//! Binary parsing utilities for reading big-endian font data.

use super::FontError;

/// A zero-copy reader over a byte slice, for big-endian binary parsing.
pub struct Reader<'a> {
    data: &'a [u8],
}

impl<'a> Reader<'a> {
    pub fn new(data: &'a [u8]) -> Self {
        Reader { data }
    }

    pub fn len(&self) -> usize {
        self.data.len()
    }

    fn check(&self, offset: usize, size: usize) -> Result<(), FontError> {
        if offset + size > self.data.len() {
            Err(FontError::UnexpectedEof)
        } else {
            Ok(())
        }
    }

    pub fn u16(&self, offset: usize) -> Result<u16, FontError> {
        self.check(offset, 2)?;
        Ok(u16::from_be_bytes([
            self.data[offset],
            self.data[offset + 1],
        ]))
    }

    pub fn i16(&self, offset: usize) -> Result<i16, FontError> {
        self.check(offset, 2)?;
        Ok(i16::from_be_bytes([
            self.data[offset],
            self.data[offset + 1],
        ]))
    }

    pub fn u32(&self, offset: usize) -> Result<u32, FontError> {
        self.check(offset, 4)?;
        Ok(u32::from_be_bytes([
            self.data[offset],
            self.data[offset + 1],
            self.data[offset + 2],
            self.data[offset + 3],
        ]))
    }

    pub fn i32(&self, offset: usize) -> Result<i32, FontError> {
        self.check(offset, 4)?;
        Ok(i32::from_be_bytes([
            self.data[offset],
            self.data[offset + 1],
            self.data[offset + 2],
            self.data[offset + 3],
        ]))
    }

    /// Read a 4-byte tag (e.g., table tags like b"head").
    pub fn tag(&self, offset: usize) -> Result<[u8; 4], FontError> {
        self.check(offset, 4)?;
        Ok([
            self.data[offset],
            self.data[offset + 1],
            self.data[offset + 2],
            self.data[offset + 3],
        ])
    }

    /// Get a sub-slice of the data.
    pub fn slice(&self, offset: usize, len: usize) -> Result<&'a [u8], FontError> {
        self.check(offset, len)?;
        Ok(&self.data[offset..offset + len])
    }
}