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// https://docs.microsoft.com/en-us/typography/opentype/spec/head

use crate::parser::SafeStream;
use crate::Font;

// This is a mandatory table, so we already know that this table exists
// and has a valid size. So we can use SafeStream.

#[derive(Clone, Copy, PartialEq, Debug)]
pub(crate) enum IndexToLocationFormat {
    Short,
    Long,
}

impl<'a> Font<'a> {
    #[inline]
    pub(crate) fn index_to_location_format(&self) -> Option<IndexToLocationFormat> {
        const INDEX_TO_LOC_FORMAT_OFFSET: usize = 50;
        let num: u16 = SafeStream::read_at(self.head, INDEX_TO_LOC_FORMAT_OFFSET);
        match num {
            0 => Some(IndexToLocationFormat::Short),
            1 => Some(IndexToLocationFormat::Long),
            _ => None,
        }
    }

    /// Returns font's units per EM.
    ///
    /// Returns `None` if value is not in a 16..16384 range.
    #[inline]
    pub fn units_per_em(&self) -> Option<u16> {
        const UNITS_PER_EM_OFFSET: usize = 18;
        let num: u16 = SafeStream::read_at(self.head, UNITS_PER_EM_OFFSET);
        if num >= 16 && num <= 16384 {
            Some(num)
        } else {
            None
        }
    }
}