Add an api to construct font data

Author: jrmuizel

core-graphics/src/font.rs

@@ -140,6 +140,126 @@ impl CGFont {
             None
         }
     }
+
+    /// Will construct a Vec<u8> containing a font that has the tables and table contents of the
+    /// CGFont. This will not necessarily be the exact same bytes as the original font but should
+    /// be functionally equivalent.
+    ///
+    /// Manually reconstructing a font is necessary because CoreGraphics does not provide a method
+    /// to retrieve the actual underlying data.
+    pub fn construct_font_data(&self) -> Vec<u8> {
+        construct_font_data(self)
+    }
+}
+
+fn calc_table_checksum(table: &[u8], skip_checksum_adjust: bool) -> u32 {
+    use std::convert::TryInto;
+    let mut sum = std::num::Wrapping(0);
+    let mut i = 0;
+    let mut chunks = table.chunks_exact(4);
+    for chunk in &mut chunks {
+        if skip_checksum_adjust && i == 2 {
+
+        } else {
+            let val = u32::from_be_bytes(chunk.try_into().unwrap());
+            sum += std::num::Wrapping(val)
+        }
+        i += 1;
+    }
+
+    // The table will be zero padded to be 4 byte aligned when written out
+    // so compute the checksum as if that were the case.
+    let mut val = [0; 4];
+    val[0..chunks.remainder().len()].copy_from_slice(chunks.remainder());
+    let val = u32::from_be_bytes(val);
+    sum += std::num::Wrapping(val);
+
+    sum.0
+}
+
+fn max_pow2_less_than_equal(a: i32) -> i32 {
+    let x = 1;
+    let mut shift = 0;
+    while (x << (shift + 1)) <= a {
+      shift+=1;
+    }
+    shift
+}
+
+// This code is inspired by the code in mozilla-central/gfx/2d/ScaledFontMac.cpp
+fn construct_font_data(font: &CGFont) -> Vec<u8> {
+    struct TableRecord {
+        tag: u32,
+        checksum: u32,
+        offset: u32,
+        length: u32,
+        data: CFData,
+    }
+
+    let tags = font.copy_table_tags();
+    let count = tags.len();
+    let mut records = Vec::with_capacity(tags.len() as usize);
+    let mut offset: u32 = 0;
+    offset += std::mem::size_of::<u32>() as u32 * 3;
+    offset += std::mem::size_of::<u32>() as u32 * 4 * count as u32;
+    let mut cff = false;
+    for tag in tags.iter() {
+        let data = font.copy_table_for_tag(*tag).unwrap();
+        let skip_checksum_adjust = *tag == 0x68656164;  // 'head'
+
+        if *tag == 0x43464620 {  // 'CFF '
+            cff = true;
+        }
+        let checksum = calc_table_checksum(data.bytes(), skip_checksum_adjust);
+        records.push(TableRecord { tag: *tag, offset, length: data.len() as u32, data: data.clone(), checksum});
+        offset += data.len() as u32;
+        // 32 bit align the tables
+        offset = (offset + 3) & !3;
+    }
+
+    let mut buf: Vec<u8> = Vec::new();
+    if cff {
+        buf.extend_from_slice(&0x4f54544fu32.to_be_bytes());
+    } else {
+        buf.extend_from_slice(&0x00010000u32.to_be_bytes());
+    }
+
+    buf.extend_from_slice(&(count as u16).to_be_bytes());
+    let max_pow2_count = max_pow2_less_than_equal(count as i32);
+    buf.extend_from_slice(&((1u16 << max_pow2_count) * 16).to_be_bytes());
+    buf.extend_from_slice(&(max_pow2_count as u16).to_be_bytes());
+    buf.extend_from_slice(&((count as u16 - (1 << max_pow2_count)) * 16).to_be_bytes());
+
+  // write table record entries
+  for rec in &records {
+    buf.extend_from_slice(&rec.tag.to_be_bytes());
+    buf.extend_from_slice(&rec.checksum.to_be_bytes());
+    buf.extend_from_slice(&rec.offset.to_be_bytes());
+    buf.extend_from_slice(&rec.length.to_be_bytes());
+  }
+
+  // write tables
+  let mut checksum_adjustment_offset = 0;
+  for rec in &records {
+    if rec.tag == 0x68656164 { // 'head'
+        checksum_adjustment_offset = buf.len() + 2 * 4;
+    }
+    assert!(buf.len() == rec.offset as usize);
+    buf.extend_from_slice(rec.data.bytes());
+    // align
+    let extra = ((buf.len() + 3) & !3) - buf.len();
+    buf.extend_from_slice(&[0;4][0..extra]);
+  }
+
+  // clear the checksumAdjust field before checksumming the whole font
+  for b in &mut buf[checksum_adjustment_offset..checksum_adjustment_offset+4] {
+    *b = 0;
+  }
+  let font_check_sum = (0xb1b0afba_u32.wrapping_sub(
+    calc_table_checksum(&buf, false))).to_be_bytes();
+  (&mut buf[checksum_adjustment_offset..checksum_adjustment_offset+4]).copy_from_slice(&font_check_sum);
+
+  buf
 }
 
 #[link(name = "CoreGraphics", kind = "framework")]
@@ -174,3 +294,19 @@ extern {
     fn CGFontCopyVariations(font: ::sys::CGFontRef) -> CFDictionaryRef;
     fn CGFontCopyVariationAxes(font: ::sys::CGFontRef) -> CFArrayRef;
 }
+
+#[cfg(test)]
+mod test {
+    use core_foundation::string::CFString;
+    use crate::font::*;
+    #[test]
+    fn construct_font_data() {
+        use std::sync::Arc;
+
+        let font = CGFont::from_name(&CFString::from_static_string("Helvetica")).unwrap();
+        let data = font.construct_font_data();
+        let data_provider = crate::data_provider::CGDataProvider::from_buffer(Arc::new(data));
+        let font = CGFont::from_data_provider(data_provider).unwrap();
+        assert_eq!(font.postscript_name(), "Helvetica");
+    }
+}