magic, XEX2_MAGIC);
        assert_eq!(header.module_flags, ModuleFlags(0x01));
        assert_eq!(header.header_size, 0x3000);
        assert_eq!(header.reserved, 0);
        assert_eq!(header.security_offset, 0x90);
        assert_eq!(header.header_count, 15);
    }

    #[test]
    fn test_invalid_magic() {
        let data = make_header(0xDEADBEEF, 0, 0, 0, 0, 0);
        let err = parse_header(&data).unwrap_err();
        assert!(matches!(err, Xex2Error::InvalidMagic(0xDEADBEEF)));
    }

    #[test]
    fn test_file_too_small() {
        let data = [0u8; 10];
        let err = parse_header(&data).unwrap_err();
        assert!(matches!(
            err,
            Xex2Error::FileTooSmall {
                expected: HEADER_SIZE,
                ..
            }
        ));
    }

    #[test]
    fn test_module_flags_display_title() {
        let flags = ModuleFlags(0x01);
        assert_eq!(flags.to_string(), "0x00000001 [TITLE]");
    }

    #[test]
    fn test_module_flags_display_multiple() {
        let flags = ModuleFlags(0x09); // TITLE | DLL_MODULE
        assert_eq!(flags.to_string(), "0x00000009 [TITLE, DLL_MODULE]");
    }

    #[test]
    fn test_module_flags_display_none() {
        let flags = ModuleFlags(0);
        assert_eq!(flags.to_string(), "0x00000000");
    }

    #[test]
    fn test_module_flags_display_all() {
        let flags = ModuleFlags(0xFF);
        let names = flags.flag_names();
        assert_eq!(names.len(), 8);
        assert_eq!(names[0], "TITLE");
        assert_eq!(names[7], "USER_MODE");
    }

    /// Test against the actual default.xex sample file.
    #[test]
    fn test_parse_sample_header() {
        let path = format!("{}/tests/data/default.xex", env!("CARGO_MANIFEST_DIR"));
        let data = std::fs::read(&path).expect("sample file should exist at tests/data/default.xex");
        let header = parse_header(&data).unwrap();
        assert_eq!(header.magic, XEX2_MAGIC);
        assert_eq!(header.module_flags, ModuleFlags(0x00000001));
        assert_eq!(header.header_size, 0x00003000);
        assert_eq!(header.reserved, 0x00000000);
        assert_eq!(header.security_offset, 0x00000090);
        assert_eq!(header.header_count, 15);
    }
}