//! the [cmap] table
//!
//! [cmap]: https://docs.microsoft.com/en-us/typography/opentype/spec/cmap

include!("../../generated/generated_cmap.rs");

// https://learn.microsoft.com/en-us/typography/opentype/spec/cmap#unicode-platform-platform-id--0
const UNICODE_BMP_ENCODING: u16 = 3;

fn size_of_cmap4(seg_count: u16, gid_count: u16) -> u16 {
    8 * 2  // 8 uint16's
    + 2 * seg_count * 4  // 4 parallel arrays of len seg_count, 2 bytes per entry
    + 2 * gid_count // 2 bytes per gid in glyphIdArray
}

impl CmapSubtable {
    fn create_format_4(
        lang: u16,
        end_code: Vec<u16>,
        start_code: Vec<u16>,
        id_deltas: Vec<i16>,
    ) -> Self {
        assert!(
            end_code.len() == start_code.len() && end_code.len() == id_deltas.len(),
            "uneven parallel arrays, very bad. Very very bad."
        );

        let seg_count: u16 = start_code.len().try_into().unwrap();
        // Spec: Log2 of the maximum power of 2 less than or equal to segCount (log2(searchRange/2),
        // which is equal to floor(log2(segCount)))
        let entry_selector = (seg_count as f32).log2().floor();

        // Spec: Maximum power of 2 less than or equal to segCount, times 2
        // ((2**floor(log2(segCount))) * 2, where “**” is an exponentiation operator)
        let search_range = 2u16.pow(entry_selector as u32).checked_mul(2).unwrap();

        // if 2^entry_selector*2 is a u16 then so is entry_selector
        let entry_selector = entry_selector as u16;
        let range_shift = seg_count * 2 - search_range;

        let id_range_offsets = vec![0; id_deltas.len()];
        CmapSubtable::Format4(Cmap4::new(
            size_of_cmap4(seg_count, 0),
            lang,
            seg_count * 2,
            search_range,
            entry_selector,
            range_shift,
            end_code,
            start_code,
            id_deltas,
            id_range_offsets,
            vec![], // becauseour idRangeOffset's are 0 glyphIdArray is unused
        ))
    }
}

impl Cmap {
    /// Generates a [cmap](https://learn.microsoft.com/en-us/typography/opentype/spec/cmap) that is expected to work in most modern environments.
    ///
    /// For the time being just emits [format 4](https://learn.microsoft.com/en-us/typography/opentype/spec/cmap#format-4-segment-mapping-to-delta-values)
    /// so we can drive towards compiling working fonts. In time we may wish to additionally emit format 12 to support
    /// novel codepoints.
    pub fn from_mappings(mappings: impl IntoIterator<Item = (char, GlyphId)>) -> Cmap {
        let mut end_code = Vec::new();
        let mut start_code = Vec::new();
        let mut id_deltas = Vec::new();

        let mut prev = (u16::MAX - 1, u16::MAX - 1);
        for (cp, gid) in mappings.into_iter() {
            let cp = (cp as u32).try_into().unwrap();
            let next_in_run = (
                prev.0.checked_add(1).unwrap(),
                prev.1.checked_add(1).unwrap(),
            );
            let current = (cp, gid.to_u16());
            // Codepoint and gid need to be continuous
            if current != next_in_run {
                // Start a new run
                start_code.push(cp);
                end_code.push(cp);

                // we might need to reach further than an i16 can take us
                // using idRangeOffset ... but we're saving that for another day
                id_deltas.push((gid.to_u16() as i32 - cp as i32).try_into().unwrap());
            } else {
                // Continue the prior run
                let last = end_code.last_mut().unwrap();
                *last = cp;
            }
            prev = current;
        }

        // close out
        start_code.push(0xFFFF);
        end_code.push(0xFFFF);
        id_deltas.push(1);

        Cmap::new(vec![EncodingRecord::new(
            PlatformId::Unicode,
            UNICODE_BMP_ENCODING,
            CmapSubtable::create_format_4(
                0, // set to zero for all 'cmap' subtables whose platform IDs are other than Macintosh
                end_code, start_code, id_deltas,
            ),
        )])
    }
}

#[cfg(test)]
mod tests {
    use font_types::{BigEndian, GlyphId, Scalar};
    use read::{
        tables::cmap::{Cmap, CmapSubtable, PlatformId},
        FontData, FontRead,
    };

    use crate::{
        dump_table,
        tables::cmap::{self as write, UNICODE_BMP_ENCODING},
    };

    fn to_vec<T: Scalar>(bees: &[BigEndian<T>]) -> Vec<T> {
        bees.iter().map(|be| be.get()).collect()
    }

    // https://learn.microsoft.com/en-us/typography/opentype/spec/cmap#format-4-segment-mapping-to-delta-values
    // "map characters 10-20, 30-90, and 153-480 onto a contiguous range of glyph indices"
    #[test]
    fn generate_simple_cmap4() {
        let mappings = (10..=20)
            .chain(30..=90)
            .chain(153..=480)
            .enumerate()
            .map(|(idx, codepoint)| (char::from_u32(codepoint).unwrap(), GlyphId::new(idx as u16)));
        let cmap = write::Cmap::from_mappings(mappings);

        let bytes = dump_table(&cmap).unwrap();
        let font_data = FontData::new(&bytes);
        let cmap = Cmap::read(font_data).unwrap();

        assert_eq!(1, cmap.encoding_records().len(), "{cmap:?}");
        let encoding_record = &cmap.encoding_records()[0];
        assert_eq!(
            (PlatformId::Unicode, UNICODE_BMP_ENCODING),
            (encoding_record.platform_id(), encoding_record.encoding_id())
        );

        let CmapSubtable::Format4(cmap4) = encoding_record.subtable(font_data).unwrap() else {
            panic!("Expected a cmap4 in {encoding_record:?}");
        };

        // The spec example says entry_selector 4 but the calculation it gives seems to yield 2 (?)
        assert_eq!(
            (8, 8, 2, 0),
            (
                cmap4.seg_count_x2(),
                cmap4.search_range(),
                cmap4.entry_selector(),
                cmap4.range_shift()
            )
        );
        assert_eq!(
            vec![10u16, 30u16, 153u16, 0xffffu16],
            to_vec(cmap4.start_code())
        );
        assert_eq!(
            vec![20u16, 90u16, 480u16, 0xffffu16],
            to_vec(cmap4.end_code())
        );
        // The example starts at gid 1, we're starting at 0
        assert_eq!(vec![-10i16, -19i16, -81i16, 1i16], to_vec(cmap4.id_delta()));
        assert_eq!(
            vec![0u16, 0u16, 0u16, 0u16],
            to_vec(cmap4.id_range_offsets())
        );
    }
}